aashto chbtw amd 1_2008

Copyright American Association of State Highway and Transportation Officials Provided by IHS under license with AASHTO Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto

Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or networking permitted without license from IHS

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To recipients of the Construction Handbook for Bridge Temporary Works, First Edition (1995): Instructions

Interim revisions have been made to the Construction Handbook for Bridge Temporary Works, First Edition (1995). They have been designed to replace the corresponding pages in the book and are numbered accordingly.

Underlined copy indicates revisions that were approved in 2007 by the AASHTO Highways Subcommittee on

Bridges and Structures. A listing of newly changed and deleted articles is included with these interim revisions as an addendum to the preface of the book.

All revised pages also display a box in the lower outside corner indicating the interim publication year. Any

non-technical changes in page appearance will be indicated by this revision box alone to differentiate such changes from those which have been approved by the AASHTO Highways Subcommittee on Bridges and Structures.

To keep your Specifications correct and up-to-date, please replace the appropriate pages in the book with the

pages in this packet.



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ISBN: 978-1-56051-397-1 Publication Code: CHBTW-1-I1

American Association of State Highway and Transportation Officials

444 North Capitol Street, NW Suite 249 Washington, DC 20001

202-624-5800 phone/202-624-5806 fax www.transportation.org

2008 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.



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1

CHAPTER 1. INTRODUCTION

SCOPE

This construction handbook has been developed for use by contractors and construction engineers

involved in bridge construction on Federal-aid highway projects. This document may also be of interest to

falsework design engineers, and supplements information found in the Guide Design Specification for Bridge

Temporary Works.(1) The content is construction-oriented, focusing primarily on standards of material quality

and means and methods. This handbook contains chapters on falsework, formwork, and temporary retaining

structures. For more in-depth discussion on a particular topic, related literature and references are identified.

Chapter Two. Falsework identifies material standards, the assessment and protection of foundations,

construction-related topics, loading considerations, and inspection guidelines. Methods for in situ testing of

foundations are identified. General guidelines regarding timber construction, proprietary shoring systems, cable

bracing, bridge deck falsework, and traffic openings are also discussed.

Chapter Three. Formwork identifies and describes the various components and formwork types

commonly used in bridge construction. Information on load considerations and design nomographs are

provided. General guidelines relating to formwork construction and form maintenance are also discussed.

Chapter Four. Temporary Retaining Structures focuses primarily on cofferdams and their

application to bridge construction. As indicated by the chapter title, however, general topics relating to a wide

range of temporary retaining structures are also addressed. Specific topics include classification of construction

types, relative costs, sealing and buoyancy control, seepage control, and protection. The construction of timber

sheet pile cofferdams, soldier pile and wood lagging cofferdams, and steel sheet pile cofferdams is reviewed.

Methods of internal bracing and soil and rock anchorage are also discussed.

Section properties of standard dressed and rough lumber, bridge deck falsework design examples,

recommended thicknesses for wood lagging, and steel sheet pile data are included as appendixes. Definitions

and related publications are identified below.

DEFINITIONS

For the purpose of this manual, the following definitions apply. These definitions are not intended to

be exclusive, but are generally consistent with the common usage of these terms.

Falsework Temporary construction work used to support the permanent structure until it becomes

self-supporting. Falsework would include steel or timber beams, girders, columns piles and foundations, and

any proprietary equipment, including modular shoring frames, post shores and adjustable horizontal shoring.

Shoring A component of falsework such as horizontal, vertical, or inclined support members. For the

purpose of this document, this term is used interchangeably with falsework.

Formwork A temporary structure or mold used to retain the plastic or fluid concrete in its

designated shape until it hardens. Formwork must have enough strength to resist the fluid pressure exerted by

plastic concrete and any additional fluid pressure effects generated by vibration.

2008 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.



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2 Interim 2008

Cofferdam A temporary watertight enclosure that allows construction of the permanent structure

under dry conditions.

RELATED PUBLICATIONS

California Falsework Manual, California Department of Transportation, Sacramento, CA.

Certification Program for Bridge Temporary Works (FHWA-RD-93-033), Federal Highway Administration, Washington, DC.

Formwork for Concrete (SP-4), Seventh Edition, American Concrete Institute, Detroit, MI.

Foundation Construction, A. Brinton Carson, McGraw-Hill, New York, NY.

Guide Design Specifications for Bridge Temporary Works (FHWA-RD-93-032), Federal Highway Administration, Washington, DC. See also AASHTO GSBTW-1 (1995) and GSBTW-1-I1 (2008).

Guide Standard Specification for Bridge Temporary Works (FHWA-93-031), Federal Highway Administration, Washington, DC.

Handbook of Temporary Structures in Construction, R.T. Ratay, Ed., Second Edition, McGraw-Hill Book Company, New York.

Lateral Support Systems and Underpinning, Vols. I, II, III (FHWA-RD-75-128, 129, 130), Federal Highway Administration, Washington, DC.

Soil Mechanics, Foundations, and Earth Structures (NAVFAC DM-7), Department of the Navy, Alexandria, VA.

Standard Specifications for Highway Bridges, 17th Edition (HB-17), American Association of State Highway and Transportation Officials, Washington, DC.

Synthesis of Falsework, Formwork, and Scaffolding for Highway Bridge Structures (FHWA-RD-91-062), Federal Highway Administration, Washington, DC.

Temporary Works, J.R. Illingworth, Thomas Telford, London, England.




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3 Interim2008

CHAPTER 2. FALSEWORK

MATERIALS AND MANUFACTURED COMPONENTS

Structural Steel Quality of Steel Steel grades greater than ASTM A 36/A 36M are generally not recommended for

falsework construction unless certified or test samples are taken. The Guide Design Specification for Bridge

Temporary Works permits the use of higher working stresses for other grades of steel, provided the grade of

steel can be identified. Identification is the contractors responsibility. If steel properties are unknown and test

samples are not taken, steel can generally be assumed to be ASTM A 36/A 36M. For reference, some of the

more common steel designations predating ASTM A 36/A 36M are provided in table 1.

Table 1. Early ASTM steel specifications.(2)

ASTM requirement

Date Specification Remark Tensile strength, lbf/in2 Minimum yield point, lbf/in2

1924-1931 ASTM A 7 (withdrawn 1967) Structural steel 55,000 to 65,000 T.S. or not less than 30,000

Rivet steel 46,000 to 56,000 T.S. or not less than 25,000

ASTM A 9 (withdrawn 1940) Structural steel 55,000 to 65,000 T.S. or not less than 30,000

Rivet steel 46,000 to 56,000 T.S. or not less than 25,000

1939-1948 ASTM A 7-A 9 Structural steel 60,000 to 72,000 T.S. or not less than 33,000

1939-1949 ASTM A 141-39 (withdrawn 1967) Rivet steel 52,000 to 62,000 T.S. or not less than 28,000

Conversion: 1,000 lbf/in2

Dimensional Tolerances Rolling structural shapes and plates involves such factors as roll wear,

subsequent roll dressing, temperature variations, etc., which cause the finished product to vary from published

profiles. Mill dimensional tolerances are identified in AASHTO M 160M/M 160 (ASTM A 6/A 6M), Standard

Specification for General Requirements for Rolled Steel Plates, Shapes, Sheet Piling, and Bars for Structural

Use.(3) This information is provided in tables 2 and 3 for general reference.

Conditioning of Salvaged Steel AASHTO M 160M/M 160 (ASTM A 6/A 6M) also provides

guidelines for the condition of plates, structural shapes, and steel sheet piling, as follows:

Plate Conditioning Plates may be conditioned by the manufacturer or processor for the removal of

imperfections or depressions on the top and bottom surfaces by grinding, provided the area ground is

well faired without abrupt changes in contour and the grinding does not reduce the thickness of the

plate by: (1) more than 7 percent under the normal thickness for plates ordered to weight per square ft,

but in no case more than in (3.2 mm); or (2) below the permissible minimum thickness for plates

ordered to thickness in inches or millimeters.




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4

Table 2. Permissible variations in cross section for W and HP shapes.(3)

A, depth, in B, flange width, in Section nominal size, in

Over theoretical

Under theoretical

Over theoretical

Under theoretical

T + T', flanges, out of square, max., in

Ea, web off center,

max., in

C, max., depth at any cross section over theoretical

depth, in

To 12, incl. 1/8 1/8 1/4 3/16 1/4 3/16 1/4

Over 12 1/8 1/8 1/4 3/16 5/16 3/16 1/4

Notes: (a) Variation of 5/16-in max. for sections over 426 lb/ft. (b) Conversion: 1 in = 25.4 mm; 1 lb/ft = 1.49 kg/m.

Table 3. Permissible variations in camber and sweep.(3) Permissible variation, in

Sizes Length Camber Sweep

Sizes with flange width equal to or greater than 6 in All 1/8 in x

(total length, ft) 10

Sizes with flange width less than 6 in All

1/8 in x (total length, ft) 10

1/8 in x (total length, ft) 5

4 1/8 in x (total length, ft) with 3/8 in max. 10

Certain sections with a flange width approx. equal to depth and specified on order as columnsa Over 45 ft 3/8 in + 1/8 in x (total length, ft - 45)

10

Notes: (a) Applies only to: W 8 x 31 and heavier, W 10 x 49 and heavier, W 12 x 65 and heavier, W 14 x 90 and heavier. If other sections are specified on the order as columns, the tolerance will be subject to negotiation with the manufacturer. (b) Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m.




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Imperfections on the top and bottom surfaces of plates may be removed by chipping, grinding, or arc-

air gouging and then by depositing weld metal subject to the following limiting conditions:

The chipped, ground, or gouged area shall not exceed 2 percent of the area of the surface being conditioned.

After removal of any imperfections in preparation for welding, the thickness of the plate at any location must not be reduced by more than 30 percent of the nominal thickness of the plate. (AASHTO

M 160M/M 160 (ASTM A 6/A 6M) restricts the reduction in thickness to a 20 percent maximum.)

The edges of plates may be conditioned by the manufacturer or processor to remove injurious imperfections by grinding, chipping, or arc-air gouging and welding. Prior to welding, the depth of

depression, measured from the plate edge inward, shall be limited to the thickness of the plate,

with a maximum depth of 1 in (25.4 mm).

Structural Shapes and Steel Sheet Piling Conditioning These products may be conditioned by the

manufacturer for the removal of injurious imperfections or surface depressions by grinding, or chipping

and grinding, provided the area ground is well faired without abrupt changes in contour and the

depression does not extend below the rolled surface by more than: (1) 1/32 in (0.8 mm) for material less

than 3/8 in (9.5 mm) in thickness; (2) 1/16 in (1.6 mm) for material 3/8 to 2 in (9.5 to 50.8 mm) inclusive

in thickness; or (3) 1/8 in (3.2 mm) for material over 2 in (50.8 mm) in thickness.

Imperfections that are greater in depth than the limits previously listed may be removed and then weld

metal deposited subject to the following limiting conditions:

The total area of the chipped or ground surface of any piece prior to welding shall not exceed 2 percent of the total surface area of that piece.

The reduction in thickness of material resulting from removal of imperfections prior to welding shall not exceed 30 percent of the nominal thickness at the location of the imperfection, nor shall

the depth of depression prior to welding exceed 1 in (32 mm) in any case except as follows:

The toes of angles, beams, channels, and zees and the stems and toes of tees may be conditioned by

grinding, chipping, or arc-air gouging and welding. Prior to welding, the depth of depression,

measured from the toe inward, shall be limited to the thickness of the material at the base of the

depression, with a maximum depth limit of 2 percent of the total surface area.

Welding Most of the ASTM-specification construction steels can be welded without special

precautions or procedures. The weld electrode should have properties matching those of the base metal. When

properties are comparable, the deposited weld metal is referred to as matching weld metal. See AWS

D1.1/D1.1M(4) for requirements. Table 4 provides matching weld metal for many of the common ASTM-

designated structural steels. In general, welding of unidentified structural steel is not recommended unless

weldability is determined.

Most of the readily available structural steels are suitable for welding. Welding procedures can be

based on specified steel chemistry because most mill lots are usually below the maximum specified limits.

Table 5 shows the ideal chemistry for carbon steels.




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Table 4. Matching filler metal requirements.(4) Welding Processa,b

Group Base metal steel specificationc Submerged metal arc

welding (SMAW) Submerged arc welding (SAW)

Gas metal arc welding (GMAW)

Flux cored arc welding (FCAW)

I ASTM A 36, A 53 Grade B, A 500, A 501, A 529, A 570 Grades 40, 45, and 50 A 709 Grade 36

AWS A5.1 or A5.5 E60XX or E70XX

AWS A5.17 or A5.23 F6X or F7X-EXXX

AWS A5.18 ER70S-X

AWS A5.20 E5XT-X and E7XT-X (except -2, -3, -10, -GS)

II ASTM A 242,d A 572 Grades 42 and 50 A 588 A 709 Grades 50 and 50W

AWS A5.1 OR A5.5 E70XXe

AWS A5.17 or A5.23 F7X-EXXX

AWS A5.18 ER70S-X

AWS A5.20 E7XT-X (except -2, -3, -10, -GS)

III ASTM A 572, Grades 60 and 65

AWS A5.5 E80XXe

AWS A5.23 F8X-EXXXf

AWS A5.28 ER80Sf

AWS A5.29 E8XTf

IV ASTM A 514 (over 2 in thick), A 709 Grades 100 and 100W (2 in and under)

AWS A5.5 E100XXe

AWS A5.23 F10X-EXXXf

AWS A5.28 ER100Sf

AWS A5.29 E10XTf

V ASTM A 514 (2 in and under), A 709 Grades 100 and l00W (2 in and under)

AWS A5.5 E100XXe

AWS A5.23 F11X-EXXXf

AWS A5.28 ER110Sf

AWS A5.29 E11XTf

Notes: (a) When welds are to be stress relieved, the deposited weld metal shall not exceed 0.05 percent vanadium. (b) See AWS D1.1/D1.1M(4), Sec. 4.20 for electroslag and electrogas weld metal requirements. (c) In joints involving base metals of two different groups, low-hydrogen filler metal electrodes applicable to the lower strength group metal may be used. The low-hydrogen processes shall be subject to the technique requirements applicable to the higher strength group. (d) Special welding materials and procedures may be required to match the notch toughness of base metal or for atmospheric corrosion and weathering characteristics. (e) Low hydrogen classifications only. (f) Deposited weld metal shall have a minimum impact strength of 20 ft-lbf (27 J) at 0 F (-18 C) when Charpy V-notch specimens are used. This requirement is applicable only to bridges. (g) Conversion: 1 in = 25.4 mm

Table 5. Preferred analysis of carbon steel for good weldability.(5)

Element Normal Range (%) Carbon 0.06 - 0.25 Manganese 0.35 - 0.80 Silicon 0.10 max Sulfur 0.035 max Phosphorus 0.030 max

Guidance with respect to workmanship, qualification, and inspection of weldable steel can be obtained

from Structural Welding Code, AWS D1.1/D1.1M.(4) Acceptable and unacceptable weld profiles prescribed by

AWS are illustrated in figure 1.




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7 Interim2008

Figure 1. Acceptable and unacceptable weld profiles.(4)

Timber Timber Quality The design values for new lumber are obtained from grading rules published by

several agencies, including: National Lumber Grades Authority (a Canadian agency), Northeastern Lumber

Manufacturers Association, Northern Softwood Lumber Bureau, Southern Pine Inspection Bureau, West Coast

Lumber Inspection Bureau, and Western Wood Products Association. Design Values for most species and

grades of visually graded dimension lumber are based on the provisions of ASTM D 1990, Establishing

Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens.

Design values for visually graded timbers, decking, and some species and grades of dimension lumber are based

on the provisions of ASTM D 245, Establishing Structural Grades and Related Allowable Properties for

Visually Graded Lumber.

The methods in ASTM D 245 involve adjusting the strength properties of small clear specimens of

wood, as given in ASTM D 2555, Establishing Clear Wood Strength Values, for the effects of knots, slope of

grain, splits, checks, size, duration of load, moisture content, and other influencing factors, to obtain design

values applicable to normal conditions of service. ASTM D 245 describes the procedures for rating lumber on

the basis of strength ratio. Strength ratio of a structural timber is the ratio of its strength to that which it would

have if no weakening characteristics were present.

Used Lumber Where the origin and grading of the material is no longer known, it should be

regraded by a qualified agency or individual. Timber should be discarded if it has been painted such that it

prevents assessment, if there is any sign of rot (fungal or chemical), if there is mechanical damage, or if there is

any undue distortion of shape. Timber should never be reused without careful inspection.




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8

Timber Characteristics Because wood is an organic material, it is subject to variations in structure

or properties or both. Some important anatomical characteristics of wood and their effects on the strength of

wood members are as follows:

Knots A knot is a portion of a branch or limb, which has been surrounded by subsequent growth of

the wood of the trunk. Knots reduce the strength of wood because they interrupt the continuity and

direction of wood fibers. They also cause local stress concentrations where grain patterns are abruptly

altered. The influence of a knot depends on its size, location, shape, soundness, and the type of stress

considered. In general, knots have a greater effect in tension than in compression, whether stresses are

applied axially or as a result of bending. Shapes of knots in various structural members and methods of

measurement are illustrated in figure 2.

Slope of Grain Slope of grain or cross grain are terms used to describe the deviation in wood fiber

orientation from a line parallel to the edge of the specimen. It is expressed as a ratio such as 1 in 6 or 1

in 14, and is measured over sufficient distance along the piece to be representative of the general slope

of the wood fibers. Slope of grain has a significant effect on wood mechanical properties. Strength, for

example, decreases as the grain deviation increases. Specimens with severe cross grain are also more

susceptible to warp and other dimensional deformations due to changes in moisture content. The

technique to measure slope of grain is illustrated in figure 3.

Checks and Splits Checks and splits are separations of the wood across or through the rings of

annual growth, usually as a result of drying shrinkage during seasoning. Checks are partial depth

fractures, while splits extend through the full cross section. If members are subject only to tension or

compression, checks and splits do not greatly affect strength, unless they occur in zones of severe grain

slope.

Moisture Content Design values prescribed by the National Design Specification for Wood

Construction (NDS) are for normal load duration under dry conditions of service.(8) Dry lumber is defined as

lumber that has been seasoned to a moisture content of 19 percent or less by weight. Green lumber is defined as

lumber having a moisture content in excess of 19 percent. Because the strength of wood varies with the

conditions under which it is used, these design values should only be applied in conjunction with appropriate

design and service recommendations from the National Design Specification.

Member Size Timber members should be generally assumed to be standard dressed (S4S) sawn

lumber unless otherwise shown on the falsework drawings. Section properties of S4S lumber are furnished in

appendix A. While these sizes are generally available on a commercial basis, it is good practice to consult the

local lumber dealer(s) to determine availability.

Typically, the dimensions of rough-cut lumber will vary appreciably from nominal, particularly in the

larger sizes commonly used in falsework construction. If the use of rough-cut material is required by the

falsework design, the actual member size should be verified prior to use.




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soils, field verification of soil strength and compressibility is more difficult. Simple tests include a dynamic

cone penetration test where the number of blows to advance a rod with a cone at the tip is recorded. The number

of blows is a rough indicator of relative density. Field density tests (AASHTO T 191) can also be performed to

determine the unit weight of the soil from which an estimate of relative density can also be obtained. The

measured unit weight can be compared with published information on maximum or minimum unit weights for

various soil types. Alternatively, the maximum and minimum unit weights can be determined by performing

laboratory tests AASHTO T 180 for the maximum unit weight and ASTM D 4254 for minimum unit weight and

calculation of relative density.

A better and more sophisticated procedure for determining the suitability of granular and mixed soil

deposits to support the footings is to perform pressuremeter testing (ASTM D 4719) or dilatometer testing in

shallow hand auger holes extended below bearing level.

Test Pits Test pits can be dug throughout the area to investigate the various soil or rock formations.

Test pits should be used to supplement other field monitoring wherever erratic or discontinuous subsurface

conditions are present. Determining the thickness and character of these deposits from a large excavation is

more accurate than from examination of small diameter samples from borings. Block samples can also be

obtained for laboratory testing.

Proof-Rolling Proof-rolling is a field observation test that can be used to indicate if and when

problem soils are located at shallow distances below grade. The procedure consists of making multiple passes

over the area with a fully loaded dump truck having a minimum weight of 20 tons (18,000 kg). As the dump

truck traverses the area, the amount of ground deflection under loading shall be observed. Deflections of 2 in

(50.8 mm) or less are indications of reasonably good support conditions. Large deflections and severe rutting

are indicative of very poor support conditions. The depth of influence of proof-rolling is likely to be on the

order of 2 to 5 ft (0.6 to 1.5 m). Any weak soil below this depth will remain undetected.

Load Testing The procedures for performing plate bearing tests are described in AASHTO T 235.

The plate load test consists of a loading plate with a minimum 12-in (305-mm) diameter with a jack to provide a

force, and with a truck or other heavy object used as a reaction. Deflections are measured with either survey

instruments or dial gauges. As the jack loads are applied, deflection readings should be taken at the design load

and at twice the design load. The test results are analyzed in accordance with figure 7. The depth of influence of

a plate load test is only about 1.5 times the diameter of the plate. Thus, larger foundations that stress the soil to

greater depths may perform differently than the plate load test would indicate.

Deep Foundations If piles are driven to support the falsework, the driving resistance of each pile should be recorded and

compared to the required driving resistance that has been developed for the project using either a wave equation

analysis or acceptable driving formula. Plumbness, length of pile installed, type of hammer and cushion, surface

alignment of the driven pile, and any other observations that could affect pile performance should also be

recorded. This data should be given to the designer for review. If a load test is required, it should be performed




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14

in accordance with ASTM D 1143. The procedure to calculate the failure load from a load test is identified in

figure 8.

If drilled piers are used to support the falsework, the strength of the soil at bearing level should be

determined in a manner discussed in the previous section for shallow foundations. The length and diameter of

the drilled shaft and bell (if used) should be recorded along with the plumbness and surface alignment. Any

other pertinent field observations that could affect performance, such as the presence of squeezing or caving

soils, water inflow, or accumulation of debris at the base of the drilled pier should be brought to the attention of

the drilled pier contractor and reported to the design engineer.

Protection of the Foundation Area Falsework foundations, in general, are set at a very shallow depth compared with those of permanent

structures. This places them within the zone affected by seasonal moisture content changes, frost action, scour,

and so forth. The area covered by the foundations under the falsework should be considered in relation to the

general topography of the surrounding ground and the likelihood of outside influences affecting it. Steps should

be taken to safeguard it, and avoid undermining conditions such as shown in figure 9. The stability of the

ground under and around the falsework foundations will depend on the ground remaining unaffected by the

following: local influences of water from water courses, extreme rainfall, melting snow, or burst water mains;

severe frosts or excessively dry and hot weather; movements of surrounding ground subjected to excavation,

filling, or other changes; and all pressures applied by adjacent construction operations.

Falsework in Streams Where supports (usually consisting of piles or piers) are installed in rivers or

streams, they should be designed to withstand the horizontal loads arising from flood conditions, applied to an

area of resistance substantially greater than that offered by the supports alone. This increase should account for

the accumulation of river debris. To minimize this accumulation and avoid the impact of larger pieces, measures

should be specified and installed upstream to divert such debris from the supports or to retain it independently.

The measures adopted will depend on the circumstances. The use of fenders, floating booms, and cutwaters

should be considered for this purpose.

Scour is likely to occur in areas of increased stream velocity. It is likely to affect the bed of the

waterway around and under the falsework and any banks, channels, or other existing features of the waterway.

Protection should be provided where such scouring forces are likely to occur.

Foundations on Sloping Ground The stability of foundations on sloping ground should be

examined by a qualified engineer specialized in soil mechanics. For rock slopes, special attention should be

given to the geometry of bedding, cleavage planes, or joint planes that might provide a sliding surface for block

failure. In many sandstone, siltstone, and mudstone formations, it is not possible to predict the shear strength at

bedding planes. Here, it is necessary to ensure that the bedding does not intersect the slope in a manner that

would permit blocks to move out of the face.

Where the requirements are such that foundation members need to be set other than level,

appropriately shaped packs should be used at the base of the vertical member. The foundation member should

be effectively prevented from moving down the slope as shown in figure 10.




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17 Interim2008

Figure 9. Washout under sill support.

(Courtesy of Scaffolding, Shoring, and Forming Institute)(10)

Figure 10. Sole plate and bracing details for falsework supported on a sloped surface.




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18

Fill Material Where falsework is to be carried on fill of unknown origin or quality, the fill should be

investigated. Fill may have abrupt variations in composition, compaction, and strength. Where falsework is

supported on a compacted fill whose properties have been determined, it is important to ensure that both the fill

and the underlying ground are protected, so that no disturbance or loss of material results from the movement of

water or environmental changes. In cases where the fill material is variable in consistency, and unable to receive

and transmit loads uniformly, a minimum depth of 18 in (457 mm) of the fill should be removed and replaced

by well-compacted and stabilized granular material of known bearing capacity.

Heavy Vibrations Deposits or layers of granular materials, if not fully compacted, are susceptible to

consolidation and settlement if subjected to vibrations either from the falsework above, from adjacent operations

(for example, piling), or the passage of heavy traffic. This condition is not accounted for by modification factors

applied to the presumed bearing pressures. Either the granular materials should be compacted, or the sources of

vibration stopped during critical stages of construction. Some uniformly graded sands and silts may also be

adversely affected by vibration from the compaction of concrete above the falsework.




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CONSTRUCTION

General In falsework construction, overall stability is a function of both internal (local) and external (global)

conditions. Internally, falsework can be subject to a wide variety of local horizontal forces produced by out-of-

plumb members, superelevation, differential settlement, and so forth. Therefore, it is necessary for the

falsework assembly to be adequately connected to resist these forces. Although friction often provides means of

load transfer, so-called positive connections eliminate, or at least reduce, the probability of underestimating

the necessary restraint. The need for positive load transfer is particularly apparent when superelevation exists or

the soffit is inclined.

Timber cross-bracing between adjacent steel beams, shown in figure 11, is commonly used for flange

support in falsework construction. In this method, timber struts are set diagonally in pairs between the top

flanges of the adjacent beams, and securely wedged into place. However, timber cross-bracing alone will not

prevent flange buckling because the timber struts resist only compression forces. A more effective flange

support method uses steel tension ties welded, clamped, or otherwise secured across the top and bottom of

adjacent beams in combination with timber cross-bracing between the beams.

Uplift can occur when falsework beams are continuous over a long span, coupled with a relatively

short adjacent span. Two common examples of this condition are longitudinal beams with short end spans and a

transverse beam with a relatively long overhang. In the longitudinal example, uplift can occur at the end

support. For the latter case, shown in figure 12, uplift can occur at the first interior post (support). Both of these

conditions can contribute to instability and, therefore, should be avoided. If uplift cannot be prevented by

loading the short span first, the end of the beam must be tied down or the span lengths changed.

In order to ensure longitudinal stability, it is necessary to provide a system of restraint to prevent the

falsework bents from overturning when the horizontal design load is applied in the longitudinal direction. This

type of restraint can be furnished by diagonal bracing between pairs of adjacent bents, or by direction transfer of

horizontal load into the permanent piers.

Timber Construction Lateral Support of Wood Beams Deep, narrow beams may fail by buckling before the allowable

bending stress is reached if they are not laterally restrained. The amount of restraint needed to ensure beam

stability is a function of the depth-to-width ratio. Blocking of soffit joists for haunches is also required.

Section 4.41 of the National Design Specification for Wood Construction provides approximate

guidelines regarding the lateral restraint of rectangular sawn lumber beams.(8) These guidelines, modified to

reflect the temporary nature of falsework construction, are as follows:

If the nominal depth-to-width ratio of a timber beam is 3:1 or less, no lateral support is needed. If the nominal depth-to-width ratio exceeds 3:1, but is not more than 4:1, the ends of the beam

should be braced at the top and bottom




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Figure 11. Timber cross-bracing between longitudinal stringers.

Figure 12. Cantilevered ledger beam at temporary pile bent.




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In general, plan bracing should be provided at the top lifts (tiers) and at least every third intermediate

lift. The plan bracing at the top tier may be omitted when the grillage used to support the permanent structure is

capable of acting as a diaphragm. When shoring a sloped surface, the tube bracing illustrated in figure 14 is

recommended.

Figure 14. Bracing detail for screw leg supporting a sloped soffit.

Screw-leg Extensions Leg load capacity for modular frames generally decreases as the screw-leg

extensions increase. Eccentric loads on screw (extension) heads should also be avoided. Variations between

various proprietary systems preclude generalizations regarding the extent of load reduction for screw-leg

extension. However, extensions at the top and bottom of a frame totaling 12 in (305 mm) generally do not

significantly affect the allowable leg capacity.




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Cable Bracing Bracing systems consisting of securely anchored cable guys are widely used to resist overturning of

falsework. In particular, cable systems provide an effective means of ensuring the stability of heavy-duty

shoring and are relatively inexpensive when compared to other bracing methods. Cable is also used extensively

as temporary bracing to stabilize falsework bents being erected or removed adjacent to traffic. However, the

effect of preloading the tower legs should be carefully analyzed before implementing this bracing technique.

The cable bracing should also be applied symmetrically to a shoring assembly to avoid unbalanced loading or

overturning.

Cables, with their fastening devices and anchorages, are manufactured assemblies as defined in the

Guide Design Specification for Bridge Temporary Works. Accordingly, and in addition to information that may

be shown on the falsework drawings, the contractor should be requested to furnish a manufacturers catalog or

brochure showing technical data pertaining to the type of cable to be used. Technical data should include the

cable diameter, the number of strands and the number of wires per strand, the ultimate breaking strength or

recommended safe working strength, and such other information as may be needed to identify the cable in the

field.

Prior to installation, cable should be inspected to verify that the type and size of the cable and its

condition (new or used) is consistent with design assumptions. Used cable should be inspected for strength-

reducing flaws, such as obviously worn, frayed, kinked, or corroded cable, which should not be permitted in

construction.

U-bolt clips must be placed on the rope with the u-bolts bearing on the short or dead end of the rope,

and the saddle bearing on the long or live end of the rope. Improperly installed clips will reduce the save

working load by as much as 90 percent. Also, the omission of the thimble in a loop connection will reduce the

safe working load by approximately 50 percent. After installation, clips should be inspected periodically and

tightened as necessary to ensure their effectiveness. General guidelines regarding the number of wire rope clips

and their spacing are shown in figure 15. However, efficiency factors and prescribed clip spacings can vary, and

the manufacturers literature should be consulted for a given application.

Extensive further review of cable bracing may be found in the California Falsework Manual(13),

Chapter 4, Stress Analysis, Section 4-5, Cable Bracing Systems.




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Figure 15. Typical installation of wire rope clip.(16)

Bridge Deck Falsework Multiple girder bridges rarely have ground-supported deck formwork. Deck casting is usually

performed using hanger beams attached to the interior girders, and cantilever brackets affixed to the exterior or

fascia girders. Figure 16 illustrates this forming method, with examples for both steel and concrete girders.

Design examples of bridge deck falsework are provided in appendix B.

The deck forms between interior stringers are generally set on joists hung from the top flange or

supported from the bottom flange. Proprietary hangers include removable brackets or coil-bolt assemblies that

remain permanently embedded in the deck slab. The embedded hangers are generally hung over the top of the

stringer, or welded to stirrups or shear studs projecting from the top surface. Welding the hangers creates a

positive connection that will prevent movement during casting. However, several States prohibit welding these

devices to the permanent structure.

In order to form the cantilevered portion of the deck slab, a needle beam arrangement or overhang

bracket can be used. As shown in figure 16, a needle beam works well for shallow steel girders where bottom

flange tension hangers can be easily attached. This support arrangement is temporarily attached to the steel

members, with no embedment anchors required in the slab.

A more common method of forming the overhang consists of an overhang bracket tied to the

fascia girder with a hanger support. Gravity loads from the formwork, concrete deck, and screed machine act

downward on the bracket. These loads create a force couple on the bracket, where tension is resisted by a

hanger support rod and compression is applied horizontally to the girder web. This compressive force is resisted

by bending in the beam web. For steel stringers, the web could buckle inward due to this out-of-plane force if




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26

(a) Bridge deck forming methods with steel stringers.

(b) Bridge deck forming methods with precast AASHTO girders.

Figure 16. Bridge deck falsework.




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the magnitude of compressive force resistance, and interior diagonal struts may be required to prevent bottom

flange rotation.

Some general guidelines regarding the use of overhang brackets are as follows:

The diagonal leg brace should bear on the web within 6 in (152 mm) of the bottom flange. The exterior stringer should have its top flange tied at regular intervals to prevent outward

rotation. Recommended maximum spacing intervals are 2 ft (0.6 m), when finishing machine rails

are located on the bracket-supported formwork, and 4 ft (1.2 m) when finishing machine rails are

on the top flange of the stringer.

Precast, prestressed concrete I-girders should have ties at 8-ft (2.4-m) maximum spacing. Steel girder diaphragm cross frames are not to be considered as ties if they do not have a top

horizontal strut.

Hardwood blocking [4 in by 4 in (102 mm by 102 mm) minimum] or the equivalent should be wedged between webs of the exterior and interior stringer within 6 in (152 mm) of the bottom

flange, located below the top ties.

Decks for cast-in-place box girders may use other hardware for supporting decks. Deck forms

supported on ledgers are typically used. The ledgers may be supported on bars/brackets cast into the girder

stems or may be nailed (shot) into, using a power-actuated nail gun.

Traffic Openings The width of a traffic opening is generally defined as the distance between the temporary railings and,

as illustrated in figure 17, the clear distance between falsework posts will be considerably greater than the

prescribed width. For a vehicular opening, no portion of the falsework should encroach into the clearance zone

established by: a vertical plane located 3 in (76 mm) behind the back edge of the temporary barrier at its base

and extending upward to a horizontal plane at the top of the rail; and a second vertical plane located 9 in (230

mm) behind the first plane and extending from the horizontal plane, at the top of the rail upward to the

falsework stringer.

Temporary construction clearances often govern layout of spans. A typical example is the required

vertical clearance over freeways in California, shown in table 6. The usual requirement is a clearance of 16 ft-6

in (5.0 m) over the traveled way, but the temporary construction clearance may be as low as 14 ft-6 in (4.4 m).

However, for a structure constructed on ground-supported falsework where a 40-ft (12-m) wide opening for

traffic is needed, an adequate depth of falsework may be 2 ft-6 in to 3 ft (0.8 m to 0.9 m). This results in a final

clearance of 17 ft-0 in to 17 ft-6 in (5.2 m to 5.3 m).




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Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m

(a) Minimum clearance diagram.

(b) Set-back distance between traffic barrier and vertical shoring.

Figure 17. Traffic openings.




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35

CHAPTER 3. FORMWORK

INTRODUCTION

Formwork is a temporary structure that retains plastic or fluid concrete until it gains sufficient strength

to support itself. The formwork system includes the sheathing that is in direct contact with the concrete, the

supporting members, hardware, and bracing.

The cost of formwork is significant relative to the cost of the in-place concrete. Therefore, the selection

and design of formwork can significantly affect the overall cost of the structure. Formwork selection is

influenced by many factors, including concrete pressures, uniformity of the structure shape, accessibility to the

structure, crane capacity, materials availability and cost, anticipated number of reuses, and crew experience.

This chapter presents an overview of formwork components and corresponding information for design.

Formwork for Concrete, published by the American Concrete Institute, provides extensive data for design.(19)

Allowable stresses for formwork materials are those used in standard structural design, except when test data

give different values for proprietary products. Precautions to be taken in the erection, maintenance, and removal

of forms are also discussed in this chapter.

FORM COMPONENTS Vertical forms are constructed from five basic components: (1) sheathing, (2) studs to support the

sheathing, (3) walers to support the studs and align the forms, (4) braces to prevent shifting of the forms under

construction and wind loading, and (5) form ties and spreaders to hold the forms at the correct spacing under the

pressure exerted by the fresh concrete. The formwork structural components and accessories should be

integrated to provide sufficient capacity in addition to easy assembly and disassembly. Typical vertical form

components are illustrated in figure 19.

Figure 19. Formwork components.(19)




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Sheathing Sheathing is the supporting component of the formwork closest to the concrete. Sheathing materials

consist of wood, plywood, metal, or other products capable of transferring the load of the concrete to supporting

members such as joists or studs. The following factors should be considered when selecting a type of sheathing:

strength; stiffness; ease of removal; initial cost; reuse potential; surface characteristics; resistance to damage

during concrete placement; workability in cutting, drilling, and attaching fasteners; weight; and ease of

handling. The design information provided here relates to plywood sheathing because it is the most common

sheathing material. Figure 20 shows horizontal plywood sheathing for a concrete bridge deck to be supported on

steel girders.

Figure 20. Plywood sheathing for horizontal formwork.

Plywood is widely used for both job-built forms and prefabricated form modules. Virtually any

exterior type of American Plywood Association (APA) plywood is appropriate for forming concrete since this

plywood is manufactured with waterproof glue. However, the plywood industry produces a product called

Plyform that is intended specifically for concrete forming. Plyform differs from conventional exterior plywood

grades in that Plyform is constructed only from certain wood species and veneers, and its exterior face panels

have thicker face plies for greater stiffness and are sanded smooth. Typical Plyform trademarks, which indicate

class, veneer grade, and conformance with applicable standards, are given in table 7.

Plyform is available in Class I and Class II, with Class I being the stronger and stiffer panel.

Structural I Plyform is stronger and stiffer than either Class I or II, and is often recommended for higher

concrete pressures. High Density Overlaid (HDO) Plyform is available in any of the three classes. The face

plies of HDO Plyform are bonded with a resin-impregnated fiber overlay, forming a hard, smooth surface that

eases removal and improves moisture resistance.




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Table 7. Grade-use guide for Plyform sheathing.(20)

Veneer grade Use these terms when specifying plywooda Description Typical trademarks Faces Inner piles Backs

APA B-B PLYFORM Class I & IIa

Specifically manufactured for concrete forms. Many reuses. Smooth, solid surfaces. Mill-oiled unless otherwise specified.

B C B

APA High Density Overlaid PLYFORM Class I & IIb

Hard, semi-opaque resin-fiber overlay, heat-fused to panel faces. Smooth surface resists abrasion. Up to 200 reuses. Light oiling recommended between pours.

B C-Plugged B

APA STRUCTURAL I PLYFORMb

Especially designed for engineered applications. All Group 1 species. Stronger and stiffer than PLYFORM Class I and II. Recommended for high pressures where face grain is parallel to supports. Also available with High Density Overlay faces.

B C or C-Plugged B

Special Overlays, proprietary panels, and Medium Density Overlaid plywood specifically designed for concrete forming.b

Produces a smooth uniform concrete surface. Generally mill-treated with form release agent.

No standard grading; for details of proprietary versions, see manufacturers specifications.

APA B-C EXT

Sanded panel often used for concrete forming where only on smooth, solid side is required.

B C C

Notes: (a) Commonly available in 19/32-in (15.1-mm), 5/8-in (15.9-mm), 23/32-in (18.3-mm), and -in (19.1-mm) panel

thickness [4-ft by 8-ft (1.2-m by 2.4-m) size]. (b) Check dealer for availability in your area.

Plywood manufactured in the United States is built up of an odd number of layers, with the grain of

adjacent layers perpendicular. Alternating the grain direction of adjoining layers minimizes shrinkage and

warping. In determining the flexural strength, shear strength, and stiffness of a panel, only those layers having

their grain perpendicular to the supporting stud are assumed to be stressed. The safe span of plywood is

therefore dependent not only on the type of plywood, but also on whether it is used in the weak direction

(the face grain runs parallel to the supports) or in the strong direction (the face grain runs perpendicular to the

supports).




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Formulas for calculating the maximum allowable pressures for plywood members based on stress and

deflection are given in table 8. Table 9 summarizes section properties for Plyform Class I and Class II, and

Structural I Plyform. Design stresses and moduli of elasticity for these plywood classes are given in table 10.

Due to the nature of plywood, the moment of inertia cannot simply be divided by half of the plywood thickness

to get the section modulus. Therefore, the moment of inertia, I, is to be used to calculate deflection and the

section modulus, KS, to calculate bending stress.

The design stresses in table 10 are given for Plyform used in wet conditions such as concrete forming.

Bending stress and rolling stress may each be increased by 25 percent under loads of short duration, though this

applies only when the number of reuses is limited. Since the limit on the number of reuses is not well defined,

the designer must decide whether to use this factor. Also, the design stresses may be higher if special conditions

exist, such as if the Plyform is well sealed against moisture so that the moisture content always remains below

16 percent. In addition to plywood strength, the designer must consider the effect of reuse on the permanent set

or deflection of the plywood.

Table 8. Formulas for stress and deflection calculations for plywood.(20)

2 spans 3 spans

Maximum allowable pressure, wb (lbf/ft2) based on bending stress

b

b 2

l

96F KSw

l=

b

b 2

1

120F KSw

l=

Maximum allowable pressure, ws (lbf/ft2) based shear stress

( )2

ss

19.2F lb / Qw

l= ( )

2

ss

20F lb / Qw

l=

Bending deflection, b (in) 4

3

b

wl

2220EI =

4

3

b

wl

1743EI =

Shear deflection, s (in) 2

2

2

se

Cwt l

127E I =

To calculate the maximum allowable pressure based on maximum allowable deflection, all., calculate b and s with w = 1.0 lbf/ft2. Then the maximum allowable pressure based on deflection, w (in lbf/ft2) is calculated as follows:

all .

s b

w =

+

Conversion: 1 lbf/ft2 = 47.9 N/m2; 1,000 lbf/in2 = 6.89 N/mm2; 1 in = 25.4 mm; 1 ft = 0.305 m. W = uniform load, lbf/ft2 Fb = bending stress, lbf/in2 Fs = rolling shear stress, lbf/in2 lb/Q = rolling shear constant, in2/ft KS = effective section modulus, in3/ft I = moment of inertia, in4/ft E = modulus of elasticity, adjusted, lb/in2 Ee = modulus of elasticity, unadjusted, lb/in2

11 = span, center-to-center of supports, in 12 = clear span, (in) 13 = clear span + in for 2-in framing, or clear span + 5/8 in for 4-in framing, in = deflection, in C = constant = 120 parallel, 60 perpendicular t = plywood thickness, in




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Table 9. Section properties for Plyform Class I and Class II, and Structural I Plyform.(20) Properties for stress applied parallel with face grain Properties for stress applied perpendicular with grain

Thickness (in)

Approximate weight (lbf/ft2)

Moment of inertia I (in4/ft)

Effective section modulus KS

(in3/ft)

Rolling shear constant lb/Q

(in2/ft) Moment of

inertia I (in4/ft)

Effective section modulus KS

(in3/ft)

Rolling shear constant lb/Q

(in2/ft)

Class I

15/32 1.4 0.066 0.244 4.743 0.018 0.107 2.419

1/2 1.5 0.077 0.268 5.153 0.024 0.130 2.739

19/32 1.7 0.115 0.335 5.438 0.029 0.146 2.834

5/8 1.8 0.130 0.358 5.717 0.038 0.175 3.094

23/32 2.1 0.180 0.430 7.009 0.072 0.247 3.798

3/4 2.2 0.199 0.455 7.187 0.092 0.306 4.063

7/8 2.6 0.296 0.584 8.555 0.151 0.422 6.028

1 3.0 0.427 0.737 9.374 0.270 0.634 7.014

1 1/8 3.3 0.554 0.849 10.430 0.398 0.799 8.419

Class II

15/32 1.4 0.063 0.243 4.499 0.015 0.138 2.434

1/2 1.5 0.075 0.267 4.891 0.020 0.167 2.727

19/32 1.7 0.115 0.334 5.326 0.025 0.188 2.812

5/8 1.8 0.130 0.357 5.593 0.032 0.225 3.074

23/32 2.1 0.180 0.430 6.504 0.060 0.317 3.781

3/4 2.2 0.198 0.454 6.631 0.075 0.392 4.049

7/8 2.6 0.300 0.591 7.990 0.123 0.542 5.997

1 3.0 0.421 0.754 8.614 0.220 0.812 6.987

1 1/8 3.3 0.566 0.869 9.571 0.323 1.023 8.388

Structural I

15/32 1.4 0.067 0.246 4.503 0.021 0.147 2.405

1/2 1.5 0.078 0.271 4.908 0.029 0.178 2.725

19/32 1.7 0.116 0.338 5.018 0.034 0.199 2.811

5/8 1.8 0.131 0.361 5.258 0.045 0.238 3.073

13/32 2.1 0.183 0.439 6.109 0.085 0.338 3.780

3/4 2.2 0.202 0.464 6.189 0.108 0.418 4.047

7/8 2.6 0.317 0.626 7.539 0.179 0.579 5.991

1 3.0 0.479 0.827 7.978 0.321 0.870 6.981

1 1/8 3.3 0.623 0.955 8.841 0.474 1.098 8.377

Notes: (a) All properties adjusted to account for reduced effectiveness of plies with grain perpendicular to applied stress. (b) Conversion: 1 in = 25.4 mm; 1 ft = 0.305 ft; 1 lbf/ft2 = 47.9 N/m2.

Table 10. Design stresses for Plyform.(20) Plyform Class I Plyform Class II Structural I Plyform

Modulus of elasticity E (lbf/in2, adjusted, use for bending deflection calculation)

1,650,000 1,430,000 1,650,000

Modulus of elasticity Ee (lbf/in2, unadjusted, use for shear deflection calculation)

1,500,000 1,300,000 1,500,000

Bending stress Fb (lbf/in2) 1,930 1,330 1,930

Rolling shear stress Fs (lbf/in2) 72 72 102

Table 10 has been increased by 25% for short duration loads. Conversion: 1,000 lbf/in2 = 6.89 N/mm26




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40

In addition to plywood, reconstituted wood materials are available for use as sheathing or as form

liners. Only those materials manufactured for forming applications, with edge sealing and surface treatment, can

be expected to endure as well as treated plywoods. Forms that are built similarly to steel plate girders, described

later in this chapter, are composed of webs, flanges, and stiffeners, with the webs in direct contact with the

concrete. Steel has high strength, stiffness, and durability, but is heavier and therefore more cumbersome to

work with. For pier caps and other applications where conduit and plumbing penetrations are limited, however,

steel formwork is often utilized if enough reuses to justify the cost of steel forms are anticipated. Fiberglass

reinforced plastic forms are strong, lightweight, can be readily fabricated to non standard shapes, and can be

extensively reused. These forms are common in the construction of round columns, as are spiral wound waxed

paper tubes and all-steel, two-piece column forms.

Structural Supports For vertical wall forms, the form ties and sheathing transfer the lateral loads from fluid concrete to

studs and walers. As with sheathing, important considerations in the selection of structural support members

include strength, stiffness, dimensional accuracy and resistance to permanent deflection, workability, weight,

cost, and durability. In proprietary modular forms, these structural supports and aligners may be made of steel,

aluminum, magnesium, or lumber. Design information for proprietary systems is available from the

manufacturer.

Almost all formwork jobs, regardless of the types of primary materials selected, usually require some

lumber. Lumber that is straight and free from defects may be used for formwork. Softwoods are generally most

economical for all types of formwork. Partially seasoned stock is usually preferred for concrete forming,

because dried lumber can swell excessively when wet and green lumber tends to dry out and warp during hot

weather, thus causing problems in form alignment. Information on the design of structural lumber is presented

in this chapter. Since lumber species, grades, sizes, and lengths vary geographically, local supplies will be the

primary source of advice for the specific materials and sizes that are available.

Lumber may be finished on all four sides and is then referred to as standard dressed or S4S lumber.

When it is used directly as it comes from the sawmill, the lumber is designated as rough. Properties of standard

lumber sizes common in formwork construction are identified in appendix B.

Guidelines discussed in Chapter 2. Falsework to ensure correct timber quality and size of material are

also applicable to formwork. Expressions commonly used to determine support spacing are provided in table 11

and general beam formulas are provided in table 12. Allowable stresses and strength factors are specified in the

NDS Supplement Design Values for Wood Construction.(21)

In addition to designing structural lumber to withstand bending and shear stresses, consideration must

also be given to bearing stresses. Allowable bearing stresses for loads applied parallel to the grain and loads

perpendicular to the grain are also given in the NDS Supplement.




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45

(a) Flat tie.

(e) Paper tie.

(b) Snap tie.

(f) Threaded bar tie.

(c) Wire panel tie.

(g) She-bolt.

(d) Pull-out tie.

(h) Coil tie.

Figure 21. Form ties.




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46 Interim 2008

Construction of forms with coil tie systems begins with the erection of one side of the form and

installation of the coil tie system as shown in figure 22. The reinforcing steel is then positioned, the closure

forms erected, and the remaining tie hardware installed. With this installation technique, the reinforcing steel is

not positioned in front of tie holes and therefore does not interfere with the tie installation. However, the coil tie

system does not provide the option of being fed through the forms. The external hardware has a high initial cost,

but can be reused.

Figure 22. Coil tie system.

Form Hangers The proprietary form hangers used with bridge deck formwork are generally the

same for cast-in-place decks supported on steel girders and on precast girders. A variety of formwork hangers

are available for the construction of bridge decks. Examples of an exterior hanger and of an interior hanger are

illustrated in figure 23.

Exterior hangers are designed to support the overhanging portion of a bridge deck on the fascia beam

of the bridge. Exterior hangers generally consist of a vertical support on the interior side of the fascia beam and

an exterior angled support typically used to support an overhang bracket on the exterior face of the beam. An

interior hanger, as shown in figure 23, may be equipped with a fixed length or adjustable coil bolt assembly.

Form hanger capacities generally range from 2,000 lbf (8,800 N) to 6,000 lbf (26,400 N).




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49

Conversion: 1 lbf/ft2 = 47.9 N/m2; 1 ft = 0.305 m; (F 32)/1.8 = C

Figure 25. Lateral pressure of concrete on formwork.

FORMWORK TYPES Bridge formwork can be divided into two categories: vertical and horizontal formwork. Vertical

formwork can be constructed using job-built systems or prefabricated systems. Horizontal formwork can be

constructed utilizing job-built, prefabricated, or permanent stay-in-place systems. These systems are defined as:

Job-Built Formwork a formwork system designed and built for a specific application, most commonly using plywood and lumber.

Prefabricated or Modular Formwork a modular system that has the durability for multiple reuses and normally is built with plywood with a metal framing. Prefabricated formwork can be built for

custom uses on special projects.




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50 Interim 2008

Stay-in-Place Formwork a formwork system designed such that the formwork is not removed after construction. This system most commonly consists of stay-in-place metal decks or precast

concrete planks for forming concrete deck systems.

Job-Built Formwork Job-built wood forms have a low initial material cost, but generally require much labor and can only be

used 10 to 15 times. The labor cost to repair and erect job-built wood forms is high compared to that for

prefabricated modular forms that have much greater reuse potential. An example of a job-built form in bridge

construction is given in figure 26.

Figure 26. Job-built formwork.

Modular Formwork The term modular form refers to all-metal forms or metal-supported-plywood systems, whose

integrated design of tie and connecting hardware is engineered to assure dimensional control, speed of erection,

and ease of stripping as well as structural integrity. Care must be taken when assembling modular forms to

ensure tight and well-aligned joints with no offsets. Also, these forms must be inspected for permanent set or

deflection that may occur after many reuses.

The most common modular forms consist of steel frames with replaceable plywood faces. This

combination provides the job-site workability of plywood and the large tie spacing and form durability of steel.

Overlaid plywood further extends the form-face wear, and yet can be nailed or cut. The most successful of these

systems utilize high-carbon steel to minimize weight. The steel portion of the form is generally designed to

protect the edges of the plywood and absorb tie loads and stripping, wracking, and lifting stresses. Since ties fit

between panel joints (instead of through the plywood), the steel frame absorbs the tie loads and the wear. All-

steel forms are practical for piers and columns since they provide great rigidity and strength and can be rapidly




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51

erected, disassembled, moved, and re-erected. A sufficient number of reuses must be expected to justify the

high initial cost. Also, special precautions must be taken when placing concrete in cold weather since the all-

steel forms provide little or no insulation protection to the concrete.

Lightweight modular forms are also made of aluminum and magnesium, but are susceptible to

deterioration from contact with fresh concrete. They should, therefore, only be used if suitably coated or as a

structural support with a separate sheathing material. Aluminum extrusions can provide bolt slots, nailer

pockets, and other special features. Aluminum beams and double-channel walers provide large gang-wall forms

that are exceptionally lightweight and straight due to the nature of the extrusions.

Stay-in-Place Formwork In areas where form removal is expensive or hazardous, the use of stay-in-place (SIP) forms may be

desirable. SIP forms help facilitate the construction of bridge decks over high-traffic areas. The additional dead

weight of the deck slab, appearance, and corrosiveness of the environment are some of the factors that should be

considered when deciding if metal or precast concrete SIP forms should be used. Ribbed metal deck and precast

concrete elements may act solely as formwork for cast-in-place concrete, or may act compositely with the

concrete and become part of the load-bearing structure. Welding to flanges in tension zones or to structural

elements fabricated from nonweldable grades of steel is generally prohibited.

Gang Forms Gang forms consist of prefabricated formwork panels that include sheathing, studs, and walers, joined

into larger units for ease in erecting, removal, and reuse. These systems are quickly assembled and permit

repetitive uses without rebuilding for efficient wall construction. Modular units are fastened to each other and to

lift brackets, lift beams, tag lines, and possibly a work platform while still on the ground. Vertical angles may

also be provided along the edges in order to attach individual gang forms with bolts or special steel clamps.

Although gang forms may be used as hand-set units, they are more commonly lifted into place by

cranes and are therefore limited in size only by the crane capacity. The use of large gang forms helps to offset

the high cost of labor, through large forms do not easily accommodate odd shapes or field adjustments.

Integration of relatively small modular panels with large gang forms maximizes the benefits of both systems.

After the concrete becomes self-supporting, the forms can be removed as large units and efficiently reused.

Lift brackets are attached to a lift beam or directly to gang form structural elements that must have

sufficient strength to withstand the inclined loads from the slings during lifting. Gang forms used in multi-lift

applications must be supported by specially designed inserts, anchors, and brackets because these are in turn

supported by freshly cured concrete.

A gang form, equipped with a working platform, is shown in figure 27. The entire unit is lifted into

place and then removed as a unit when the concrete has gained sufficient strength. Gang forms are well suited to

the construction of walls as shown in figure 28.




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52 Interim 2008

Figure 27. Assembled gang form.

Figure 28. Gang form for wall construction.




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53

Plate Girder Forms Plate girder forms, such as the one shown in figure 29, are well suited for the construction of bridge

pier caps. These systems are capable of forming concrete while structurally spanning between supports with no

intermediate shoring. In many applications, these panels also do not require external walers. The large tie

spacing and high pressure capacity provide form tie cost advantages in spite of the high form cost and weight.

Larger plate girder modules create fewer joints to seal, align, and finish. The most significant cost-savings result

is from the self-spanning capabilities of this system, which makes bridge pier construction possible while

minimizing the amount of falsework.

In plate girder form systems, the web of the steel girder doubles as a form face. The steel ribs of the

girder serve as web stiffeners to support the weight of the form and concrete. They also act as beams to transfer

the horizontal pressures of the liquid concrete from the form web to the form top and bottom flanges. The plate

girder forms come in modules that are bolted together, as needed, for specific project. Proprietary bolting

hardware allows the transfer of flange forces between individual modules, thereby allowing the formwork

system to span between supports without intermediate shoring. Examples of plate girder forms are given in

figures 29 and 30.

Conversion: 1 ft = 0.305 m; 1 in = 25.4 mm

(Courtesy of Economy Forms Corporation)

Figure 29. Plate girder form spanning between two supports.




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54 Interim 2008

(Courtesy of Economy Forms Corporation)

Figure 30. Plate girder forms used to form a bridge pier.

CONSTRUCTION

It is essential that formwork is erected as designed. The assumptions made in the design of the

formwork, such as rate of concrete placement, should be designated on the shop drawings and confirmed during

construction. Guidelines that apply to the safe construction of formwork are as follows:

In addition to inspection prior to concrete placement, inspection should continue during the pour to ensure early recognition of possible form displacement or failure. A supply of extra bracing

materials necessary in an emergency should be readily available.

Construction materials, including concrete, must not be dropped or pile

aashto chbtw amd 1_2008

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