planning, design & construction

7/27/2019 Planning, Design & Construction

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PLANNING, DESIGN AND CONSTRUCTIONOF

STRUCTURAL CONCRETE BLOCK MASONRYK.VENKATESH BABU.B.E;

CIVIL AND STRUCTURAL ENGINEER

INTRODUCTION:

Concrete blocks were in use as an alternative to clay bricks since early 1970 in India. Due to thedevelopment that has taken place in raw materials, production process and availability of Hi-Tech machines,concrete blocks are being manufactured with control over its properties like size, shape, strength, waterabsorption, surfac~ finishes and textures, color, acoustical properties, thermal and insulation propertiesetc ... The availability of such high quality blocks enables the designers to make use of the blocks efficientlyresulting in greater economy and improvement in quality of construction. This has led to the use of concreteblocks as exposed load bearing masonry for high rise structures. The use of structural hollow concrete blockmasonry in multistoried structures has led to considerable economies compared to conventional methods ofconstruction, but the requirements of this method need to be borne in mind by the Engineers and Architects.GENERAL PROPERTIES OF ENGINEERED CONCRETE MASONRY UNITS:

Exposed concrete block should be dense and water tight and of such shape that it conveniently produceswater tight walls and that will not require water proofing or painting. Also it must liberally meet the minimumload and tensile strength requirements for a specific use.Additional characteristics of a high quality blocks are

Strength and design to withstand abnormal stresses when laid in wall. Sufficient impermeability to withstand serious wetting by heavy rain and the ability to dry out

quickly afterwards. Adherence to consistent quality, uniform size and accurate dimensions. Finished unit dried to minimum moisture content. Freedom from discoloration, popping or staining. Pleasing appearance eliminating the necessity of painting except for decoration.

The physical properties of 190mm thick two cored hollow structural concrete blocks are as below:(if Size of full blocks(ii) Dimensional tolerance(iii) Weight of full block(iv) Yard dry density(v) Water absorption(vi) Net characteristic crushing

Strength on flange area(vii) Thermal resistance(viii) Estimates sound transmission class

390mm x 190mm x 190mm3mm17.5 Kg.2100 - 2200 Kg/Cum.3 to 5%

15 Mpa. to 50 Mpa.0.38 to 0.33 M2KfW46 to 58 dB

...


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BEARING WALLSTRANSVERSE SHEAR WALLS

-2-PLANNING METHODS:

The design of a concrete block masonry wall depends on its required appearance, economy, strength,insulation and acoustical characteristics.I. PLANNING LAYOUT OF WALLS AND COLUMNS:The layout of walls and columns involves important considerations such as functional requirement of thestructure, internal arrangement of components, opening size and locations, requirement 'Of modularplanning, idealization of structural concepts, provisions for shrinkage and cracking control and weatherresistance. All deserve careful planning if the wall is to successfully serve its intended purpose. Masonrystructures gain stability from the support provided by cross walls, floors, roofs and other elements. Loadbearing walls are structurally most efficient when the structure is so planned that the eccentricity of theloading is minimized. When the stability of a building is entirely dependent on the masonry walls, it isessential that the wall be properly arranged to resist lateral loads from any direction. This may be achievedby using

I. Double cross wall arrangement.II. A cellular wall arrangement.III. A complex wall arrangement.

Fig. Some examples of two-directional bearing/shear wall layouts


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BEARING WALLS

TRANSVERSESHEAR WALLS

Fig. : Examples of multi-directional bearing/shear wall layouts.

The cross wall arrangement is usually associated with one way spanning floor or roof slabs whereas theother forms permit the use of two way slabs. These latter forms lead to economics in the floor system andalso improve the robustness of a building, giving greater protection against progressive collapse if a loadbearing wall is accidentally damaged.II. DIMENSIONAL PLANNING:

The modular nature of masonry units tends to impose a stricter discipline on, lengths, heights andparticularly the thickness of walls and piers than is found in other conventional materials.Modular planning is a method of coordinating the dimensions of various building components to simplify thework and thus lower the cost of construction. In modular plan for concrete masonry construction allhorizontal dimensions are given in multiples of half the nominal height of the block.The width of doors and window openings as well as wall length should be multiples of 8".

...


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-4-DESIGN METHODS:

DesiQnconcepts:The design concepts and philosophies are to be well understood before attempting design of structures. It isgenerally believed that masonry structures are suited primarily for low rise structures. Concrete block wallhas specific advantage over load bearing brick masonry or RCC framed structure as the hollowcores of theblocks can be reinforced with steel and grouted with concrete, it enables the designer to adopt anyone ofthe following structural system to suit the functional and architectural requirement of the building.

a. Load bearing masonry system.b. Framed structure with reinforced block masonry columns and beams.c. A hybrid system of load bearing masonry walls and reinforced block masonry columns.

Unreinforced masonry is designed to act basically in compression and is assumed to be incapable ofcarrying tensile stresses. Vertical loads are transmitted by load bearing walls from storey to storey down tofoundations. Each floor must be designed to carry its own dead load and live loads and possibly somebending moments arising from the assumed continuity of walls and floors. Lateral loads due to winds aretransmitted directly to the load bearing walls and indirectly from the floors. These lateral loads are resistedby the load bearing walls acting as continuous vertical cantilevers from the top of the building and give riseto tensile and compressive vertical bending stresses under flexure.The concrete block masonry walls are grouted and reinforced to supplement the strength of masonry incompression, tension and shear forces. Reinforced masonry increases the vertical axial load carryingcapacity and also enables to use the shear wall concepts to resist the lateral forces due to wind andearthquake loads. The distribution of lateral forces between shear walls in a load bearing structure willdepend upon the in-plane flexibility of the floor system and the arrangement and relative stiffness of theshear walls. The in-plane action of a floor system or diaphragm is analogous to that of the web in a plategirder. In this analogy the floor slab constitutes the web, the floor joists and beam function as stiffeners andthe walls or bond beams act as flanges.The designer must also ensure that there is a reasonable probability that catastrophic collapse of the loadbearing structure will not occur under the effect of misuse or accident. Damage from over loads should notbe disproportionate to the nature of the overload. Research has shown that provided accidental damageprovisions are considered at the design stage, the possibility of progressive collapse can be avoided withlittle or no extra cost to the resulting structure.The structural design shall provide for the safety, serviceability and proper functioning of the structure as awhole and its component parts, including each masonry member under the.action of the loads and forces. Itshall ,

a. Be in accordance with accepted principle of mechanics.b. Provide for all loads and forces to be transferred through the structures to the foundation.c. Provide all necessary compatibility between each masonry member and the structural members

giving vertical and/or lateral support to that masonry member.d. Be based on the material strength properties of the masonry.e. Beconsistent with the following assumptions

i. Plane sections remain plane under bending actions.ii. A recognized stress - strain relationship for the masonry.iii. The tensile strength of unreinforced masonry shall be taken as zero except for

stresses resulting from wind loads and similar forces of short term transient nature....5


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Members composed of more than one type or grade of unit or mortar shall be designed for the propertiespertaining to the least favorable combination of components used or if appropriate shall be designed forcomposite action taking account of the different material properties of the parts.DESIGN FOR COMPRESSION:

Factors affecting load carrying capacity of masonry walls are(i) The cross sectional properties of the wall based on its bedded area.(ii) The characteristic compressive strength of the masonry.(iii) The wall slenderness ratio.(iv) The effective eccentricities of the load.

A member shall be ,designed such that the following relationship is satisfiedFd:: KFo

a. For ungrouted hollow units masonry:Fo = Cm fm Ab

b. For grouted hollow units masonry:(i)

(ii)Where

if no testing is done:Fo = Cm [fm Ab + Kc (fc/1.3) Ac]if testing is done:Fo = Cm fmg Ad

Fo = basic compressive capacity of the cross sectionFdCm

=

=design compressive force.Capacity reduction factor.

< 0.45 Ksz for compression.Ksz = size reduction factor

= 0.75 + Ag/500000 for members in compression, with or withoutsimultaneous bending and whose overall cross sectional areais less than 125 000 mm2 where Ag= overall cross sectionalarea of the member '

= 1.00 for other members.K = a reduction factor for slenderness and eccentricity given in the

following table ....


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-6-

Slenderness Reduction Factors (K)ccentricity to thickness ratio (elltw).1 0.2.3.33111111 I1.00.79.87.94.58.73.79.38.60.67.32.56.40.94.73.81.88.54.68.79.34.56.67.29.52.64 ..88.67.76.83.49.63.79.31.51.67.25.48.64 0.82.62.70.77.45.59.72.27.47.67.22.44.64 0.76.56.64.71.40.54.67.23.43.63.18.40.61 0.70.51.59.66.35.49.62.20.39.58.15.35.56 0.64.45.53.60.31.44.56.16.34.52.11.31.51 0.58.40.47.55.26.39.51.13.30.47.08.27.46 0.52.35.42.49.22.34.45.09.26.42.04.23.42 0.46.29.36.43.17.29.40.06.21.37.01.19.37 0.40.24.31.38.12.24.35.02.17.33.15.32 0.37.210.28.35.10.21.33.15.30.13.30

Kc ==

======= 1.4 for hollow concrete masonry units of density greater than 2000 kg/Cum.1.2 for all other masonry.characteristic cylindrical compressive strength of grout in Mpa.characteristic compressive strength of masonry.characteristic compressive strength of grouted masonry.the bedded area that is within the zone of dispersion under consideration.the cross sectional area of grout.design cross sectional area of the member.Slenderness ratio calculated as explained in the slenderness section.

Evaluation of slenderness ratio:

The slenderness ratio of a member about a principal axis shall be as follows:(a) Other than as in (b) below

Sr = (av H) / (Kt t)

(b) For a wall that is laterally supported along one or both of its vertical edges, the least of

= Slenderness ratio= Overall thickness or depth of the member's cross section perpendicular to

(i)(ii)Hi) SrAv

SrSrSr

===

(av H) / (Kt t)(ah L) /tav H/3t Kt + ah U3t


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= 0.85 for a member laterally supported at both top and bottom androtationally restrained at only one of these or

= 1.00 for a member laterally supported and rotationally free at both top andBottom; or

= 2.00 for a member laterally supported and rotation ally restrained at only atits bottom.

Ah

Kt

=

-=

1.00 for a member laterally supported along both vertical edges(Regardless of the rotational restraint along these edges); or2.00 for a member laterally supported along one vertical edge, andunsupported along its other vertical edge.

thickness coefficient derived from the following tableTHICKNESS COEFFICIENTS FOR WALLS STIFFENED BY MONOLITICALL Y ENGAGED PIERS

THICKNESS COEFFICIENT (Kt)Pier Thickness ratio (twptt)

1or more.0.4.0.0.3.7.0.2.4.0.1.2 1.0.0.0

Notes: 1. Pier spacing is taken as the distance between centre line of piers.Characteristic Compressive Strength of MasonryThe value of the characteristic compressive strength of masonry (fm) used in design shall be as follows:(a) For masonry other than special masonry

F'm = Kh fmbWhere fmb = the value derived from the following table

CHARACTERISTIC COMPRESSIVE STRENGTH OF MASONRY (fmb)

Type of Mortar050500 Composition1:1:6.6.4.4.3.0.7.8

Kh = Factor reflecting the influence of the ratio of masonry height to mortar bed jointthickness.


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BEDDED AREA:

The bedded area of the masonry member shall be the contact area of the mortar with the masonry uwhich shall not include the depth of any raking of joints ..In hollow unit masonry the contact area of the mortar shall be the face shell bedded area meas~ aminimum thickness of the face shell and shall not include the area of the webs in the units.

GROUTED AREA:

The cross sectional area of grout in a masonry member shall be taken asAc = Ad -Ab

The load carrying capacity of a 3m height wall is as per the above is(i)(ii)

M30 Grade block wall in C:L:S-1:1:6M20 Grade block wall in C:L:S-1:1:6

232.7 KN/M.190.4 KN/M.

REINFORCED MASONRY:DESIGN FOR COMPRESSION:Reinforced masonry members of uniform cross section subject to compressive forces shall be proportioso that

Fd s..Cr K (fm Ab + Kc (fc/1.3) Ac + fsy As)Where

CrFsyAs

===

0.7characteristic strength of reinforcement steelArea of steel.

DESIGN FOR BENDING:Reinforced masonry members subject to bending shall be proportioned so that(a) Md ~ Cr fsy Asd d {1- [ (0.6 fsy Asd) I (1.3 fm b d) ] }

Where bDAsd

===

width of masonry membereffective depth of the memberthat portion of the cross sectional area ofthe main tensile reinforcement.

(b) As/bd ~ (0.29) (1.3 fm) I fsy ...9


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-9-CONCENTRATED LOADS:

Concentrated load acting on concrete masonry shall be assumed to disperse through the masonry atangle of 45 degrees from the perimeter of the bearing area of the load but this dispertion is not to extend(a) In to the dispersion zone of an adjacent concentrated load on the member, or(b) Beyond the structural end of the masonry in the memberLoad capacity under a concentrated loadThe design compressive force acting on any given cross section shall include the design concentrated loplus that portion of the other compressive forces that acts on the cross sectional area (Ads) undconsideration.

The design bending moment acting on the same cross section under consideration shall include the bendinmoment if any from the design concentrated load plus that portion of the bending moments from other loaand forces that acts on that cross sectional area.In addition the member shall be designed to satisfy the following equation for each cross section within tzone of dispersion of the concentrated load:Fd < Kb FoWhereFoFdKb

===

basic compressive capacity of the cross sectiondesign compressive forceconcentrated bearing factor

Concentrated bearing factorThe value of the concentrated bearing factor (Kb) shall be(a) For cross sections at a distance greater than 0.25H below the level of the bearing of the

concentrated load on the memberKb = 1.00

(b) Fo~ cross sections at a distance within 0.25H below the level of the bearing of the concentratedload on the member

(i) other than as in (ii) below (including ungrouted hollow unit masonry)Kb = 1.00 .(ii) in solid or cored unit masonry, or in grouted masonry

0.55 (1+0.5 a1/L)Kb = ----------------------------------- or

(Ads/Ade) 0.33Kb = 1.50 + a1/L whichever is less, but Kb is less than 1


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-10-WhereAds

====

bearing or dispersion area of the concentrated load at the design cross sectionunder consideration.effective area of dispersion of the concentrated load in the member at mid height.distance from the end of the wall or pier to the nearest end of the bearing area.clear length of the wall or pier.

When the masonry is not capable of resisting the concentrated loads a bearing pad of reinforced concrete 1suggested. Care should be taken under the supports of beams and cantilevers which may have significantdeflection under the load resulting in severe edge stresses. The use of bearing pads will help to prevenspalling of the face of the masonry. Heavy concentrated loads on walls otherwise lightly loaded can producesignificant differential strains resulting in premature vertical cracking adjacent to the loaded area. Bed joinreinforcing immediately below the loaded area extending beyond the zone of influence is one means ocontrolling the cracking.CONSTRUCTION OF ENGINEERED CONCRETE MASONRY UNITS:

Masonry structures have been built thousand of years and methods passed on from generation tgeneration by instruction and example. The techniques of construction of structural concrete masonry shabe adopted by engineers suitable to the site conditions. The recommended method of constructionexplained below.Estimation of materials:

Estimation of material requirement shall be first job in the construction of block masonry. The following armaterial requirement for the construction of unreinforced plain concrete block masonry per unit area of onsquare meter in Cement lime mortar 1:1 :6.Concrete blocks of size 390mm x 190m x 190mmCementSandLime

Keepinq the units drr

12.5 Nos.0.08 Bags.0.55 Cft.0.1 Cft.

When delivered to the job, concrete masonry units should be dry enough to comply with specified limitationfor moisture content. Concrete masonry units should never be wetted immediately before and durinplacement, a practice contrary to the general construction of clay brick masonry. When moist concretmasonry units are placed in a wall, they will shrink with loss of moisture. If this shrinkage is restrained, asusually is, stresses develop that may cause cracks in the walls. Hence it is important that the units be keat or dried .

.. .11


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-11-Mortar:

Mortar for concrete masonry is designed not only to join masonry units into an integral structurepredictable performance properties, but also to:(1) Effect tight seals between units against the entry of air and moisture(2) Bond with steel joint reinforcement, metal ties, and anchor bolts, if any, so that they perform

integrally with the masonry;(3) Provide an architectural quality to exposed masonry structures through color contrasts or

Shadow lines from various joint-tooling procedures; and(4) Compensate for size variations in the units by providing a bed to accommodate tolerances on

Units.

Masonry mortar recommended for exposed unreinforced plain masonry is one part of cement, one parlime and six parts of clean well graded sand with sufficient quantity of water to produce a plastic, workamixture. Probably the most important quality of a masonry mortar is workability because of its influenceother important mortar properties in both plastic and hardened states. Good mortar is necessary for gworkmanship and proper structural performance of concrete masonry.Mortar beddinq:Two types of mortar bedding are used with concrete masonry;

(i) Full mortar bedding.(ii) Face-Shell mortar bedding.

Full bedding is used for laying the fist or starting course of block on a footing or foundation wall as welfor laying solid units. It is also commonly used when building concrete masonry columns, piers and pilasthat will carry heavy loads. Where some vertical cores are to be solidly grouted, such as in reinformasonry, the webs around each grouted core are fully mortared. For all other concrete masonry workhollow units, it is common practice to use only face shell bedding. Also the head (vertical) joints of bhaving plain ends are mortared only opposite the face shells .. For bed joints, all concrete block shouldlaid with the thicker part of the face shell up. This provides a larger mortar-bedding area and makesblock easier to lift. A mechanical spreader can be used to speed production, especially on long, strawalls. For head joints, mortar is applied only on the face shell ends of the block. Some masons buttervertical ends of the block previously placed; others set the block on one end and butter the other end belaying the block. Time can be saved by placing three or four block on end and then buttering their veredges in one operation. Each block is brought to its final position and pushed downward into the mortarand sideways against the previously laid block so that the mortar oozes out of the head and bed jointsboth sides of the face shell. This indicates that the joints are well filled.Bondinq of plain Masonry:All the concrete blocks in a masonry wall must be bonded to form a continuous mass. Although sevmethods may be used to achieve bonding between blocks the most prevalent method of bonding in a siwall unreinforced concrete block masonry is


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Usually two cored hollow concrete blocks are laid in running bond with an over lap of half the block lengas the hollow cored are vertically continuous and vertical joints are minimum. Staking bond wlll useplaces like columns, mullions or piersLavinq UP a wall:First Course:On jobs where more than one mason is working, the footing or slab foundation must be level so 1tlat emason can start his section of the wall on a common plane and the bed joints will be uniformly s1Jaightwthe sections are connected. If the foundation is badly out of level, the entire first course should bebefore masons begin work on other courses. Before any units are laid , the top surface of the concfoundation should be clean. Laitance is removed and aggregate exposed to ensure good bond omasonry foundatiqn. Then full mortar bed is spread and furrowed with a trowel to ensure plenty of malong the bottom edges of the block for the first course. I the wall is to be grouted, the mortar bedding fofirst course should not fill the area under the block cores; grout should come into direct contact withfoundation.

Make Height of .all 10fil concrele masonryu~il.o~e\Jl1ila~d one Jointequal200mm

Un corner uni t wi th2.!!!Ind flu~.h


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-13Corners:The corners of the walls are built u higher, usually four or five courses higher, than the course being laid athe center of the wall. As each course is laid at a corner, it is checked with a level for alignment for beinleveland for being plumb with a level or straightedge to make certain the the faces of all block are in thsame plane.Between corners:After each corner at each end of a wall have been laid up, a mason's line (string line) is stretched tightlfrom corner to corner for each course and the top outside edge of each block is laid to this line. The linemoved up as each course is laid.Care must be taken to keep the wall surface clean during construction. In removing excess mortar thaoozed out at the joints, the mason must avoid smearing soft mortar onto the face of the unit, especially if thwall is to be left exposed or painted. Any mortar droppings that do stick to the wall should be almost drbefore they are removed with a trowel. Then, when dry and hard, most of the remaining mortar can bremoved by rubbing with a small piece of concrete masonry and by brushing.

Trimmingooi t is leveUedby

taWinq ",ilh aJ

trower

Buttering

standunit ",end.10place mortarfor vert ic aI joint

;:~ine 10 la~unit to,,)

Fig. : Couele masonry laying details

Positioning

... 14


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-14-Closure unit:When installing the closure unit, the mason butters all edges of the opening and all four vertical edges oUileclosure unit before he carefully lowers the unit into place. I any of the mortar falls out, leaving an oPenjointhe mason remove the closure unit, apply fresh mortar and repeat the operation. Closure unit 'locationsarstaggered throughout the length of the wall.Toolinq Mortar ioints:Weather tight joints and neat appearance of concrete masonry walls are dependent on proper tooling;i.ecompressing and shaping the mortar face of a joint with a special tool. After a wall section has been laid anthe mortar has become thumbprint-hard, the mortar joints are usually considered ready for tooling. Upohardening, mortar has a tendency to shrink slightly and pull away from the edges of the masonry unitProper use of a jointing tool restores the intimate contact between the mortar and units when they were lato make weather tight, uniform, sharp and clean lines. Horizontal joints should be tooled before verticjoints.

looting Q horizontal jointconcave type

Tooling a vertical joint

Fig.

Mortar WITS are trimmed ofaf t


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Electrical concealment and Joineries fixation:The service lines like electrical conduits can be concealed though the hollow cores during construction ofthe wall as shown in the figure below. The joineries like doors and windows shall be fixed to the masonryusing conventional clamps or with anchor fasteners to the block masonry and the door and window jamsand sill shall be grouted with concrete._____X

p.'".c conduit pipe

\>fire mesh

switch box.

Section'xx'

r.c,c slab

-switch box--- '--' '' ---pvc conduit 'pipe

.I'""HI,I,

Elevation ELECTRICAL CONDIUTING

200X200 hatfunitt jron hold fast -

~~. ~"~. ID'~entire height of theopening.

Elevation~ ID OIGJJ~

o

i

I

ore grouted upto1~~i~h~i.iW~nitron hold fas t

DOOR FIXATION DETAIL

planning, design & construction

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