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Mick Leso – Structural Option 2006 Penn State AE Senior Thesis

EXISTING CONDITIONS 7

STRUCTURAL

The structural system of the James C. Renick School of Education Building is split into two

sections. However, the two sections do not follow the same scheme as the two architectural

“bars”. The split allows for the cantilever to disengage from the rest of the building. Using an

expansion joint, the Renick building technically becomes two separate structures during

design analysis. The cantilever section, which is the focus of this report, will be referred to as

the northern wing, while the rest of the building is the southern wing1.

Stewart Engineering, Inc. located in Charlotte, North Carolina, completed the structural

design. Using RAM Structural and RISA 3-D for computer analysis, the structural system was

determined to be adequate for the design loads located within ASCE 7-98. The Renick

Building must follow the North Carolina State Building Code, a form of the 2000 International

Building Code (IBC). When completing the computer investigations, the structural system was

examined using both design methods, the load resistance factor design (LRFD) and the

allowable stress design (ASD). Under careful consideration, it was determined that seismic

loads contributed to larger forces on the building than wind loads. For one thing, the Renick

Building is only three stories high. Also a factor is the nearby epicenter of a disastrous

earthquake during the late 1800s, Charleston, South Carolina. For this reason, it was a

challenge to provide substantial strength, especially for the cantilever, under many load

combinations.

The structural system includes several features. The huge cantilever trusses located in

the north building have diagonal wide flange bracing members that carry forces from both

gravity and lateral loads, and help to distribute them into wide flange girders and columns

that are designed to resist moments and bear axial compression. Diagonally placed HSS

rectangular tubing is used in both vertical and horizontal directions to provide bracing against

lateral loads without adding too much weight to the structure. Two shear walls at the base of

the cantilever carry gravity loads from the overhang, yet also stabilize the cantilever from

lateral vibration and help to dissipate loads down to the foundation and soil below. A

concrete slab on composite metal decking works together with shear studs that are welded to

the floor-supporting steel beams, creating a stronger floor system and ultimately reducing slab

thickness and beam sizes. A shallow foundation system carries the total building loads while

saving money on the amount of concrete used during construction. 1 Enlarged Structural Plans for the North Wing located in Appendix C



Applicable Building Codes: • 2002 North Carolina State Building Code (2000 IBC with Revisions) • ACI 318-99 Requirements for Structural Concrete • ACI 530-99 Requirements for Masonry Structures • AISC Manual of Steel Construction, ASD, 9th Edition • AISC Manual of Steel Construction, LRFD, 3rd Edition • ASCE 7-98 Minimum Design Loads for Buildings • American Welding Society D1. 01-98 • SJI-92 Standard Specs, Load & Weight Tables for Steel Joists & Joist Girders

Materials:

• Structural Steel ASTM A992, Grade 50, Fy = 50 ksi HSS ASTM A500, Grade B, Fy = 46 ksi Miscellaneous Steel ASTM A36, Fy = 36 ksi Reinforcing Steel ASTM A615, Fy = 60 ksi

• Concrete - Lightweight f’c = 4000 psi (elevated slabs)

Normal weight f’c = 3000 psi (slab on grade, walls, footings)

• Bolts ASTM A325 – ¾” diameter

Load Combinations: Strength Design – ASCE 7-98 (sect 2.3.2, p.5) & IBC 2000 (sect 1605.2, p. 296)

• LRFD-1 1.4(D+F) • LRFD-2 1.2(D+F+T) + 1.6(L+H) + 0.5(Lr or S or R) • LRFD-3 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W) • LRFD-4 1.2D + 1.6W + 0.5L + 0.5(Lr or S or R) • LRFD-5 1.2D + 1.0E + 0.5L + 0.2S • LRFD-6 0.9D + 1.6W + 1.6H • LRFD-7 0.9D + 1.0E + 1.6H

Allowable Stress Design – ASCE 7-98 (sect 2.41, p.5) & IBC 2000 (sect 1605.3, p. 298)1 • ASD-1 D • ASD-2 D + L + F + H + T + (Lr or S or R) • ASD-3 D + L + (Lr or S or R) + (W or 0.7E) • ASD-4 0.6D + W + H • ASD-5 0.6D + 0.7E + H • ASD-6 D + L • ASD-7 D + L + S + E/1.4 • ASD-8 D + L + W • ASD-9 D + L + W + 0.5S • ASD-10 D + L + S + 0.5W • ASD-11 D + L + S + 0.71E • ASD-12 0.9D + 0.71E

1 ASD Method not accounted for in this report

D = Dead Loads S = Snow Loads H = Soil Loads # L = Live Loads W = Wind Loads F = Fluid Loads # Lr = Roof Live Loads E = Earthquake Loads R = Rain Loads #

T = Self-straining Force (temperature) # # - Not in the scope of this report



GRAVITY SYSTEM Design Loads:

• Dead Loads

1st Floor (psf) 4” Normal weight slab on grade 50.0 (4/12)ft*150pcf Floor Finish 1.0 From ASCE-07 51.0 TOTAL 2nd/3rd Floor Concrete slab on metal deck1 41.0 From USD Manual pg. 38 Suspended ceiling grid 2.0 From ASCE-07 5/8” Gypsum board panels 6.0 From ASCE-07 MEP 5.0 From ASCE-07 Floor Finish 1.0 From ASCE-07 55.0 TOTAL Roof Built-up roofing 2.0 From ASCE-07 ¾” Fiberboard 1.1 1.5 psf per 1” thickness 3” Rigid insulation 4.5 0.75 psf per ½” thickness Suspended ceiling grid 2.0 From ASCE-07 MEP 5.0 From ASCE-07 Concrete slab on metal deck2 31.0 From USD Manual pg. 38 45.6 TOTAL

• Live Loads

(psf) Roof 20 Classrooms 40 Live Load Reductions: Office 50 Applicable to floor live loads only. Office w/ Partitions 80 Not permitted where Live Load 100 psf. Upper Floor Halls 80 All columns minimal reduction = 20%. Public Areas, Lobby 100 If support area > 400 sq. ft., then reduce loads Ground Floor Halls 100 in beams, columns, walls, foundations Stairs 100 L = Lo(0.25+(15/SQRT(KLL*AT)) Storage 125 KLL=Live load element factor (Table 1607.9.1) Mechanical 150 AT= Tributary area

• Snow Loads

pf=0.7CeCtIpg Pg 17.0 psf Ce 1.0 Ct 1.0 I 1.1

pf 13.09 psf < 15.0 psf3 pf 15.0 psf

1 3¼” Light-weight concrete slab on 2”- 20 Gage composite metal deck 2 2” Light-weight concrete slab or 2” cellular concrete on 1½”- 22 Gage metal deck 3 Minimum per North Carolina based IBC Code



Typical North Building Structural Plan

The north building is a steel braced frame structure with two enormous two-story trusses

running parallel to each other1. The trusses actually resemble that of a cantilevered bridge’s

supporting members2. They work together with seven steel columns and a pair of 12” thick,

reinforced concrete shear walls to provide the bulk of the strength in the northern wing3.

The floor system, shown below, at the second and third levels, consists of a 3¼” thick,

4000 psi lightweight concrete slab on three-inch, 20-gage, Type-NS composite metal deck.

The decking spans north-south resting on 40’ long steel wide flange bbeeaammss44 that connect the

ggiirrddeerrss of the two cantilever trusses. These beams, spaced every ten feet on center, are

W21x50s, W21x44s, or W24x55s. Two rows of W14x34 jjooiissttss span perpendicular to the beams to

provide extra support for the slab decking. Two sshheeaarr wwaallllss form the central core to the

cantilever structure, and enclose a fire escape stairwell for that portion of the building.

The roof consists of two-inch, lightweight cellular concrete on 20-gage metal deck. It

slopes east or west, away from the cantilever’s centerline at ¼” per foot. The roof beams are

bent W21x44s, that follow the slope of the roof and are also spaced at ten feet on center.

The length of the cantilever is 52’-6”, requiring two hefty girders at each level to support

the floor weights and help distribute loads from the diagonal braces. The second floor and

1 See next page for Truss Elevation 2 See page 27 for Cantilever Bridges 3 Enlarged North Building Plans located in Appendix C 4 Colors represent members in typical plan, this page



_ Temporary Supports _

Cantilever Elevation

roof girders are W14x211s and the third floor girders are W14x90s. As specified by the

engineers, the cantilever trusses are to be constructed prior to shipping them to the site. Each

truss will be assembled in two pieces.

Piece 1, shown below, consists of the second and third floor girders along with the

second floor spliced columns and braces. Piece 2 will be built with only the roof girders to

connect the third floor columns and braces. The girders, or truss chords, are to be fabricated

in the steel plant at a length desirable to the fabricator. The engineers gave a suggestion for

the splice location of these girders, recommending lengths of 38’-1½”, 39’-11”, & 29’-1½” at

each floor level1. It was also recommended that each piece be disassembled into three

separate parts for easier transportation to the site.

None of the proposed chord lengths can cover the entire spread of the overhang on

their own, calling for a splice at some point along the unsupported length. To provide

adequate strength for a split requires sturdy shear splice connections2. In fact, these

connections were designed with 88 bolts, 72 of which bolt the two girders together with splice

plates located on both top and bottom of the wide flanges. Two shear plates, one on each

side of the girders’ webs, require the other 16 bolts to complete the connection.

When construction of the cantilever begins, the only parts shown above that will be

standing are the 17-foot high, ground floor columns. The girders are not spliced at the column

locations, so the entire truss will be bolted down with the use of ¾” cap plates at the top of

each column and stiffener plates located between the girder’s flanges2. This will result in a

substantial uplift force in the column on the far left once the truss pieces are in place. For this

1 Splice Locations found in Enlarged Truss Elevations on pages 19 & 20 2 Splice Connection Detail located in Appendix E



reason, two temporary supports are placed under the cantilever until the entire structure is

bolted in place. The two shoring members, to be supplied by the construction team, should

be capable of enduring at least 125 and 200 kips of axial load, respectively.

The columns that come preassembled at the second and third floor levels are also

bolted with base or cap plates to the flanges of the continuous girders. Except for the two

W14x132 columns at grid line 3, they are all W14x43s. Most of the vertical bracing members

are also W14x43s. Welded into the girders and bolted to the columns using panel-point

connections with various bolt assemblies, these braces deliver axial forces to the truss chord.

The bracing connections, in one way shape or form, involve a gusset plate that is welded at

full penetration to the girder and bolted to either double angles or double shear plates that

are welded to the column. The brace is then attached to the gusset plate by bolting splice

plates to the brace’s web and claw angles to the brace’s flanges1.

The steel columns at ground level are attached to the concrete footings with base

plates that range in size from a 1’-2” square, at one inch thick, to 1’-8”x1’-10”, two inches thick.

The base plates are all bolted with four A36 or A307 anchor bolts, one at each corner, located

two inches from the edges. Soil samples showed that a shallow foundation system would work

exceptionally well, resting on soil with a bearing pressure of 3000 psf; especially if the depth

does not go beyond a 24” footing thickness for frost protection reasons. The shallow footings

are made of 3000 psi, normal weight concrete and range in depth from 1’-6” to 2’-6”. A four-

inch deep slab on grade consists of 3000 psi, normal weight concrete and contains 6x6-

W2.1xW2.1 welded wire mesh reinforcement at 1½” down from the top of the slab. It shall rest

on top of a four-inch thick layer of compacted NCDOT #57 stone.

As listed in the subsurface exploration report, the allowable net bearing pressure can

actually be designed for up to 5000 psf, if the footings rest on hard residual material below 790

feet. This happens to coincide very well with the positioning of the columns that support the

huge cantilever loads. The building site slopes from a height of 813’ above sea level at the

southwest corner down to an elevation of 790’ on the northeast side, at the cantilever

columns. There are seven cylinder shaped caissons underneath these columns to help drive

the extreme point loads from the cantilever trusses well below the surface to solid rock. The

values of these vertical loads could reach as high as 1200 kips. The six-foot diameter caissons

are all to be approximately 12’ deep, depending on the depth of rock below ground level.

There is an issue concerning differential settlements in the building’s foundation due to the 1 See connection details in Appendix E



Typical South Building Structural Plan

combination of deep caissons and shallow footings. However, the soil below an elevation of

790’ was found to be a small layer of highly consistent residual soil on top of partially

weathered rock, so settlements should be limited. In fact, the soil report insists that the crucial

settlement will occur during construction and can be estimated to be less than ½”.

The south building is a steel frame structural system as well; however its layout is much

simpler than the north building. The floor slab on metal deck at the second and third levels is

identical to the north building. On the other hand, the south building has a more typical grid

layout with girders spanning 25’, 30’, or 35’ between columns. The girders and columns come

in various sizes since they were designed cost effectively depending on each member’s

specific loads. The wide flange columns range in size from W8x24 to W14x132, with additional

ten-inch diameter, A53 Grade B structural pipes located in the lobby for aesthetic reasons.

The girders and columns are welded together with simple moment connections in the north-

south direction to provide lateral support and carry floor loads. The second and third floor

beams, all W21x44s, span either 42’-6” (in the west wing) or 40’ (east wing); barring exceptions

where they are split by floor access openings, mechanical shafts, or lateral bracing.

The roof structure is also slightly different

(typical plan, left), using an array of

bbeeaammss1, ggiirrddeerrss, and ooppeenn wweebb sstteeeell jjooiissttss

to carry the cellular concrete plates on

metal decking. Typical roof supports are

30K9s and 28K8s in the east/west

direction, spanning the 42’-6” or 40’,

respectively, between girders. Although,

anywhere there is a ccoolluummnn, the beams

and girders connecting to it are wide

flanges. Beams are connected to the

weak axis with simple shear connections

since they only carry a small tributary area of floor weight. Moment connections, blue

triangles at the end of a girder, are used for the girders running north/south, just as for the

second and third levels. There are a few exceptions in the typical roof grid, such as the atrium

ceiling and extra structural strength at locations of mechanical equipment2.

1 Colors represent members in typical plan, this page 2 See Structural Plans in Appendix B



LATERAL SYSTEM Design Loads:

• Wind Loads

V 90.0 Basic Design Wind Velocity (ASCE7-98) Iw 1.15 Importance Factor II Occupancy Classification C Exposure Category

GCpi +/-0.18 Internal Pressure Coefficients

Main Wind Force Resisting System:

Vy North Building South Buildng Roof 4.11 7.08 3rd Floor 12.53 21.72 2nd Floor 13.70 23.98 Foundation 7.64 13.22 Total Base Shear 38 kips 66 kips Overturning Moment 809.77 ft-k 1405.61 ft-k



Vx North Building South Buildng Roof 16.10 36.05 3rd Floor 31.46 70.44 2nd Floor 29.90 66.95 Foundation 14.54 32.57 Total Base Shear 92 kips 206 kips Overturning Moment 2218.72 ft-k 4967.91 ft-k

Components and Cladding: As required by the North Carolina State Building Code, all building

components and cladding engineered by the component manufacturer are to be designed

by the manufacturer’s engineer to comply with the basic design wind velocity, importance

factor, and exposure listed on the previous page.



• Seismic Loads Seismic Force Resisting System: South Building:

E/W Direction – Steel Concentrically Braced Frames R = 3 N/S Direction – Steel Moment Frames R = 3

North Building: E/W Direction – Steel Braced Frames & Concrete Shear Walls Rmin = 3 N/S Direction – Steel Concentrically Braced Frames R = 3

Design Base Shear: Per Section 1617.4 of IBC Analysis Procedure: Equivalent Lateral Force Procedure

Importance Factor 1.25

Use Group II Design Category B

Site Class C SS 24.9% S1 11.1% Fa 0.0120 Fv 0.0169 Sms 0.0030 Sm1 0.0019 Sds 0.0020 Sd1 0.0013

Direction North Building South Buildng N-S Ry 3.0 3.0 Cdy 3.0 3.0 Vy 95 kips 375 kips E-W Rx 3.0 3.0 Cdx 3.0 3.0 Vx 95 kips 375 kips



Typical North Building Structural Plan (continued next page)

The north building’s lateral system can be summed up in one word: truss. This design

distributes lateral forces into axial loads along truss chord and web members.

There are trusses spanning the length of the cantilever horizontally at the floor levels.



Typical North Building Structural Plan



There are trusses running vertically.

Truss Elevations – Grid Line B



Truss Elevations – Grid Line C



The lateral system is broken down into two separate systems, the north/south direction

and the east/west direction.

The north/south lateral system must be extremely rigid since the cantilever’s overhang

spans in that direction. Strong enough winds could cause the cantilever to deflect and tilt

downwards. The stiffness of the trusses in this system also contributes to the amount of

vibration the cantilever will see during an earthquake. Two different types of trusses, one

vertical and one horizontal, make up the bracing in the north/south direction. The vertical

bracing, shown on the previous four pages, is located at grid lines B and C. These trusses

closely resemble a cantilever bridge truss. The two pieces, as they are labeled, are to be

brought to the site preassembled into three different sections each. The sections are split at

each of the splice locations, and once on site, they will be bolted together into the

magnificent trusses as shown. The trusses consist of top and bottom chords that are either

W14x211s or W14x90s, depending on which floor level. The web members are either W14x43s

or W14x90s.

There are also braces at grid line D, to provide stability at the ground floor1. The reason

for this bracing can be explained by comparing the trusses at grid line B and C. Grid line B has

an extra column, at grid line 4.4, and a cross-brace, at the ground floor level between grid

lines 4.4 and 4, that is not found in grid line C’s elevation. The architectural plans2 show that

located between grid lines B and D on the ground floor is a large lecture hall that can not

afford to have a column or cross bracing interrupting the space. Because of this, the brace

must be relocated to grid line D. These two chevron braces are HSS 6”x6”x½” steel tubes.

The horizontal bracing utilizes HSS rectangular tubes bolted in place below the metal

decking at each floor level. Most of the braces are 13’-5” long, Grade 46, HSS 8”x8”x¼” tubes

that are bolted on site. The large cantilever-girders are paired with W14x34 beams to make up

truss chords around the rectangular tubing. The principal depth of the trusses is nine feet,

essentially spanning the entire 106’ length of the north building.

The east-west direction is a mixture of two, 12” thick, reinforced concrete shear walls

and vertical tube bracing3 at grid line 4.9. It also utilizes the floor truss web members as

additional bracing. The two shear walls, located at grid lines 3 and 4, are 46’-5” high, 22’ long

1 Bracing elevation VF#7 found in Appendix D 2 Architectural Plans found in Appendix A 3 Bracing elevation VF#6 found in Appendix D



with reinforcement at each face composing of #5 vertical bars @18” O.C. and #5 horizontal

bars @ 12” O.C. The tube braces are HSS 6”x6”x½” steel tubes.

The south building has a similar lateral bracing system. In the east/west, there are five

additional chevron bracing locations1. Grid lines 5, 7, 9, 11, and 13 all have inverted-V braces

at each floor level over a 27’-

1” span between grid lines B.5

and C. The ground floor

braces are 21’-9” long,

second floor 19’-7”, and third

floor 18’-5”. Each floor has

two HSS 6”x6”x½” rectangular

tubes, one to brace in each

direction. The tube brace

connections in the Renick

Building entail bolting gusset

plates to the web of either the

columns or girders. Then, on

site, the rectangular tubing is

field welded to the gusset

plates with 1-inch wide fillet

welds on both sides.

In the north/south direction,

there are simple moment connections

located at the splice between the

main floor girders and columns. The

welds, as shown here, are to be field-

welded, bevel plug or slot welds. The

beam web is also bolted to the

column flanges.

1 See Bracing Elevations in Appendix D

mick leso – structural option 2006 penn state ae senior … · mick leso – structural option...

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