Faculty Consultant : Dr. Linda Hanagan Date: Wednesday ... · dimensioning. Alternative Floor...

Technical Assignment #2 Pro/Con Structural Survey of Alternative Floor Systems Executive Summary

Technical Assignment #2

Pro/Con Structural Survey of Alternative Floor Systems

Executive Summary:

The purpose of this report is to compare the advantages and disadvantages of various alternative floor system designs against the existing floor system in the Hyatt Center. Many factors can be used for comparison such as weight, costs, ease of construction, material benefits, structural benefits and setbacks as well as impacts on other systems within the building and compliance with the design criteria. Alternate systems will be designed and compared to the existing to establish which systems are viable as alternative gravity systems.

The existing composite floor system and Alternative #1 – Open-web steel joist with composite

deck; and Alternative #4 – One-way concrete slab and beam; are feasible options. Alternative #2 – Non-composite beams with form deck; and Alternative #3 – One-way concrete pan-joist; are not viable alternatives.

Alternatives:

Existing: Composite beams with composite metal deck Smallest member depths, but susceptible to vibration problems. Highly efficient system for long-spans and heavy loading.

Alternative #1: Open-web steel joist with composite deck

Lightest system weight of survey, however very susceptible to vibration problem. Speed and ease of erection but many joists increase crane movement and possibly

construction time.

Alternative #2: Non-composite beams with form deck Deep members required to span long direction to maintain deflection criteria. Lightweight system however imposes on mechanical and wiring systems.

Alternative #3: One-way concrete pan-joist

Large deep girders required to resist pan reactions also imposes on mechanical system.

Lease susceptible to vibration however was found to be heaviest system in survey which greatly effects foundation and gravity columns size.

Alternative #4: One-way concrete slab-and-beam

Ability to conform to unique bay configurations, but a heavy system which impacts foundation and column sizes which may become architecturally unacceptable.

Deep beams and girders result in increased floor heights and overall building height.

Patrick L. Hopple Faculty Consultant : Dr. Linda Hanagan Date: Wednesday, October 27, 2004 H y a t t C e n t e r ( 7 1 S o u t h W a c k e r D r i v e : C h i c a g o , I L )

1 of 14 Technical Assignment #2 Pro/Con Structural Survey of Alternate Floor Systems

Patrick L. Hopple Faculty Consultant - Dr. Linda Hanagan Date: Wednesday, October 27, 2004

H y a t t C e n t e r ( 7 1 S o u t h W a c k e r D r i v e : C h i c a g o , I L )

Introduction - Technical Assignment #2 – Pro/Con Structural Survey of Alternative Floor Systems

The purpose of this report is to compare the advantages and

disadvantages of various alternative gravity floor systems against the

existing gravity floor system. Many factors can be used for comparison of alternative

systems such as weight, costs, ease of construction, material benefits, structural benefits and

setbacks we well as impacts on other systems within the building. Alternative systems will

be chosen by typical industry use in commercial office high-rises, conservatively designed

with baseline loads and bay dimensions then measured against existing systems before

conclusions will be made on whether a system is to be investigated further or if the existing

flooring system is the best viable alternative.

Overall Structure: (An overview)

Composite System: The Hyatt Center is a 49-story commercial office

building in the center of the Chicago business

district. The entire building is composed of a 49-

story tower and a 7-story mezzanine.

The mezzanine structure and the tower have

some similar characteristics in terms of employed

gravity structural systems. The mezzanine is

comprised typically of rectangular bays (36’x46’)

and use specially detailed open-web steel trusses

and wide-flange beams with web-openings in the

retail areas to accommodate ducts and overhead

electrical runs. The trusses are comprised of

back-to-back angles for the diagonals and

verticals with chords of “WT” sections which

support an 11-inch slab on composite metal deck




and 4 ½” – ¾” diameter shear studs. Lateral loads in the mezzanine

are resisted by rigid moment frames in both directions. My focus for

this report is primarily on the tower of the Hyatt Center.

The tower structure consists of an oval-shaped footprint on the

site which produces some difficulty when trying to find a square bay.

However, the tower was designed to allow for a constant span dimension from the core to

the perimeter spandrels. This was accomplished by designing the central core walls to

mimic the profile of the curved façade, in doing so a constant beam span (varying from 40’

at the base to 43’ in upper levels) can be obtained across the floor plate, resulting in the

same beam spans. A constant perimeter span of 38’-3 ¾” was also upheld by the radial grid

dimensioning.

Alternative Floor Systems: Four alternate floor systems were chosen based on common practice in commercial

high-rise design:

1. Open-web joist system with composite metal deck 2. Non-composite wide flange beams with non-composite form deck 3. One-way concrete pan joist system (ribbed slab) 4. One-way concrete slab-and-beam system

The systems chosen above were selected based on a most practical/feasible approach in terms of constructability and least weight for the existing building layout. Because the Hyatt Center has typical bays which are trapezoidal-shaped reaching 42ft in the N-S direction and ranging 31ft-38ft in the E-W direction, many systems can be ruled out as possible solutions. One such floor system is a waffle slab which is common in high-rise applications for its less weight and easy of construction. Waffle slabs typical are utilized in square bays upwards of 38ft, but without square bays the two-way action becomes quite complex to analyze which causes the system to be dropped. The issue of long spans also becomes and issue when dealing with concrete construction. To be economical and a feasible alternative, a two-way slab can be placed in a relatively square bay with typical bays of no further than 30ft. After this point the slab and reinforcement become quite heavy which eliminates the advantages of using a two-way slab in the first place. In conclusion, although bays could be squared and minimized the architecture would discourage this approach as much of the expansive view would be obstructed causing vast differences in the aesthetics appearance.




Design Methodology: To simplify the design process and promote consistency in the

structural survey between floor systems a simplified bay was used

instead of the actual trapezoidal bay as seen below. Since various

loading is present throughout the structure, a typical plan was used

to find the typical bay and limit loading on the bay. This typical plan was repeated

numerous times in the upper 35th through 46th floors of the Hyatt Center and is

representative of a large portion of the building. The simplified bay was squared-off

providing 36ft x 42ft spans and assumed to be representative of each bay throughout the

entire building. This assumption results in highly conservative design methodologies

however, with the unique bays and massive size of the Hyatt Center this approach will be

consistent among each system. Design loads were also kept relatively similar (and limited)

between each system to provide a baseline measurement for comparing member sizes.

Design Superimposed Dead Loads:

Raised Floor 15 psf

Ceiling/MEP 5 psf

Office 40 psf

Design Live Loads (ASCE 7-02):

Office 50 + 20 psf partitions




Once the loading was determined to be fairly consistent across

the design of each system, members were arranged according to two

design methods. One was to keep the N-S (radially-away-from the

central core) arrangement of the members to ensure that in-plane

stiffness of the horizontal diaphragm was similar and adequate to the

existing system which provides transmission of lateral force to the central core. The second

method was to arrange members along the least span dimension (38 ft E-W) and assume

that shear wall-frame interaction of the columns and girders contributed to most of the

lateral load distribution along with the rigid diaphragm. Both arrangements have their pros

and cons which were taken into consideration which affected member sizing. Concrete

beams are typically not efficient in long-span conditions such as in the Hyatt Center;

therefore, design of pan joists in the short direction was implemented and one-way slab-

and-beam were arranged in the long direction.

The first alternative system, open-web joists with composite deck, was assumed to be

spaced equally along the 38’ bay which resulted in a spacing of 4’-9”. Such a close spacing

ensured that the joists could span the 42ft dimension while having adequate capacity to

hold the heavy loads. Hand calculations of required loads and stresses as well as the design

of the deck and joists can be found in Appendix B of this report.

The second alternative system design, non-composite wide-flange beams with form

deck, can be found in Appendix C of this report. The controlling factor in designing non-

composite systems is meeting deflection criteria. A limit of L/360 was set for each design

and required member properties were found and checked against the applied loading.

Non-composite sections do not have the versatility of concrete slab-and-composite beam

interaction to resist heavier loads; therefore the depth of members will most likely be

greater.

The last alternative systems, concrete pan-joists and one-way slab-and-beam, can be

found in Appendix D and E of this report, respectively. To ensure sufficient fire

protection, a 5” minimum slab was checked for stresses from the applied loads and

members were designed using the CRSI Design Handbook 2002. Member sizes were




checked using ACI 2002 load factors as compared to ACI 1998 load

factors in the handbook.

Detailing such as reinforcement placement, shear stud quantities,

connections and slab reinforcement were neglected in the overall

design of each system due to the broad focus of this report. Once

members systems were finalized, the advantages and disadvantages of each system were

compared and conclusions were formed about the validity of each system as a viable

alternative.

Issues addressed in determining validity include overall constructability, weight

increase/decrease, impact of each system on other building elements (foundation,

mechanical, electrical, etc.). Also a large concern in recent years, especially in new

commercial offices is vibration issues caused by lighter systems. Each system will be

discussed and a summary will conclude with which systems warrant further investigation by

a more refined analysis as an alternative floor system.




Alternative Flooring System Survey

Existing Composite Flooring System: System Description: Slab: 5 ½” (2 ½” cover) LWC (115pcf) f ’c= 4 ksi Deck: USD 3” LOK-FLOOR, 20 gage Beams: A992 composite wide-flange sections W18x50 (40), cambered 1 ¼” Girders: A992 composite wide-flange sections W18x50 (40), cambered 1 ¼”

Perimeter Beams: A992 composite wide-flange sectionsW27x84

Depth: 23.5” total depth interior; 32.2” total depth exterior spandrel

Design Results: (Wt./SF = 18.13psf) Design Loads: 72psf (DL), 112psf (LL) Member Sizes: (# shear studs) Beams: W18x50 (40), cambered 1 ¼” Girders: W18x50 (40), cambered 1 ¼”

Perimeter Beams: W27x 84 (30) (due to L/600 deflection criteria)

Pros/Cons: The versatility of a composite floor system is due to

the inherent strength of the concrete slab in compression and the ability of the steel member to span long distances which are prevalent in the Hyatt Center. Advantages of composite systems include reduction in material cost, on-site labor, and construction time as well as simple repetitive connections, reduced structural depth of members and lower building mass. The combination of a composite steel deck with composite beams provides for a highly efficient system which can be repeated several times and become very cost effective, especially in a high-rise building where reductions in floor depths can result in cumulative savings in curtain walls, wiring, ductwork, plumbing, etc. which can be quite considerable.

Composite sections have greater stiffness than the individual (slab-and-beam) and therefore load resistance is higher, vibration issues are minimal and the beam arrangement in composite floor systems creates a very rigid horizontal diaphragm which provides stability for the overall building as well as distributing lateral loads to the core.




Alternative #1 – Open-web steel joist with composite deck: System Description: Slab: 4” (2 ½” cover) LWC (115pcf) f ’c= 4 ksi Deck: USD B LOK, 20 gage Joists: Vulcraft Open-web joist Perimeter Beams: Vulcraft Joist-Girder Depth: 32”+4”=36” total

Design Results: Design Loads: 86psf (DL), 70psf (LL) Member Sizes: (depth (in)) Joists: 32LH13 (32”+4”) @ 4’-9” o.c.

Joist-Girder: 38G8N20K (32”+4”) USD: 5’span = 400psf LL > 70psf (service) Max single span = 5.14’ > 4.75’ Max 3-span = 6.91’ > 4.75’ Overhang = 2’-3” > 2’-1” (20 gage) TLAPPLIED= 874plf < TLALLOW = 974plf LLAPPLIED= 532plf < LLALLOW = 710plf Wt/S.F. = 7.17psf

Pros/Cons: A main advantage of an open-web joist system is the overall light weight of the system. A

light floor system will ultimately have the least impact on foundation and column requirements for a high-rise as well as crane sizing since heavier beam loads will not have to be lifted. The typical weight per area of this system is comparable to composite systems. Mechanical ductwork can be integrated through openings within the truss webs.

Construction and erection of joists can be quick and very repetitive if no unusual circumstances arise such as irregular bays or framing, such a fast erection can speed installation of composite deck behind the erection and individual trades can work almost immediately after erection is complete, therefore, construction time is minimized.

Disadvantages of open-web joists include issues pertaining to fireproofing. Also close spacing of joists may possibly hinder crane placement and slow the erection process.

Vibration is a major concern with this system. The lack of weight able to contribute to damping as compared to a wide-flange section causes light-framed floor systems to vibrate excessively with simple movement of just one person if additional partitions and other heavy loads such as file systems are not considered.

Connection into core wall will require extensive detailing of embedded plates.




Alternative #2 – Non-composite beams with form deck: System Description: Slab: 6” (4” cover) NWC (145pcf) f ’c= 4 ksi Deck: USD UF2X, 18 gage (44- W4.0 x W4.0) Beams: A992 Wide-flange sections (non-composite) @ 9.5ft o.c. Perimeter Girder: A992 Wide-flange sections (non-composite) Depth: Interior (35.9” total); Spandrel

(44.2” total)

Design Results: Design Loads: 75psf (DL), 70psf (LL) Member Sizes: (depth (in)) Beams: W30x90 (29.5”)

Perimeter Girder: W40x149 (38.2”) USD: 9.5’span Max 3-span = 253psf TL >202psf TL BEAM: MU= 423.4k’ < ΦMN = 1060k’ GRD: MU= 868.2k’ < ΦMN = 3620k’ Wt/S.F. = 23.0 psf

Pros/Cons: Main advantages of non-composite

wide-flange beams include long-span efficiency, a good strength to weight ratio and fast erection with minimal foundation requirements due to lighter system densities as compared with concrete systems.

Non-composite systems have the ability to be custom-fit for use in irregular bays such as the trapezoidal bays in the Hyatt Center. The efficiency of this system is in repetitive member use and connection designs to provide quick erection around the central core.

A non-composite system however lacks the advantages of composite action between the concrete slab in compression and inherent long-span efficiency of structural steel causing member depths to be greater than a composite system.

Vibration can be a concern with a non-composite floor system. The lack of stiffness found in composite systems as well as less weight able to contribute to damping as compared to concrete floor systems may cause non-composite wide-flanges beams promote vibration issues if additional issues like partitions and other heavy loads such as file systems are not considered.

Spray-on fireproofing or a rated ceiling assembly is needed unlike the fireproofing which concrete systems provide.




Alternative #3 – One - way concrete pan-joist: System Description: Slab: 5” Slab NWC (145pcf) f ’c= 4 ksi fy= 60ksi Forms: 30” forms = 6” ribs @ 36” o.c. Pans: 20” Deep pans + 5” slab = 25” deep (36’ span) Girder (h x b): 42” x 20” f ’c= 4 ksi fy=

60ksi

Design Results: Design Loads: 60psf (DL), 70psf (LL) Reinforcement: Ext. Span (int. span) Slab: AsMIN= #3 @ 12” o.c. Joist: Top bars: #6 @ 11” (#5 @ 8”) Bot. bars: 2 - #7 (2 - #6)

Girder: Top: 7-#14 2 layers Bottom: 4-#11 2 layers JOIST: LDALLOW= 195psf > LDAPPLY =

184psf GRD: MU= 1158k’ < ΦMN = 1818k’ LDALLOW= 11.3klf > LDAPPLY = 11.12klf Wt/S.F. = 125psf

Pros/Cons: Pan-joist floor systems have the advantage of being the most effective for reducing dead

weight of floors for long spans. Pan-joist construction can be quite fast and efficient when similar forms can be replicated and reused over and over again which reduces costs and saves on construction time.

Concrete construction in general has advantages of having no lead time, late changes can be made without much impact on schedules or costs and no additional fireproofing is needed unlike in steel construction.

A major disadvantage, evident in this system is the inefficiency in long spans. Girder depths of 42” provide no space for ductwork or wiring unless floor height is increased. Use of such a heavy system will have drastic impacts on foundation designs, crane and hoisting demands during construction and also cause column sizes to become so massive, they become architecturally unfeasible unless high-performance concrete is used.

This system will definitely impact the column and foundation requirements causing use of deep foundations and high-performance concrete to be used in the Hyatt Center.

Complex framing and bay sizes cause slow onsite construction because custom forms are needed which also congest site space during construction.




Alternative #4 – One - way concrete slab-and beam: System Description: Slab: 6” Slab NWC (145pcf) f ’c= 4 ksi, fy= 60ksi (spacing = 10ft) Beams (h x b): 32” x 18” f ‘c=4ksi, fy= 60ksi Girder (h x b): 32” x 20” f ’c= 4 ksi, fy = 60ksi

Design Results: Design Loads: 75psf (DL), 70psf (LL) Reinforcement: Slab: AsMIN= #3 @ 12” o.c. Beam: Top bars: 4 - #14 Bot. bars: 2 - #11

Girder: Top: 4 - #14 Bottom: 3-#14 JOIST: LDALLOW= 6.2klf > LDAPPLY = 2.74klf +MU= 337.8k’ < ΦMN = 736k’ --MU= 602.8k’ < ΦMN = 1002k’ GRD: LDALLOW= 7.8klf > LDAPPLY = 7.2klf +MU= 611.3k’ < ΦMN = 795k’ --MU= 860k’ < ΦMN = 1019k’ Wt./SF = 107.7 psf

Pros/Cons: A big advantage of one-way slab-and-beam is

the ability to span long and short dimensions as well as form non-square configurations which are typical in the Hyatt Center. Vibration is typically not an issue with one-way concrete slab-and-beam systems due to the overall increase in weight as compared to the existing system.

Concrete construction in general has advantages of having no lead time, late changes can be made without much impact on schedules or costs and no additional fireproofing is needed unlike in steel construction.

The main disadvantage of this system is the required depth of members to resist the given loads. An increase of 14.5” compared to the existing system will cause conflicts in ductwork and wiring configurations unless the floor height is increased. Increases in building height will also impact the lateral load resisting system and increase the required sizes of gravity columns used to support such a heavy flooring system. Further analysis into use of high-performance concrete in flooring and column designs would have to be performed




This system will definitely have an impact upon the column and foundation requirements possibly causing use of deep foundations and high-performance concrete to be used in the Hyatt Center.

Conclusions:

Of the systems surveyed I left many flooring options out of the survey which are typically used in high-rise design. Waffle slab, post-tensioned concrete slab, and pre-cast hollow plank were considered in the initial stages of this report but research into each system and my design methodology helped weigh each as viable alternatives.

First, a waffle slab is typically a very efficient flooring system which has the advantage of

reducing the overall weight of the system by incorporating square pans as the formwork. The two-way action of a waffle slab is best suited for use in long-spans and heavy loading conditions, both of which are design criteria for this report. I chose not to explore this system due to the irregular bay which is present in the Hyatt Center. Logically no easy way to construct the waffle slab can be found. Instead of each waffle section being aligned with the end girders as in normal square bays, the trapezoidal bay will result in conflicting rib alignment. The lack of constructability in a “real-case” scenario leads me to neglect a waffle-slab in my evaluation.

Next, a post-tensioned slab system was initially considered as an alternative due to its

ability to span long dimensions and relatively small slab depth and reduced weight which




fit the design criteria of this survey. However, due to limited

knowledge and research of post-tensioned slabs I chose to neglect

this type of floor system. One drawback of using a system would be

tendon location around the central core. If tendons were to be radial

offsets about the curvilinear profile, then this system warrants further

investigation as a possible solution. Tendon orientation

perpendicular to the core wall may involve construction issues such as where tendons

would need to be placed in the core and what sequence would be needed to minimize the

impact on other trades, especially the jump-form used in the core wall construction.

The last alternative system I chose to neglect in my evaluation was pre-cast hollow

planks on steel wide-flange beams. The design of the Hyatt Center, with the central core

resisting all lateral loads, relies heavily on the horizontal diaphragms to distribute lateral

loads to the core shear walls. By incorporating pre-case planks into the floor design, we no

longer can assume a rigid diaphragm in the plane of the floor. Drastically reducing the in-

plane stiffness of the rigid diaphragm causes the structure to behave differently than

originally analyzed. A complete redesign of the lateral force resisting system would result if

pre-cast planks were used in place of a rigid floor slab.

After considering the four alternative floor systems I believe the best system to be used

in the Hyatt Center would be the existing composite steel beams with composite deck.

Looking into each alternative I found that many of the designed members were very deep

and large which resulted in heavier systems.

A one-way concrete slab-and-beam system is a feasible option to be investigated further

due to the ability to accommodate irregular bay configurations. Investigating use of high-

performance concrete for gravity structural members may provide the key element to

enable a switch to an all concrete building. One high-rise in particular, 311 South Wacker

Drive, is amongst the world’s tallest buildings. What makes this building unique is the use

of high-performance concrete in the building columns with compressive strengths in the

order of 8000-15,000psi. Concrete of this high-strength leads to a very brittle failure mode

which has to be considered during further investigation, however, the combination of high-




performance concrete with a one-way slab and beam system warrants

further investigation as a viable alternative.

An open-web steel joist system with composite decking is also

another system that should be analyzed in-depth. Many high-rise

buildings designed today are designed with long-spanning joists due

to the lightweight and ease of erection. Issues such as stiff girders between core wall and

exterior columns to transmit lateral loads may have to be addressed since the relative

stiffness of the rigid diaphragm will be reduced with use of light framed joists. Given the

design criteria, the open-web joist system with composite decking may work and warrants

further investigation into feasibility as an alternative system.

And lastly, the existing composite floor system is the best possible solution for the

flooring system considering the design criteria. Considering the grand scale of the Hyatt

Center, 50 stories (1.7 million gross square feet), a need for the most efficient floor system

needs to be utilized so costs of construction are the most economical. The slightest change

in the number of shear studs in a bay for example, which when replicated over 50 stories

on the same bay of 3 beams, has the possibility to drastically change cost figures. Also the

reduction in beam depths which results from the use of a composite system allows

mechanical ducts and other systems to be run in the ceiling plenum space, resulting in an

efficient floor sandwich.




Appendix: Appendix: Pages: Description:

A 1-2 Design Bay and Loads

B 3-4 Alternative #1: Open-web steel joist with composite deck

C 5-6 Alternative #2: Non-composite beams with form deck

D 7-9 Alternative #3: One-way concrete pan-joist

E 10-12 Alternative #4: One-way concrete slab-and-beam

F 13 Project Grid Dimensions and Floor plan

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Appendix A

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Appendix B

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Appendix C

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Appendix D

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Appendix E

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Typical bay used and simplified for design and comparison of alternative floor systems.

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Appendix F

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