Thin Slab Floor Systems for High rise Towers

Thin Slab Floor Systems for High Rise Towers

By Steven M. Baldridge, P.E., S.E., and Jeffrey Yoders

SEPTEMBER 2012

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2 PDH Professional Development Advertising Section — Bentley Systems, Incorporated

Professional Development Series

T he primary goal of the project team behind the Trump International Hotel & Tower at Waikiki Beach Walk was to provide the finest

accommodations and the best Pacific Ocean and Diamond Head views in Waikiki, Oahu, Hawaii. Baldridge & Associates Structural Engineering (BASE) created a structure that fulfills its purpose without interfering with the views of the ocean or the saleable space — and allows for additional valuable floors due to the innovative thin floor slab system used in it. Bentley Systems’ RAM Concept slab design software played an integral role in this process.

The Trump International Hotel & Tower at Waikiki Beach Walk is a $442 million, 38-story, 775,000-square-foot luxury hotel completed in 2009 that posed several diffi-cult design challenges. It is the flagship

property of the Waikiki Beach Walk rede-velopment. Architectural function was to be the highest priority, and while structural form was of secondary importance to the naked eye, its hidden innovations made the stunning postmodern form of the finished project possible. The structure is located in a seismic area with strict codes for reinforce-ment and limits for building height.

Constrained by a 350-foot height limit in Waikiki, the team of BASE, architects Guerin Glass and Benjamin Woo, and the joint venture general contractor A.C. Kobayashi/Kiewitt Building Group was challenged to include all 38 floors in that 350-foot-frame by limiting each floor slab’s thickness while still maintaining acceptable sound transmis-sion, vibration, and deflection characteris-tics. The height constraints were achieved with thin slab post-tensioned technology

coupled with 3D analysis to verify perfor-mance.

The structural engineers at BASE used Bentley RAM Concept to design 19 unique floor slabs to be as thin as possible. RAM Concept includes an integrated design process for post-tensioned concrete slabs, including full 3D finite-element analysis of a design and drawing-ready rebar detailing. Using RAM Concept, the engineers at BASE were able to virtually test the performance of each floor in the design stage.

The height restrictions also posed chal-lenges for the structure’s lateral load-resisting system. The project incorporated innovative steel plate-reinforced link beams to achieve the required strength and stiff-ness of the building. Thanks to the use of the thin floor slab design, the lower floors had 6-inch slabs and the upper condo floors

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Learning ObjectivesThis article provides basic information about: • how to properly design and specify thin floor slabs to allow

for extra useable space in a high-rise project;• how floor transitions were used to shift loads on the Trump

International Hotel & Tower at Waikiki Beach Walk;• how to use a lateral load-resisting system that uses steel

plate-reinforced link beams to achieve required strength and stiffness; and

• how to specify a composite system of concrete surrounding a steel plate with shear studs in place of link beams in areas with tight floor-to-floor distances.

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Thin Slab Floor Systems for High Rise TowersBy Steven M. Baldridge, P.E., S.E., and Jeffrey Yoders

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Professional Development Advertising Section — Bentley Systems, Incorporated PDH 3

with longer spans used 6-1/2-inch slabs. Using RAM Concept allowed BASE quick exploration of various slab design alterna-tives, leading to an optimized design with a banded slab at the corridors. The design

resulted in a very efficient structure requir-ing only 6.1 pounds per square foot (psf) of reinforcing steel and 0.56 psf of post-tension tendons.

The hotel/condo tower contains five floors

of parking, a pool and spa deck, 17 floors of hotel rooms, and 15 floors of condominium units all in 38 stories. These diverse building uses led to 19 structurally unique floors. As in most vertical, mixed-use projects, the opti-mum column and wall layouts for each use rarely match the supporting levels below. More than 70 supports required a transition with some elements shifting in plan several times throughout the height of the build-ing. The original architectural design also had a lack of structural depth that, in many cases, prevented the use of conventional transfer girders.

Transition of load was required from the very top of the tower. The 38th-floor pent-house units were inset 8 feet from the build-ing’s perimeter to provide wrap-around balconies on each end. The roof of those penthouse units was required to support heavy mechanical loads in the center and rooftop terraces on the perimeter. To achieve the appearance of column-free space in the penthouse units, 3-inch steel posts were hidden within the window mullion system and used to support the roof structure. To transfer loads from these mullion columns outward to the supporting columns below, the penthouse floor’s post-tensioned slab was thickened to create a transfer slab at the structure’s perimeter.

The postmodern design had an area of architectural relief at levels 35 through 37 that required another offset in plan and the addition of a glazed, non-structural façade. Concrete walls 8 inches thick were hidden in the partition walls between the units in those floors to provide structural support. This required four more transfer conditions, provided at the 34th floor by creating a deep corbel that shifted the load approxi-mately 2 feet outward to the exterior shear wall.

At the transition from the 462-condo-minium units to the hotel, additional floor space was gained by extending a portion of the building’s southeast side by 6 feet. The building’s diagonal walls provided support for the hotel floors. The floor area above this transition was supported by 24-inch-diameter concrete columns. To accomplish

The Trump International Hotel & Tower is the anchor of the Waikiki Beach Walk development in Oahu, Hawaii. Photo: Vito Palmisano/www.vitopalmisano.com

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this transition, the round columns were cast integral with the diagonal walls for a full story. As the residential tower drops down to its podium

at the pool and recreation deck level, discontinuing the expressed diagonal walls gave the building the dramatic architectural expres-sion desired. The tower appears to float over the podium. This effect was achieved by changing the support structure to smaller columns, offsetting portions of the seventh floor inward, and creating a three-story vaulted space at the south end of the building above the sixth floor infinity pool. Nineteen walls were discontinued at this transi-tion. As in the condominium level transition, the diagonal walls were cast integral with the supporting rectangular columns for one full story.

The parking requirements at levels two through five mandated the transfer of an additional 15 columns. This was accomplished with post-tensioned transfer girders in both downturn and upturn conditions, depending on available space. A final set of transitions accommodated a 56-foot-wide roadway and loading area through the north side of the building. Sloped columns shifted four tower columns out of the loading dock space. The columns above the load-ing dock were transferred by 12-foot-deep, post-tensioned girders. With 36 levels of structure above, careful planning of camber and tendon stressing sequences was required.

Lateral design considerationsOahu is a moderate seismic zone subject to infrequent hurri-

BASE proposed replacing steel flanges in some areas with rebar and keeping only the web as a vertical steel plate. Photo: BASE

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canes. The size and prominence of the project justi-fied a more vigorous approach to determining wind loads and a wind-tunnel study was performed by RWDI Consulting Engineers & Scientists. Because of the height of the structure and its relatively light weight — overall structural weight was reduced by 30 percent by reducing column, wall, and foundation requirements due to the thinner floor slabs — seismic forces were reduced to the point where wind forces governed the design of nearly every lateral element. Consequently, the wind study was well worth the invest-ment.

Ordinary concrete shear walls are employed as the building’s basic lateral system as the project falls in Seismic Design Category C. A stair core toward one end of the building is balanced by the exterior wall on the opposite end, with a central elevator core between them. The geometry of all three main lateral elements is governed by the build-ing’s architectural demands and thus all three change form several times along the building’s height. This complicated both the design and construction of these walls, particularly the concrete forming system. Adjustable, self-climbing forms were used on all three sets of walls, but only above the parking levels, where the variation of the geometry required hand-set forms.

Two link beams cross the central corridor at both the elevator and the stair cores. The extremely tight floor-to-floor distances left little height for these beams, resulting in the shear demand exceeding the maximum concrete capacity in many of the links. Steel, wide-flange beams were considered as an alternative but were rejected because of anticipated congestion issues in the tight area.

To eliminate the conflict between the vertical reinforcing steel and the steel beam flanges, BASE proposed replacing the flanges with rebar and keeping only the web as a vertical steel plate. The result-ing composite system, with concrete surrounding a steel plate with shear studs, has been used internationally and on a small number of projects in the United States, but is not yet recognized by the International Building Code and, at the time, had limited support-ing literature. A design procedure was developed using basic engi-neering principles along with research from the University of Hong Kong.

Local building authorities approved this performance-based approach through a third-party review process. This link beam system allowed the building to perform properly without additional structural elements, and the plates were installed successfully by A.C. Kobayashi/Kiewitt with minimal construction issues.

For the property to take full advantage of the ocean views, the resi-dential units were rotated 40 degrees from the building axis toward the ocean end of the structure. In place of columns, structural walls were inserted into the partition spaces between units to serve as the primary gravity elements. With every square inch being valuable, sellable space negotiations with the architect reduced the structural wall thickness from 8 inches to 7 inches, and finally to 6-1/2 inches. These 19 diagonal walls joined the three main lateral elements, but could not continue to the ground. The walls landed on rectangular columns at the eighth floor, creating significant discontinuity in

This RAM Structural steel concept showed that changing the support struc-ture to smaller columns, offsetting portions of the seventh floor inward, and creating a three-story vaulted space at the south end of the building above the sixth floor infinity pool achieved the desired architectural effect of making the condo and hotel seem from afar as if they are floating above their podium. Image: BASE

To transfer loads from these mullion columns outward to the supporting columns below, the penthouse floor’s post-tensioned slab was thickened to create a transfer slab at the structure’s perimeter. Image: BASE

This RAM Structural transparency shows where the entire structural load is being carried on this floor. Image: BASE

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the lateral stiffness of the structure. Structural analy-

sis revealed that the continu-ous lateral elements received an additional 3,000 kips of lateral shear at the transition floors due to the prying effect at the base of the diagonal walls.

To alleviate the problem, horizontal gaps were introduced in the ends of the walls at each story, leaving a continuous wall length sufficient to carry the gravity loads.

The gaps reduced the in-plane flexural stiff-ness of the walls, dramatically reducing the excess shear transferred at the transition floors, and the structural requirements were met without noticeably changing the archi-tectural design.

Even with all of the vertical load transi-tions, this structure required less concrete and reinforcing steel per square foot than any other recently constructed tall build-ing in Honolulu. Having opened in late

2009, it has been recognized as one of the most innovative and iconic structures in the islands.

Steven M. Baldridge, P.E., S.E., is the president of Baldridge & Associates Structural Engineering (BASE), the firm he established in 1995. Jeffrey Yoders is technology editor of Structural Engineer.


Transfer girders distribute the load of the Trump International Hotel & Tower at Waikiki Beach Walk. Photo: BASE

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1. The engineers at BASE were constrained by what local building code on the project?

a) 350-foot height limit in Waikiki b) Local seismic load requirements c) Concrete core demands d) All of the above

2. The structural design required: a) Several lofted ceilings b) 19 unique thin floor slabs c) Castellated beams d) Structural Insulated Panels

3. The architectural design emphasizes: a) The Bauhaus style of architecture b) The division between the hotel and condominium units c) The best Pacific Ocean and Diamond Head views in

Waikiki d) The building as a sculpture

4. The design resulted in a very efficient structure requiring:a) Almost no steel rebarb) 6.1 pounds per square foot (psf) of reinforcing steel and

0.56 psf of post-tension tendonsc) Less concrete than other hotel/condo towers of the

same sized) All of the above

5. The building’s diverse uses (condo, hotel, and parking) led to:

a) A mixture of condo and hotel units on some floors b) Thinner floor slabs c) Some units without an ocean view d) 19 structurally unique floors

6. The roof of the penthouse units was required to support heavy mechanical loads in the center and rooftop terraces on the perimeter. To achieve the appearance of column-free space in the penthouse units, 3-inch steel posts were:a) Placed in the middle of the unitsb) Hidden within the window mullion system and used to

support the roof structurec) Placed in partitions between the two penthouse unitsd) Connected to load-bearing girders on the sides of the

building

7. To transfer loads from mullion columns outward to the supporting columns below:a) The penthouse floor’s post-tensioned slab was thickened

to create a transfer slabb) Additional wall supports were inserted within each

penthouse unitc) An additional loft floor was built to transfer the loadd) None of the above

8. At the transition from the condominium to the hotel, additional floor space was gained by extending a portion of the building’s southeast side by 6 feet. The building’s diagonal walls provided support for the hotel floors. The floor area above this transition was supported by 24-inch-diameter concrete columns. To accomplish this transition:a) The columns were supported by concrete partition walls

on the above floorb) The diagonal walls ended at the beginning of columnsc) The columns were cast integral with the diagonal walls

for a full storyd) Another, thicker transfer floor was necessary

9. What did BASE specify as an alternative to link beams near the stair and elevator corridors?a) Steel, wide-flange beamsb) Replacing the steel flanges with rebar and keeping only

the web as a vertical steel plate c) High-density rebar-less concreted) None of the above

10. The 19 diagonal walls that joined the three main lateral elements, yet could not continue to the ground, landed on rectangular columns at the eighth floor, which created significant discontinuity in the lateral stiffness of the structure. How was this problem alleviated?a) More columns were added to stiffen the structure

through the 10th floorb) Girders were added to provide stability on all floors and

hidden in the window mullionsc) Horizontal gaps were introduced in the ends of the walls

at each story, leaving a continuous wall length sufficient to carry the gravity loads.

d) All of the above.

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Thin Slab Floor Systems for High rise Towers

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