Introduction

Reinforced concrete was chosen as the structural system by the design team for several important reasons on this residential building. The floor to floor heights were minimized with the use of thin, post-tensioned concrete slabs, which allowed the Owner to cost effectively maximize the number of floors in the building. In addition, concrete provided the necessary stiffness for the Outrigger-Braced lateral system while providing the speed of construction necessary to meet the project schedule demands.

Wind and its dynamic effects governed the structural design of this 700 foot tall concrete residential tower. Originally conceived as a 40 story tower – its concrete core and outrigger lateral system and structural layouts were established, and then the height was then extended to 61 stories. Recognizing the structure’s stiffness would now not be sufficient to keep accelerations within acceptable limits (the key for lateral design of tall towers) – at least not without increasing costs substantially and yielding impractical architectural space layouts – Halvorson and Partners (HP) sought to economize the structure at hand and introduce a secondary damping system to insure accelerations in the tower due to wind met standard criteria.

With the assistance of the Boundary Layer Wind Tunnel (BLWT) Laboratory, HP studied options for different combinations of structural stiffness and damping to develop a cost effective structural solution that incorporates a Tuned Liquid Damper (TLD) at the top of the building. The TLD is the latest approach to controlling perceptible motion in tall, slender buildings offering low cost, low maintenance and high performance over a broad range of wind conditions. The Elysian is one of the first residential towers in the United States to incorporate this efficient technology.

Project Description

The Elysian Hotel and Private Residences is 638,000 square feet of mixed-use space with retail, parking, hotel, and condominium functions located on E. Walton Place between Rush Street and State Street, just minutes from Chicago’s iconic Magnificent Mile. The west side of the site consists of a four-story (60 feet tall) steel structure above grade containing retail facilities, office, restraint, fitness, pool and an open motor courtyard. To serve residents and hotel visitors, four levels of below grade parking are provided under the courtyard. A 61-story (685 feet tall) reinforced concrete tower structure marks the east side of the site. A one level basement, 16 feet deep, will be constructed under the tower for mechanical and office usages and the ground level will contain lobby, retail and service facilities. There will be five stories of hotel amenity space such as a spa, restaurant, meeting rooms, and ball room facilities. The next 19 stories will contain hotel room space with balconies. Mechanical floors are provided at Levels 27 and 28 and above the penthouse level. The tower columns are transferred at level 28 with a 6’-0” thick slab to accommodate the building set back at the east and west sides of level 29. Residential condominium space is provided in the upper portion of the tower at level 29 through 59. Column transfers at level 51, 52, 57, and 59 are supported by post-tensioned concrete beams or thickened slabs. A Tuned Liquid Damper (TLD), consisting of four reinforced concrete tanks with water, is located at the top of the building to control building accelerations. The Elysian Hotel and Private Residences is one of the first residential buildings in the United States to integrate a TLD into the structural design. Additionally, the superstructure will be clad with insulated pre-cast concrete panels and metal panels. All designs were completed in accordance to the Chicago Building Code.

Foundations

For this 700 foot tall tower, it was originally predicted that foundations would extend to rock. However, working closely with the geotechnical engineer, an aggressive scheme was devised to bear at Chicago’s ‘hardpan’ clay later – located approximately 85’ below the site’s grade – with an allowable bearing capacity of 32 ksf. Belled drilled piers (or belled ‘caissons’, as they are referred to in Chicago) range in size from 2’-6” to 7’-0” with bell sizes from 4’-0” to 20’-0”, using 8,000 psi concrete. The key was making sure total and differential settlements were acceptable. And based on settlement predictions – this system was determined to be feasible with special construction considerations and settlement monitoring.

The center core shear walls are supported with a 12’-0” thick mat foundation supported by a group of eight caissons with 7’-0” shafts and 20’-0” bells. The caissons resist lateral base shears from the core shear walls. Reinforcing for the mat consists of #11 top and bottom reinforcing bars and #9 shear reinforcing bars. An 8,000 psi mix design was specified for strength. Mega-columns, 6’-0” by 10’-6” located at the north and south perimeter sides, are supported by mega-grade beams 12’-0” wide by 16’-0” deep. 228-#11 reinforcing bars are provided for the bottom and 114-#11 for the top reinforcing. Shear reinforcing consisted of #7 bars with 14 legs. An 8,000 psi concrete mix design was specified. Because of the thickness of the mat and mega-grade beams, concerns for the heat of hydration and maximum differential temperature between the center of the members and their surfaces became apparent. To prevent the potential for unwanted cracking, mass concrete provisions were specified. These included the maximum temperature during the curing process and maximum temperature gradient between the center of the members and the surfaces. A self-consolidating, low heat of hydration mix design was provided by the contractor to satisfy these provisions. Thermal sensors were used to monitor the temperatures within the members for a specified time period.

Four levels of parking are located at the west side of the site. The lowest level is 47 feet below grade. Because of lateral earth pressures at this depth, 30 inch thick reinforced concrete slurry walls were designed with 4,000 psi concrete. The walls provide lateral earth support for the below grade parking while also supporting the gravity loads from the low rise steel building above. Under the footprint of the tower on the east side, a one level basement has 14 inch thick perimeter reinforced concrete walls to resist lateral earth pressures and transfer lateral base shear from the tower superstructure to the soil.

During construction, the temporary earth retention system for the below grade parking structure consisted of four levels of internal steel pipe bracing with wide flange whalers. These members braced the concrete slurry walls until the permanent 10 inch two-way reinforced concrete flat slabs were constructed at each level. The pipe bracing spanned diagonally at the corners and 122 feet in the north-south direction. Steel sheeting was installed at the tower basement perimeter and large diameter pipes were used to brace the sheeting internally. The bracing spanned 96 feet in the north-south direction. All the temporary earth retention systems were designed by the contractor.

Floor Systems

At the four levels of below grade for parking, a 10” two-way reinforced flat slab with drop panels will be utilized, with 6,000 psi concrete strength. Since the slab resists horizontal earth pressures acting simultaneously with vertical gravity loads – a special buckling analysis was done to insure the floors were sufficient for this interaction.

For the typical tower floors - columns are located only at the perimeter, which provides open floors between the perimeter and center core walls. (This also directs the gravity load to the core walls and perimeter columns engaged with outriggers – to help these elements avoid uplift under wind loads.) The typical hotel level floor plate is 97 feet in the north-south and 126 feet in the east-west directions, while the typical condominium floor plate is 86 feet in the first and 112 feet in the latter. An 8 inch two-way, post-tensioned concrete flat plate supports typical residential live loads. The slab spans 34 feet from perimeter to core wall and 50 feet between concrete outrigger walls. The concrete strength is 5,000 psi. Slab deflections were limited to the span divided by 360 for live loads, or the span divided by 240 for total long term sustained loads after partitions are installed. Perimeter slab deflections were limited to ½ inch or less where it supports the precast building façade.

At the transition between hotel and residential at level 28 – a 6’-0” two-way transfer slab at the east and west sides transfers the 29-story residential perimeter columns above to the offset hotel columns below. The slab is reinforced with #11 bars top and bottom along with #9 shear reinforcing at the columns (where required for punching shear resistance). For constructability purposes, the transfer slab was poured in two separate sections. The first was a 2’-0” thick bottom section followed by the remaining 4’-0” thick top section several days later. The first 2’-0” thick bottom portion was designed by the contractor with post-tensioning to assist in supporting the wet weight of the remaining 4’-0” thick pour section. Halvorson and Partners (HP) designed the slab portions to behave compositely by adding vertical shear reinforcing throughout the slab footprint to transfer horizontal shear forces. The top of the slab was intentionally roughened per ACI requirements to provide shear-friction between the two pours and the concrete strength is 6,000 psi for both pours. HP also evaluated the effects of the post-tensioning on the slab behavior for the final condition where the composite slab sections together resist the applied transfer column forces. Close communication and coordination with the contractor was needed for the construction of level 28 to be successful.

Additionally, a full story deep beam is provided below level 7 to transfer two columns, each supporting 50 stories of load from above. The 5’-8” wide by 19’-6” deep transfer spans approximately 56’-0”. High strength concrete of 12,000 psi is provided and reinforcing consists of #11 top and bottom bars with #7 shear reinforcing.

Gravity System

The perimeter reinforced concrete columns and the interior core walls comprise the gravity system by resisting the loads within their tributary areas. All cladding will be supported directly by the perimeter reinforced concrete columns and selected floor slabs.

Lateral System

The tower uses an efficient ‘core and outrigger’ lateral system. However, as the tower grew from its original 40-story design to its final 61-story configuration – there was no opportunity to re-configure the structure or significantly increase structural sizes. In order to make the structure work for the final height – HP aggressively optimized the given structure for strength and introduced a damping system to reduce accelerations due to wind to meet standard criteria. See the next section for more.

The lateral load resisting system consists of an 18 inch thick central reinforced concrete shear wall surrounding the core elements and four reinforced concrete “mega-columns” located on the north and south sides of the tower. The central shear wall has outside to outside dimensions of 21’-6” in the north-south and 56’-6” in the east-west directions. The mega-columns are 6’-0” by 10’-6” at the base and transition in size throughout the height of the building. Outrigger shear walls in the north-south direction connect the core to the mega-columns. These walls start at Level 7 and terminate at Level 40 on the south side and Level 51 on the north side to create open spaces for the top condominium units. The walls beginning at Level 7 allows for flexibility in the space layout for the hotel functions. Reinforced concrete link beams will be provided at the openings in the shear walls. The widths of the link beams typically match the thickness of the shear walls at the central building core. Link beams between the core and the outrigger walls are typically wider than the walls. Lateral forces are resisted by the Outrigger-Braced system in the north-south direction and the central shear walls in the east-west direction. The concrete strength for the mega-columns, core shear walls, outrigger walls and link beams transitions from 12,000 psi at the base to 8,000 psi to 6,000 psi to 5,000 psi at the top of the building. Overall building drift is H/800 at the roof of the building yielding a relatively stiff building.

Tuned Liquid Damper

As described above, when the tower height increased significantly – it became necessary to introduce a damping system to keep accelerations due to wind within standard criteria, in lieu of major structural revisions. Wind tunnel testing was conducted by the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario (BLWT) on this slender tower, with a height to width ratio greater than 7. As anticipated for its extended height, the structure’s stiffness alone was insufficient to bring accelerations within acceptable limits. With the assistance of the BLWT, HP studied options for different combinations of structural stiffness and damping to develop a cost effective structural solution that incorporates a Tuned Liquid Damper (TLD) at the top of the building.

The TLD is the latest approach to controlling motion in tall, slender building offering low cost, low maintenance and high performance over a broad range of wind conditions. The Elysian is one of the first residential towers in the United States to incorporate this technology. Water-filled concrete tanks fitting within the central core wall footprint will be located at the top of the building. The movement of the water will be controlled with internal steel baffles that produce a sloshing effect. The baffles are composed of simple steel tube sections in a regular spacing. The water motion works opposite the building motion to control accelerations produced from wind forces. The Elysian’s TLDs will be tuned in two directions to mitigate motion perception.

Photos courtesy of Halvorson and Partners.

Case Studies:

Cultural Buildings
Educational Institutions
Green Buildings
Healthcare
Hospitality
ICF Buildings
Luxury Residential
Mixed Use
Office Buildings
Religious Structures
Tilt-Up Buildings

Owner/Developer: Elysian Development Group, LLC Project Manager: Golub & Company Architect: Lucien Lagrange Architects General Contractor: McHugh Construction Company Structural Engineer: Halvorson and Partners (HP) Geotechnical Engineer: Ground Engineering Consultants (GEC) Wind Tunnel: Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario (BLWT)

 
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