Citizens Bank

31

Citizens Bank Center Tower
Dallas, Texas

Architects: Omniplan Harrell + Hamilton
General Contractor: Thos. S. Byrne, Inc.

The Dallas Chapter of the A.I.A. awarded the Citizens Bank Center Tower with the coveted 25 Year Award for Lasting Architecture. When accepting the Award, the Architect credited Thomas Taylor and Datum Engineers for their major contribution to the success and architectural expression of this project.

The Citizens Bank Center Tower is one of the architectural-engineering landmarks of Richardson, Texas. Although the original plan was for a steel frame covered with precast concrete veneer panels, the final design turned out to be an all-precast structure. The cost was somewhat lower than for the steel alternative.

It was to be a building with a strong visual identity that expressed the client's commitment to longterm ownership. That's what the architect's program for the 13-story Citizens Bank Center Tower, located in Richardson, Texas, originally called for. And that preliminary architectural concept was to use precast concrete; it met the program requirement and never changed from the original schematic stage to completion of construction. What did change, though was the structural concept-a change which considerably improved the detail appearance of the building.

At first, it was thought that the only economical solution to the framing of the building was to construct a steel structure with columns 35 ft (11 m) on centers on the perimeter of the building-and to cover the structure with precast-concrete veneer panels. At first this approach appeared to be the only practical and/or economical solution, but there were visual drawbacks: the two columns at the third points of each building elevation would have to be set back into the office space to allow the window glass to conceal them in elevation, and the precast veneer panels would have to be fabricated in small sections (which would require numerous vertical joints). The problem was that such joints would detract from the appearance of a solid beam spanning from corner to corner.

The idea of using precast concrete as a structural element was explored by Datum Engineering, both from a technical and practical standpoint. Precast concrete spandrel beams 2-ft (0.6-m) by 5-ft (I.5-m) by 105-ft (32-m) -long were too heavy to handle in the local precast plant. So lightweight Haydite aggregate was selected to reduce the total weight of each beam to 60 tons (54,000 kg). (These beams span between corner columns and support the floor load along with their own weight.) The 60-ton beams also seemed to be the maximum weight that two 150-ton (136,000-kg) crawlers could lift to the roof at the site. As a result, the 60-ton weight limit began to dictate certain decisions. For example, the roof and second-floor spandrel beam were to be 8-ft (2A-m) deep (for architectural expression), but the extra 3-ft (0.9-m) depth of beam was conceived to be attached-in the field-to a standard 5-ft deep, 60-ton beam, thereby creating joints that can be seen in the top of the roof beam.

Concern about camber reflected in final design
Control of beam deflection and camber was a major concern in the building's design. The positive camber in the beams could not be visually objectionable; but it would have to be high enough so that neither initial nor long-term deflection would be objectionable. In order to control beam deflection within acceptable Visual limits two major design decisions were made. First, it was decided to cast sleeves into the columns to allow for their vertical post-tensioning at the site; this would weld the precast columns and beams into a rigid-frame which in turn would prevent beam rotation at the support. It was also decided to prestress and post-tension beams at the plant; prestressing was to be done as soon as the concrete obtained a 4,000 psi (28,000 kN/m2) compressive strength to a precise predetermined amount. The post-tensioning was to be delayed until after 28 days of plant curing-and then be stressed only to the pre-calculated camber. Because the beam deflection and camber were so important, creep and shrinkage characteristics of the proposed lightweight concrete mix were closely analyzed-and were used in the calculations.

Corner columns were to be fairly simple (2-ft or 0.6-m thick, L-shaped precast-concrete columns with conventional reinforcement and sleeves for the post-tensioning tendons).

Based on Datum's preliminary calculations, the precast concrete manufacturer also made preliminary cost studies for both the precast veneer system originally conceived and the alternate structural precast concrete concept. When the costs of steel, fireproofing, and caulking were added to the precast veneer cost, the total estimate was actually higher than the structural precast concrete concept. A then-recent agreement with ironworkers in the Dallas area to erect both steel and precast elements allowed the floor system to remain composite steel beams mated with a composite metal deck and lightweight structural concrete. (Both deck and concrete had the necessary two-hour UL rating; and the beams at 10 ft or 3 m on centers were spray fireproofed.) The most economical floor system, therefore, could be used with either structural system.

New for Dallas
Because the precast structural system was fairly new and innovative for the Dallas area, it was decided to complete plans and specifications for precast concrete fabrication as quickly as possible-and bid the precast before the rest of the building trades were bid. If cost problems developed, it was believed that they could be worked out (or the system could be abandoned without having to rebid all of the other trades). The bids did come in higher than the budget allowed for, but certain changes were made (detail, specification items, concrete mix, and architectural finish) that brought the cost of precast fabrication into budget. (The decision to prebid the precast fabrication not only turned out to be a sound decision in controlling the cost of the project, but also allowed the owner to award a contract on the precast concrete early-a circumstance which give the precast manufacturer some lead time to construct forms and prepare shop drawings.) The erection of the precast was bid later with the balance of the structure.

It was determined that the most economical solution to constructing the walls around the core-the elevators, stairs, mechanical room and toilets-was to utilize the walls as load-bearing elements that would support gravity loads from the floors, and provide the three- and four-hour fire rating required by many of them. By analysis, this concrete core would provide 40% of the lateral stability of the structure in inter-action with the post-tensioned precast exterior frame (which would provide the remaining 60%).

Because 10 to 12 weeks were required by the fabricators before structural steel or precast could be delivered to the site, it was decided to detail the core of the structure to allow for slipform construction techniques. This would permit core construction to be completed before the steel and precast were delivered so that erection could proceed without interruption.

Timing turned out well
That kind of thinking worked out quite well with the actual construction schedule. During the time required by the fabricators for the mill order, shop drawings and fabrication of structural steel and precast concrete elements, the slipform core was slipped to completion. With the core finished and standing free, the steel erector was free to proceed with steel and precast elements without coordinating with the general contractor's efforts to conventionally form and cast the concrete walls.

At the site, the aforementioned crawler cranes then hoisted the main precast girders and other precast and structural-steel elements to their final position. The structure was erected one floor at a time; once four of the L-shaped corner columns were in place, then each girder (along with the steel floor beam) was set. After the grout between the column and girders was set, prestressing rods (extended through sleeves in the girders and columns) were post-tensioned to 6,700 psi (46,000 kN/m2).

In addition to vertical sleeves for post-tensioning, the girders and columns each have dual-function steel embedments at both ends, on the top horizontal surfaces. Each fitting consists of a bearing plate with a short wide-flange stub attached-end up. After jacks tensioned the vertical rods, nuts were turned down against the plate to maintain the tension. The wide-flange stubs project up into a box formed in the bottoms of girders and columns; their purpose is to transfer horizontal shear from girder to column, and vice versa.

Final details
To totally complete the shell, the only remaining items-after the structural portion of the project was completed-were concrete coatings and window-glass installation. Except for tenant improvements, the entire shell was then completed.

As it turned out, actual beam cambers and deflections measured in the field turned out quite close to calculated amounts. Also, there were no real fabrication, shipping, erection or tolerance problems with the system. Cost of the building was held within the budget limitation of $22/ft2.

A prizewinner (one among 19) in the Prestressed Concrete Institute awards program, the bank tower is a prime example of where the architectural and structural systems are attractively and efficiently wed. Result: a high quality building at considerable savings. The PCI jury commented: "…as is so often the case with outstanding buildings of precast concrete, restraint, simplicity and economy of design were watchwords."