The Operations/Technology Services Center for the First National Bank of Omaha, Omaha, Nebraska, was added to Omaha's building inventory when it was completed in the summer of 1999. Located on a site bounded by Capitol Avenue, 16th Street, Chicago Street, and 14th street, the three-story, 180,000-square-foot) facility houses electronic and paper operations for the bank, and operates 24 hours per day, 365 days per year. The architects and structural engineers of HDR Architecture, Inc., Omaha, encountered some interesting design criteria in this project, including one that required the structure to withstand tornado-induced loads.
Structural Framing System
To achieve functional efficiency, large floor plates are provided in order to allow entire divisions to be accommodated on one level. The footprint of the building is essentially rectangular, with five bays spaced at 40 feet in one direction and 11 bays at 30 feet in the other direction. The structural framing chosen for this project is conventionally reinforced cast-in-place concrete. Wide-module joists are utilized for the floor and roof framing. The joists, which are 15 inches wide by 24.5 inches deep, are spaced 5 feet 8 inches on center and run parallel to the 40-feet direction. A 4.5-inch thick slab spans the distance between the joists. The girders are typically 29 inches wide by 33 inches deep, and the spandrel beams are offset 2 inches from the exterior columns. Twenty-inch square columns are utilized for the full height of the building.
The structure resists lateral forces as a moment frame through bending of the columns and the beams. The lateral load criteria established for this building are described below.
The structural design of the facility follows the 1994 edition of the Uniform Building Code as amended by the City of Omaha. Typical office space is designed for a live load of 50 psf and an additional dead load of 20 psf to account for moveable partitions. Computer areas and mechanical room floors are designed for a live load of 200 psf. To accommodate the computer flooring, which is approximately 20 psf, the floors are depressed 18 to 24 inches in some areas. In addition to the live and superimposed dead loads, portions of the floor are designed for vaults. The vaults, comprised of 5-inch thick precast concrete walls lined with sheet steel, occupy one full bay. The snow load on the roof is 30 psf plus drifting.
In accordance with the 1994 UBC, the building is designed for earthquake forces corresponding to Seismic Zone 1. Since this is a region of low seismic activity, no special detailing is required for the members of the ordinary moment-resisting frame.
Due to the importance of certain services within this facility, the structure is also designed to resist tornado-induced wind forces. Based on previous experience and research, the main structural frame is designed for a wind velocity of 150 miles per hour (mph), while "hardened" walls protecting essential rooms in the facility are designed for a velocity of 200 mph. Note that the forces generated by the 150 mph wind are actually 2.8 times greater than those generated by the 90 mph wind that is required by the local building code. The exterior of the building is comprised of precast concrete panels clad in stone veneer, and provides a hardened shell for the structure. Openings consist of tempered and laminated glass to prevent penetration of windblown objects.
All concrete for this project is supplied by the Ready Mixed Concrete Company, Omaha. Normal weight concrete with a specified compressive strength of 4,000 psi is utilized for the entire building, except for the interior columns in the first story which are 6,000 psi. Both mixes contain ¾-inch limestone. The 6,000-psi mix also contains a high-range water reducer and Class C fly ash. Note that fly ash is not included in the mix for the floors and roof to reduce the chance of shrinkage and to improve finishability. A 3,000-psi sand and gravel mix with fly ash and a high-range water reducer is used as fill for the piling pipes. Approximately 20,000 cubic yards of concrete is used in this project.
Based on the type of use and the gross square footage, the code requires Type I fire-resistive construction for this facility. These requirements are met with the structural system of cast-in-place concrete columns, beams, and slab. Fire resistance ratings of the columns are three hours, while those for the floors and roof are two hours. The exterior walls are designed to resist fire for a period of four hours. Separated from the primary building by the exterior walls are the stair shafts and tunnel connection to the parking structure which are Type II non-rated. The entire facility is protected with an automatic fire sprinkler system and fire alarm.
Accessibility and Security
The entire facility is designed to be accessible to individuals with disabilities. The ground floor and first level are accessible from grade, and four passenger elevators connect all three levels. A ramp is also provided at the southeast entry to the first level. Overall security of the facility was an important design criterion that was considered in the preliminary stages of the project. To provide a secure separation from street activity, the building is setback from the street frontage a distance of 60 feet. Also, two levels of access to the building and mail deliveries are separated from the armored car deliveries. In addition to limiting the number of employee entrances along an entrance corridor from the surface parking lot, access to the building is limited to a tunnel connection from the parking structure. A screen wall secures the entire perimeter of the site.
Concrete Versus Steel Framing
In the preliminary design stages of this project, both concrete and structural steel framing schemes were considered. The general contractor, Hawkins Construction, Omaha, determined that the cost of the concrete frame was virtually the same as the cost of the steel frame. The main reasons why the concrete system was chosen are as follows:
The inherent continuity of the concrete frame made it easier to transmit the lateral loads throughout the structure. The steel frame would require full penetration welds at the beam-column joints to achieve the required level of lateral resistance. In lieu of moment connections, X-bracing could be used, but this would have obstructed usable space.
The lead time that was expected for delivery of the structural steel was long. Construction start-up time would be nearly eliminated by using concrete. Also, with the concrete system, there would be no need to wait until the structural frame was complete in order to install electrical, mechanical, plumbing, heating, ventilation, air-conditioning (HVAC), interior partitions, etc. The aforementioned work could be started while the concrete frame is still progressing upwards, unlike a steel system.
Lastly, one of the design criteria established by the owner was that a significant amount of remodeling and changes in layout would likely occur over the life of the structure. By using concrete framing, changes could be easily accommodated without any concern about losing and subsequently reapplying fireproofing that would be required for the structural steel frame.
First National Bank of Omaha, Omaha, Nebraska
Architect and Structural Engineer:
HDR Architecture, Inc., Omaha, Nebraska
CECO Concrete Construction Corp., Merriam, Kansas
Ready Mixed Concrete Co., Omaha, Nebraska