Above (63k) and below (49k): 
Interior views of the Gantcher Family Sports & Convocation Center at Tufts University in Medford, MA.

The building was brought into service within an 11-month schedule at a firm price and desired quality level using a rigid frame structural solution.

Butler Heavy Structures demonstrated its specialisation in single-source engineering and fabrication expertise for the 10,000-seat Mitchell Center at the University of South Alabama, Mobile, AL. 

structural engineer considers a number of factors before recommending a particular type of structural framing needed to achieve the desired clear span within a proposed indoor sports or entertainment facility. Ruling out exotically framed mega domes or fabric-roofed structures that present their own unique engineering considerations, structural steel trusses or welded three-plate girders normally offer efficient alternatives to achieving a desired clear span within typically smaller stadia.

Appropriate Applications

The adoption of a truss, versus welded three-plate girders, as the primary framing members is normally dictated by the width of the clear span, the building height, local wind and snow load conditions, foundation costs and configuration of spectator seating. As a general rule, welded three-plate members (referred to as 'rigid frames') offer more economy than trusses but their application is limited to regions with lighter snow loads or when the clear span runs less than 200 feet.

Otherwise, the depth of the beams can become aesthetically objectionable and intrude into the sightlines of spectator seating at higher levels around the perimeter. Rigid frames required to achieve exceptionally long clear spans also impose generally higher foundation and erection costs.

The Gantcher Family Sports & Convocation Center at Tufts University presents an excellent example where a rigid frame structural solution was acceptable for the 70,000sq.ft. building, which has a 200ft. clear span. The facility primarily supports indoor track and field competitions, practices during inclement weather for outdoor sports, and concerts and convocations with seating for up to 6,000.

Both of the facilities addressed in this article were engineered and supplied by Butler Heavy Structures, a business unit within Butler Manufacturing Company. Butler Heavy Structures explored several structural options for the 200' x 306' building that was designed to Factory Mutual Class I-60 design criteria. A structural truss and heavier girder system were both examined before deciding on the adopted concept.

In this instance, the framing consists of modified rigid frames with primary spans on 24', -4" centres. The flat end wall condition was required for the building's hip roof. Because of the long clear span achieved with rigid frames, the column bases had to extend several feet below the level of the finished floor and are carried on W-12 shapes used as tie members to resist horizontal thrust forces. With this structural solution, the facility achieved the aesthetic and functional goals set down by Sasaki Associates, the project's architects.

In Mobile, Alabama, a framing concept utilising structural trusses proved more efficient than the rigid frame concept originally proposed for the 10,000-seat Mitchell Center at the University of South Alabama, Mobile, AL. The multipurpose facility replaced a 3,500-seat gymnasium that had served as the university's basketball venue for 25 years.

The construction project followed a preliminary study that concluded the school's venerable gymnasium would not accept enough expansion to accommodate the increased seating, leased suites and range of support amenities sought in a larger facility. A number of municipal and university arenas were visited to identify features incorporated into a design that was eventually submitted to the bid process. Cost, constructability, and spectator viewing issues soon radically altered the design concept for the structural framing.

Interior of the Mitchell Center (55k) during a recent event.

The University's Construction Department served as in-house general contractor for the project. It was the largest ever undertaken on the 1,200-acre campus. Harvey Gandler Associates, a Mobile architectural firm, was engaged to design the facility after performing the expansion study on the gymnasium. The project was initially bid as a dome structure with alternate quotes accepted for rigid frame proposals that retained the brick veneer and stucco. When a bid submitted for a rigid frame alternative ran nearly $l million less, the focus immediately shifted away from a dome.

Single Source Engineering

Butler Heavy Structures again demonstrated its specialisation in single-source engineering and fabrication expertise for large, complex structures. A series of meetings ensued, with members of the project team, revealing potential concerns and the rigid frame design was discussed more at length. Butler offered to develop an even more improved design that could be executed within the Alabama statutory cost limit for a construction change order.

The original alternative proposed a rigid frame design that would use welded three-plate columns to support a 10ft. deep roof truss to achieve the desired 266' clear span. Although functional, the tapered columns were a concern because they raised sightline issues affecting nearly 100 seats. Butler Heavy Structures therefore recommended replacing the rigid frame concept with a braced truss rafter system using single-shaft W30 and W36 mill sections for columns.

Substantial Savings

This adopted alternative reduced shear reactions at the base of the columns by 50%, which yielded substantial savings in foundation costs. Structural trusses also simplified the concourse framing and facilitated the project's multi-phased erection sequence.

The structural truss system applied to the stadium shares much in common with the large hangars and high-walled structures that Butler Heavy Structures routinely designs for other projects located in areas with unusual wind or dead load conditions. Since the university campus would be potentially exposed to hurricanes, the structural system was an efficient design solution for withstanding a 100mph wind speed. Additional load considerations were presented by the roof structure, which features an integrated; show lighting and rigging grid capable of supporting 100,000 lbs. of rigging loads.

Aerial view (76k) of The Mitchell Center while under erection shows the intricate structural truss system used as the framing method.

By integrating this element directly into the building structure, it was more cost effective to fabricate and erect. The roof trusses were constructed with a flat bottom chord that easily accepted the integration of the rigging grid, the catwalks, lighting, scoreboard and speaker systems.

The design change to a truss also resulted in an 8ft. reduction in the height of the roof structure compared to the rigid frame concept. This allowed for the economic integration of the parapets at the eaves.

Resolving Design Issues

Changing the design to a braced frame also allowed the erector more latitude during the construction sequence. The adopted scheme permitted the erection of columns, seat framing, and the pre-cast concrete seat decks prior to setting the roof trusses that span the structure. The original rigid frame concept would have made this prohibitive. The net effects for the truss system adopted to frame the building were the resolution of all design issues while simultaneously gaining cost and schedule savings, improved jobsite safety and better coordination with other trades.

The resulting effort produced a truly admirable - and more functional - facility. Mitchell Center seats 10,200 for basketball and will accept 1,400 additional floor seats for concerts, commencements and other events. The building has a total of 210,000 sq. ft. on three levels. Offices, three classrooms, eighteen ticket windows, a 200-seat meeting/banquet room, locker and training rooms are on the First Level, with labs and offices at grade. The suites and spectator seating are primarily accessed on the Second Level Concourse. The concourse has six permanent, and twelve portable concession stands, and men's/women's lavatories (150 stalls) at each corner, supplemented by three others on grade level. The Third Level is used for storage, with some surplus space there envisioned as a sports bar.

The facility has sixteen hospitality suites with eleven season seats for basketball. The suites can be secured to independently market their seating for concerts and other special events. The facility is equipped with a Daktronic octagonal video scoreboard and marquee system and a 14,000 watt Bose sound system. One thing is certainly evident in this admirable facility - arenas on college campuses are incorporating an ever-increasing level of sophistication.

Both of these building programs underscore the importance of working closely with a design/fabricator in the early planning of a facility. The approach permits thorough cost/benefit analysis of framing alternatives and improved cost projections.