Wide-Open Spaces

Brian H. Caudle, P.E

Photo by Mark Green

new age in multipurpose March and need a weather-tight arena- venues began when Reliant like setting for the rodeo and its concert Houston’s Reliant opened in Hous- events. The solution to satisfy these di- ton, TX last August. Reliant verse needs was to build the NFL’s first Stadium features the Stadium is home to two retractable roof stadium. Amajor tenants: the Houston Texans and This modern venue is located less largest retractable roof the Houston Livestock Show and than 200 yards from another milestone in the U.S., made Rodeo. These two tenants have vastly structure, the . The As- possible by innovative different requirements. The Houston trodome opened in 1965 as the first en- Texans are the newest team in the Na- closed football or baseball stadium and engineering and high- tional Football League and prefer to was called the “Eighth Wonder of the play their games on natural grass turf World.” The curved steel lamella dome strength steel. in an open-air setting. However, the supports a 640’ roof span, which more Houston Livestock Show and Rodeo than doubled any previous roof span. fill the seats from late February to mid- After years of hosting thousands of his-

Modern Steel Construction • July 2003 torical sporting and entertainment events the venerable old stadium is ready to share the lime light with an- other trend-setting showplace, Reliant Stadium. Signaling a new standard for stadi- ums throughout the world, Reliant Sta- dium opened to rave reviews from players, fans and the media. Reliant Stadium is the NFL’s largest stadium, covering over 12 acres and comprising 1.9 million sq. ft. With 12,000 tons of cooling capacity, it is the largest indoor air-conditioned space in Texas and, other than multi-terminal airports, Re- liant Stadium is the state’s largest pub- lic assembly space. It is also the first NFL stadium with an operable roof, and at 4.25 acres, it is the largest such roof in the . The translu- cent fabric roof creates an instant archi- tectural landmark and a positive new image for the City of Houston.

RETRACTABLE ROOF— Schematic drawing of Reliant Stadium roof structure.The roof system was extensively tested A STEEL AND FABRIC KITE to withstand wind uplift, unbalanced loading, lightning strikes and corrosion damage. It is the Design of the distinctive retractable first roof of its kind to be constructed in the United States.The tri-chord-truss-framed roof splits and glides toward end zones on supertrusses. roof was the most challenging aspect of the project for Walter P. Moore, the structural engineer for Reliant Sta- dium. Imagine constructing a giant lightweight kite, one football field wide and three football fields long—and then try to control that kite in hurricane force winds! The operable portion of the roof consists of two enormous but elegant fabric-clad panels that ride along two parallel supertrusses. The two bi-parting panels abut over mid- field and dock over each end zone. Each 385’-by-250’ panel is framed with five tri-chord trusses spanning across the width of the playing field. The 24% translucent Teflon-coated fiberglass fabric is tensioned between the tri- chord trusses. Walter P. Moore worked closely with Uni-Systems, the mechanization engineers, to create the computer-con- trolled moving roof. Each tri-chord truss is supported on a carrier beam with two 36”-diameter steel wheels rid- ing on a single steel rail. A computer- ized system of 40 5-horsepower, 460-volt, three-phase motors power the 40 steel wheels that can open or close the roof panels in less than 10 minutes. The 6”-tall, 175-lb. crane rail rests on a concrete slab at the top chord of the su- A 400-ton crane and a 650-ton crane perform a tandem lift of a fabricated side section of the pertrusses. The operation of the roof is east supertruss. The scoreboard box truss (still supported on erection towers) can be seen framing into the left side of the concrete supercolumn.

July 2003 • Modern Steel Construction tive to its supporting rail For further economy, the concrete position. This lateral motion rail slab atop the supertrusses was had to be accommodated in made composite with the truss top a relatively short vertical chord. The composite steel/concrete distance of 16’. At the east supertrusses on Reliant Stadium are end of the retractable roof believed to be the largest ever used in a tri-chord trusses, a four-bar building structure. linkage was integrated into To speed construction, the su- the truss to form its connec- pertrusses were erected in large, pre- tion to the transport carrier fabricated pieces, starting at each beams riding along the rail. column and working toward the cen- This linkage allows free lat- ter. The final pieces were custom-fabri- eral movement of the east cated to fit perfectly. In total, there are Photo by Wes Thompson end of the roof trusses while more than 1.5 linear miles of primary so quiet that fans hardly notice it is keeping the transporter car- long-span trusses in the roof of Reliant moving overhead. rier wheels vertical. This concept al- Stadium. To accommodate the vertical toler- lows only modest lateral thrust loads ances of the 1,000’-long rail, an innova- and reduces potential wear on the HIGH STRENGTH STEEL tive two-wheel transporter system was wheels and rail. This linkage system Throughout the roof, ASTM A913 designed. A “plug ‘n play” electrical provided an economical, easily con- Grade 65 steel was used to reduce ton- system allowed the tri-chord roof structed solution to a very challenging nage, contributing greatly to the econ- trusses to be quickly erected at the design problem. (Read more about omy of the Reliant Stadium structural north end of the stadium and immedi- stress relief in long-span roof systems system. ASTM A913 Grade 450 was se- ately moved south along the rail with a in the article “Stress Relief” in Novem- lectively used in the long-span trusses minimum of field electrical connec- ber 2002 issue of Modern Steel Construc- to reduce steel weight and cost. tions. Erecting the tri-chord trusses in tion at www.modernsteel.com.) As with all compression members, one location then moving them on the truss members in Grade 65 steel rely on rail allowed reuse of the erection tow- THE SUPERTRUSS maintaining short unbraced lengths for ers and saved time by not having to An elaborate, but efficient, steel economy. Several strategies can be em- move the cranes, further speeding con- structural system supports this enor- ployed in large trusses to keep un- struction. mous roof. Two massive supertrusses braced lengths in an economical range Inside of each carrier beam are hy- clear span 620’ between concrete su- for Grade 65 steel usage. The primary draulic scissor clamps that can be tight- percolumns along either field sideline. strategy used on Reliant Stadium was ened around the rail as soon as the roof The bottom chord of each supertruss is the use of built-up laced compression panels stop moving. This first-of-its- gently arched to accommodate for members in the supertrusses. In total, kind computerized clamping system sight lines of the seating bowl, creating roughly 2950 tons of A913 Grade 65 keeps the steel and fabric panels from a truss with minimum depth of 50’ at steel was used in Reliant Stadium, “flying away” during high winds mid-span and a maximum depth of 72’ which resulted in an overall savings of at the supercolumns. In order to find an approximately 740 tons. FOUR BAR LINKAGE economical design for a truss with the The full economic benefit of using One of the design and engineering mid-span depth shallower than the Grade 65 is difficult to quantify, but innovations developed for the Reliant depth at the supports, the structural fabrication, handling, and erection sav- Stadium roof was a unique stress-relief engineer tied the truss rigidly to the su- ings due to reduced weight did mag- four-bar linkage detail. The design of a percolumn, thereby achieving a rigid nify the pure material tonnage savings. long-span roof system to accommodate frame action between the truss and its In addition, the lighter steel weight led potential restraint forces caused by supporting columns. to savings in the supercolumns and temperature, creep, shrinkage, founda- The maximum factored moment at foundations. The light steel weight was tion settlement and construction toler- the top of each supercolumn is 800,000 crucial in fitting the supertruss base ances is always a significant challenge. kip-feet. The transfer of load from the plates in the very limited bearing area This is particularly true for a roof that supertruss to the supercolumn was ac- available on top of the supercolumns. moves on a parallel rail system. complished through six large built-up The lighter steel weight made possible The roof structure rides on rails steel columns supported on base plates by using Grade 65 steel also helped to along the supertruss that can vary sig- large enough to distribute the compres- control differential settlement between nificantly in the distance between sive loads and to accommodate the re- the supercolumns and the bowl con- them, caused by supertruss movement quired number of 2½”-diameter, courses. under lateral load, temperature, and 20’-long, ASTM A354, Grade BD an- construction tolerance. When all ex- chor rods. The lateral shear was trans- EXCEEDING GOALS pected tolerances and deflections were ferred through steel “drag” plates with The owners had multiple goals in added up, it was determined that one 2½”-diameter ASTM A354, Grade BD designing and building Reliant Sta- end of the truss had to be able to move anchor rods. dium, each of which was successfully laterally plus or minus 21” inches rela- achieved:

Modern Steel Construction • July 2003 The erection towers have been removed from beneath the west supertruss. The lacing of the truss chords and vertical web members are visible in this picture. The first fabric panels have been installed on the left end of the main truss span.

I The specific needs of two tenants conditioned environment. The total CONTRACTOR were met. The Livestock Show & stadium structure utilized over 17,000 Manhattan/Beers, a joint venture of Rodeo has a roof to avoid concert tons of structural steel. Its structural Manhattan Construction Co., Houston, rain-outs. The Houston Texans have design incorporated several unique and Beers Skanska, Inc., Atlanta their open-air venue with a portable features to make it functional and af- grass playing field. fordable. The stadium was built within ROOF MECHANIZATION I The roof will support 170,000 lb. of budget and in less than 30 months, CONSULTANT audio equipment, video equipment proving that Houston will have accom- Uni-Systems, Inc., Minneapolis or other rigging loads for the rodeo plished its goal of setting the standard and other major events, comparable in once again. # STEEL FABRICATOR to any modern arena. Hirschfeld Steel Co., San Angelo, TX I Superior amenities are offered to the Brian H. Caudle, P.E., is a principal and (AISC member) spectator—maximized sight lines, managing director of Austin Structural wide concourses, themed entertain- Operations at Walter P. Moore, and served STEEL ERECTOR ment areas, 8,200 club seats and 177 as a project manager for Reliant Stadium. Derr Steel Construction Co., luxury suites. Euless, TX (AISC, SEAA member) I The Houston Texans sold all 60,000 LEAD STRUCTURAL ENGINEER of their season tickets, with a wait- Walter P.Moore and Associates, Inc., STEEL DETAILER ing list established. The club is ex- Houston Dowco Consultants, pected to generate about $200 (NISD member) million in revenue this season. ARCHITECTS I The project was completed within Houston Stadium Consultants (HSC), ENGINEERING SOFTWARE budget and on time—in a short 30 a joint venture of Hermes Architects, SAP 2000, AutoCAD months. Houston Reliant Stadium’s unique re- and Lockwood Andrews Newnam, DETAILING SOFTWARE tractable roof and its incomparable fan Houston Xsteel (initial model imported from amenities promise not only to bring Walter P.Moore SAP model) NFL football back to Houston in great ARCHITECTURAL STADIUM style, but also to create a new home for CONSULTANT BENDING SERVICES the largest rodeo event in the world – Sports Design Architect: HOK Sport+ Bendco, Inc., Pasadena, TX all in an open-air or fully enclosed, air- Venue+Event, Kansas City, MO

July 2003 • Modern Steel Construction