Dennis M. Wittry, P.E., S.E., Dennis R. Tow, P.E., and Lanson Nichols describe the design strategy to contain the enthusiasm and noise generated when Bronco fans get together for a game at Mile High Stadium, Denver, Colorada, USA.

Storm clouds gather at Mile High Stadium (56k).
© 2002 Jackie 
Shumaker

t disrupts the quarterback's signals and unnerves the kicker. It puts doubt in the opponent's psyche and ringing in his ears. In Denver, Colorado (USA), the noise generated at Mile High Stadium - fondly known as "Rocky Mountain Thunder" - has helped Denver's NFL World Champion Broncos to a league-best, home-field record through the years. Their fans even set a mark of their own in October 2000 when they established the world record for crowd noise at 128.7 decibels - a deafening roar roughly equivalent to a jet engine on the runway or a gunshot blast.

Less than three years after a successful November 1998 public funding referendum, the US$404 million, 76,503-seat INVESCO Field at Mile High was delivered early and under budget by the design-build joint venture of HNTB and Turner Construction Company. The architects, led by HNTB Sports Architecture, and prime structural engineer, Walter P. Moore, were charged with not only replacing a beloved landmark but also recreating the legendary Rocky Mountain Thunder of the old stadium.

Besides creating a modern, curving version of the signature horseshoe shape of Mile High Stadium, the designers expanded on a unique feature that helped create the noise - steel treads and risers. The entire east sideline stand and the third deck of the other portions of Mile High Stadium were constructed with steel treads and risers, to minimise the weight in later additions to the old stadium. 

Fans in those portions of the old stadium soon learned to turn support into a full body activity, jumping and stamping on the steel beneath their feet to generate the fearsome low frequency rumble. The new stadium extends this experience to almost the entire audience, as all three bowls of the new stadium are constructed with 3/16"-thick bent steel plate seating risers. Only small portions, at the disabled patron seating areas and the first row of the two upper decks, are concrete.
   

Structural underpinning.
  

INVESCO Field at Mile High contains some 12,000 tons of structural steel, including 550 tons of 'tusk' light standards and lighting trusses, 4,500 tons of subframing in the seating bowls, and 2,000 tons of steel plate treads and risers. It includes approximately 8,700 club level seats, 124 suites, eleven party suites, over 400 points of sale for concessions, thirteen elevators, fifteen escalators, three video boards (including one that measures 96' by 27'), and over 500 televisions. The seats are wider than Mile High Stadium (ranging from 19" to 21") and have more legroom (33" typical row spacing). The total square footage of the new stadium is over 1.7 million square feet, more than twice the size of the old stadium, with much wider concourses. The stadium is ADA-compliant, and has 730 pairs of ADA-accessible spaces and companion seats.

Design Challenges

The twelve acres of exposed steel plate in the new stadium presented significant design challenges, especially to address potential corrosion, vibration control and long-term maintenance. Stresses due to thermal expansion and contraction of the steel plate are reduced through the use of eight expansion joints spaced proportionally around the seating bowls, with six bays of framing between each joint. Only the two centre bays of each section are constrained to framing below, allowing the two outer bays on each end to move independently of the shaded structure beneath, while still preventing sway of the structure due to occupant and earthquake loads.

Corrosion and maintenance concerns were addressed through careful analysis of the existing stadium, considering such factors as metallurgy, climate, and wear during service.

The end result is a design incorporating positive-drainage and minimal penetrations through the continuously welded watertight treads, with a secondary sub-roof below the treads above areas with finished interior space. The plates are 'killed' A36 material, which most closely resembles the metallurgical composition of the old stadium. No special maintenance is planned, as the old stadium has shown that a protective patina will develop on the surface of the steel during its service life by the combination of foot traffic and the mild acids from the inevitable spills of beer and sodas.
  

The riser plates are supported on a subframing system of rolled wide flange structural steel sections. Each tread is supported on stringers spaced at distances of up to 16 feet. The stringers span the slope of the bowl between girders, which in turn span between rakers on the column grid lines. The rakers at the lower bowl are precast concrete, while the middle and upper bowl rakers are steel. Stringers are typically 14" to 30" deep-rolled steel beams, and girder sizes vary from 24" to 36".

Due to the long spans of the steel seating framing and their light weight, in comparison with conventional precast seating units, fan-induced vibration was a significant concern in the design of the steel seating framing. The structural engineer analysed each subframing member throughout the entire stadium for dynamic response to two excitation modes. The first mode simulated fans rhythmically jumping in the stands, and was represented by a 30lbs/sq. foot live load under harmonic excitation with a frequency of 3 Hertz and a dynamic load factor of 25 percent.
  

The second mode simulated fans stamping their feet by increasing the frequency to 5.5 Hertz and decreasing the dynamic load factor to 5 percent.

Dynamic Loading

Rather than setting a minimum fundamental frequency of vibration criteria for all members, the design limited the effective peak acceleration of the structure, which is the most important factor in determining human perceptibility to vibration. The effective peak accelerations in the stringers were limited to 5 percent of the gravitational acceleration (5 percent grams) for both jumping and stamping. The effective peak accelerations in the girders were limited to 7 percent grams for jumping, but were not limited for stamping, due to the large tributary area for the girders.

Published research has shown that large groups of people cannot maintain unison with higher frequency activities such as stamping. That is why the stamping forces on the girders produce a somewhat random 'noise' dynamic loading on the girders, and do not rhythmically shake or sway the structure.
  

Although minimising vibration was essential, it was also important not to eliminate it. To ensure that every Broncos fan sat in an acoustically 'live' section of seating, designers took special care to individually design each subframing member to somewhat 'tune' the structure. These individual analyses required well over 1,000 different designs.

Each element was checked for dynamic response, lateral-torsional buckling resistance, flexural resistance, and deflection. 
  

Fireworks light up the 
night sky at Mile 
High Stadium (70k).

© 2002 Jackie 
Shumaker

To maximise the economy and accuracy of all of these individual design elements, Walter P. Moore created a customised program to execute the analyses, perform the designs, and link the results in a database format.

With one season in the new stadium, the fans have quickly adapted. A new chapter in the history of Broncos football has begun, and although no official measurements have yet been taken, Rocky Mountain Thunder lives on. With such fan-friendly amenities available in this state-of-the-art facility, the Broncos will undoubtedly continue their 31-year-long streak of home sellouts.

And, with the resounding roar of Rocky Mountain Thunder, INVESCO Field at Mile High - the stadium built for sound - will surely help the Broncos maintain their rank as one of the NFL's best home teams. 
  

Who's who at INVESCO Field

Dennis M. Wittry, P.E., S.E., a Senior Associate with Walter P. Moore, served as structural project manager for INVESCO Field at Mile High. Dennis R. Tow, P.E., an Associate with Walter P. Moore, served as structural project engineer for the bowl framing of INVESCO Field at Mile High.

Walter P. Moore provided all structural engineering services for the new stadium, except for the ramp design, which was performed by The Sheflin Group of Denver. Walter P. Moore maintains offices in Houston, Dallas, Tampa, Atlanta, Orlando, Kansas City, and Austin, and provides diverse building-related and sports-related structural, civil, traffic, and parking engineering services throughout the United States and internationally.

Lance Nichols is an Associate Vice President with HNTB Sports Architecture and served as senior project manager for INVESCO Field at Mile High. HNTB Corporation, an international consulting firm with more than 2,500 employees, provides architectural, engineering, planning and design-build services to clients from offices in more than 60 cities. Current sports projects include five minor league ball parks that all opened this spring, a US$150 million stadium modernisation for Ohio State University, design and master plan services for Purdue University and a new arena for the University of Southern California.

Associate architects on INVESCO Field at Mile High were Fentress Bradburn Architects Ltd., and Bertram A. Bruton & Associates.
  

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