metal-panel cladding around the rim of the upper deck. The facade of the Denver Broncos building is wrapped in a sinuous lat- ticework of aluminum, glass and metal panel curtain wall. The new stadium is also unique in Pin Their that the seating treads and risers con- sist of 3/16” thick bent steel plate. Two factors led the designers to choose bent steel plates in lieu of the Future on Steel usual L-shaped precast concrete sec- tions. First, the entire east sideline of the existing Mile High Stadium uses By Dennis R. Tow, P.E., steel treads and risers in order to Michael S. Fletcher, P.E., S.E., save weight in the world’s largest and Lanson B. Nichols movable-seating structure. The east- ern section (all three tiers of seating) can retract 145’ to make room for a left field during baseball season. Secondly, in the geometrical layout of the new stadium each column grid line skews relative to the adjacent column grid lines. In most modern stadiums, the sideline and end zone seating sections are linear, with either a 90-degree segmented curve through the corner, or a linear cor- ner turned on a 45-degree chamfer. HNTB and Walter P. Moore defined the grid system of the new stadium as a doubly symmetric 48-sided polygon based on broad-radius arcs at the sidelines and end zones with tighter arcs through the corners. The con- tinually curving seating rows created by this grid system provide better sightlines and field proximity for sta- dium patrons and generate the appearance of a smoothly curved seating bowl. As a result of this design feature, each successive row of seats is slightly longer than the previous row throughout the stadi- n November 1998, in the afterglow HNTB Design/Build Inc. and Turner um. This would have substantially of back-to-back NFL World Construction Company. increased the number of different IChampionships won by their precast riser members required and Designed by HNTB Sports beloved Broncos, taxpayers in decreased the economic viability of Architects, in association with Denver and six surrounding counties precast. Therefore, steel risers Fentress Bradburn Architects Ltd. voted to replace the famous Mile became an excellent and affordable and Bertram A. Bruton and High Stadium. Slated to open in time alternative. Associates, and with structural engi- for the 2001 NFL season, the new neering by Walter P. Moore, the new With over 12 acres of exposed $364 million, 76,125 seat stadium is stadium exhibits a sleek modern steel plate in three seating bowls, a design/build project headed by facility, full of weeping curves, maintenance of the structure was a exposed HSS structural steel and major factor in design. The solution View of typical pinned connection for HSS columns at upper View of same typical pinned connection, as built (existing bowl (rendering generated from 3D CAD model). Mile High Stadium is in the background). leaves the top surface of the plates riser section with a second bend cre- simulated fans rhythmically jumping unpainted. Extensive testing and ating the heel of the tread. The front in the stands and was represented by metallurgical analysis of the existing of each tread bears directly on the 2” a 30-psf live load under harmonic stadium indicated that the steel return of the section below, and a excitation with a frequency of 3 Hz treads were killed A36 material. Also, 3/16” continuous fillet weld seals the and a dynamic load factor of 25%. due to Denver’s naturally dry cli- sections together. An automated The second mode simulated fans mate, the only corrosion problems in welding machine that runs around stamping their feet by increasing the the treads at the old stadium exist at the nosing of each tread makes fillet frequency to 5½ Hz and decreasing locations where water became welds, allowing for a total weld the dynamic load factor to 5%. trapped on top of the plate by poor length of over 17 miles. Rather than setting a minimum drainage or badly adhered traffic A subframing system of rolled fundamental frequency of vibration coating. wideflange sections supports the riser for all members to satisfy, each mem- Based on this information, the plates. Vertical L3×3 stubs on ber, analyzed individually, allowed riser plates of the new stadium con- stringers spaced at distances of up to consideration of the individual loads sist of killed A36 steel, and each 16’ support each tread. The stringers and stiffness. Limitation of the effec- tread was detailed with a 1/2” per span along the slope of the bowl tive peak acceleration was chosen as tread drainage slope. Defining the between girders, which in turn span the design criteria. The effective slope per tread instead of per foot between rakers on the column grid peak accelerations in the stringers helped in keeping these consistent lines. Precast concrete makes up the were limited to 5% of the gravita- and made the detailer’s job a little rakers at the lower bowl, while the tional acceleration (5% g) for both easier. Furthermore, all proposed middle and upper bowl rakers are jumping and stamping. The effective traffic coatings are being tested in the steel. Stringer sizes vary from peak accelerations in the girders field. To provide a watertight system, W14×22 to W30×124, and girder were limited to 7% g for jumping. penetrations through the riser plates sizes vary from W24×55 to The effective peak acceleration in were minimized, all welds are contin- W36×280. the girders due to stamping was not uous and a secondary subroof is pro- limited due to the large tributary Due to the combination of long vided between the riser plates and area for the girders. Research has spans and high live load to dead load any finished interior spaces below. shown that large groups of people ratio, vibration was a significant con- cannot maintain unison with higher Butt-welds join the treads in sin- cern in the design of the seating frequency activities such as stamping. gle row sections at the change in framing. Each subframing member alignment at each grid line. During throughout the entire stadium was The post-tensioned cast-in-place fabrication, each section is bent to analyzed for dynamic response to two concourse framing is separated into form a 2” return at the top of the excitation modes. The first mode eight midrise buildings to relieve cap plate. Two 1” clevis plates spaced 2” apart are welded to the cap plate, with a 4” diameter hole in each cle- vis plate at 18” from the end work point of the column. The clevis plates fit on each side of a 1½” thick half-round gusset plate. The gusset plates, 24” in radius, center on the end work points of the columns with a matching 4” diameter hole on a radius of 18”. A 4” diameter round stock pin fits through the holes in View of club (middle) and lower bowl framing and both levels of suites at north the gusset and clevis plates, with ¼” endzone. thick plate washers serving as spacers between the plies and six inch diam- thermal, creep, and shrinkage stress- eral analysis of the concourse fram- eter, 1¼” thick cap plates on each es, and to isolate lateral wind and ing. Slip-critical bolts field bolt all end of the pin. The column load is earthquake loads. To accommodate connections. transferred through bearing on the differential thermal movements The raker frames in the upper plates and shear in the pin. Six-inch between the riser plates and the sub- bowl are one of the signature items diameter pins were used in the con- framing below, the connections of the new stadium. The raker beam nections of some of the more heavily between the treads and the stringers consists of a tapered wide flange sec- loaded columns. consisted of either fixed or slip-capa- tion built-up from 1”×26” A36 flange ble details. The slip connections The gusset plates at the upper end plates and ½” A36 web plates vary- allowed differential movements of the columns are field bolted to the ing in height from 25” to 66”. The around the bowl between the treads underside of the built-up raker beam raker beam has a straight taper along and the stringers underneath. Rigid with slip critical bolts in oversized its lower length, with a curved taper fixed connections were used in the holes. The lower gusset plates bear at its upper end. Since the height of two center bays of each of the build- on cast-in-place pedestals, which are the upper bowl varies around the sta- ing sections, with the slip connec- 4’ tall extensions of the cast-in-place dium, the radius of the curve at each tions in the remaining bays (those concourse frame. The pedestals raker varies in order to maintain a bays within two grid lines of a build- increase concourse circulation space constant work point at the lower end ing expansion joint). Longitudinal by raising the lower end workpoint and a vertical depth of 72” at the bracing between the bowl framing of the columns above the headroom upper end. At the four raker frames and the concourse frame was also required for the patrons, and they supporting the scoreboards and video located in the center bays of each also elevate the pin connections to boards in the northern corners of the building section. eye level of the patrons on the upper stadium, the flanges become 2” thick concourse. The raker frames at the middle in the upper portion of the raker bowl extend from the upper suite beam to support the extra weight.