metal-panel cladding around the rim of the upper deck. The facade of the Broncos building is wrapped in a sinuous lat- ticework of aluminum, glass and metal panel curtain wall. The new 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 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 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 , 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. Two brace columns occur at the level down to the club level, with a The longer raker beams at the side- center raker frame of each building cantilever extending out over the lines and end zone consist of a bolted section, and extend from near the lower suites. These frames consist of field splice for ease of shipping and upper end of the raker beam to the W33 raker beams with W14 columns erection. outermost pedestal of the adjacent at the fulcrum of the cantilever. A raker frames. These brace columns HSS 24×½ columns that lean heavy W24 shape serves as a strut stabilize the upper bowl for lateral toward and away from the field in from the club level out to the end of and erection loads. Walter P. Moore the plane of the raker supported the the W33. The W24 extends beyond used three-dimensional CAD models upper raker beams. The connections the W33 to support the spandrel to describe and define the complex at the ends of the HSS simplify func- member at the front fascia of the geometry of the raker frames and the tion, erection and appearance. bowl, which contain the only struc- undulating shape of the rear of the Rather than having a complex weld- tural precast members in the middle upper bowl. ment between HSS at odd angles, the and upper bowls. Since the raker designers chose a true-pinned con- Tolerance in erection of the upper frames effectively tie together two nection. Three feet from the end bowl raker frames is provided by the levels of the cast-in-place concourse work point of each column, the HSS rotation of the pins and the oversized frame, they were included in the lat- 24 section terminates into a 1” round holes in the gusset plate connections.

Modern Steel Construction / October 2000 A rigid welded connection detail would have locked in the raker frame geometry, without sufficient erection tolerance. At the upper end of each raker beam is a pair of built-up wideflange “tusks” three feet apart. These tusks, typically 30” deep with ½” thick webs and ¾”×10 ½” flange plates, curve 40’ upward and 18’ inward over the upper bowl. A 5’-4” deep Vierendeel truss connects the tops of the tusks, with HSS 18 bottom chord, View of north (enclosed) endzone upper bowl, taken from south (open) endzone. HSS 12¾ top chord, and HSS 8 5/8 verticals at 9’-2” maximum spacing. increased the effectiveness of the football season, Schuff Steel will This truss supports the distributed design and added value to the pro- have supplied and erected approxi- sound system and banks of field ject. mately 12,000 tons of structural steel, lighting and the catwalk that services including the circulation ramps, ele- Some of the modifications that them. Walter P. Moore also vator core areas, concourse infill were suggested by Schuff and incor- employed three-dimensional CAD framing, scoreboards and curtain porated into the design by Walter P. models in defining the geometry of wall framing. the truss members, since the light Moore include a combined bolted truss varies in elevation and location and sleeved connection between A variety of software was used to similarly to the top of the upper each light truss segment and the design this project. The stringers and bowl. Due to the slope of the tusks, tusks and moving the connection girders of the stadium seating sub- the top chord of the light truss leans between the tusks and the upper framing was designed with an MS in closer to the field than the bottom bowl rakers to the top of the raker Visual Basic/MS Excel program writ- chord, which further complicated the beam. A collaboration of the design- ten specifically for this project to geometry. ers and contractors also resulted in accurately predict the dynamic some minor material cost savings by response of the subframing members. Exposed structural steel exists as a specifying A36 material instead of RISA-3D was used to analyze the common theme at all vomitories grade 50 steel for some of the deeper club level raker frames as well as to leading into the seating bowls. The (W27 and above) and heavier (over performed the preliminary analysis stringer on both sides of each vomi- 100 pound footweight) sections in for the upper bowl rakers. SAP-2000 tory ramp and stair is exposed, and the seating subframing. Since the was used for the final analysis of the the ends of the riser plates are closed design of many of these members upper bowl raker frames, including with a vertical bent plate. The vomi- was controlled by vibration, the the tusks, the lighting trusses and the tories within each bowl make sure change in material strength did not scoreboards; it was also used for that all stringers adjacent to vomito- affect member size. design of the building frames. RAM ries are of a similar size (i.e. S-Beam was used to design the steel- × Three NISD members, BDS W21 6½” in lower and upper bowls, framed infill floors of the concourses. and W18×6” in middle bowl), and Detailers (Brisbane and , the horizontal return on the bent Australia), Coast Detailers (Topeka, The project is financed primarily closure plate matches the flange KS) and Steel Draft (Woodland, CA) by the continuation of the 0.1% sales width of the stringer underneath. supplied the detailing for the project, tax originally used to construct Coors with the Brisbane office of BDS Field for baseball’s . As part of the design/build detailing the bulk of the bowl fram- The Football Club is process, the prime fabricator and ing (more than 3,000 sheets of shop contributing 25% of the project’s erector, AISC-member Schuff Steel drawings). In addition to the 2,000 cost, and any proceeds from the pos- Company of Phoenix was brought tons (over a half million sq. ft.) of sible sale of the naming rights will into the team at an early stage to 3/16” steel plate, Schuff Steel was reduce the public’s debt. In addition assist in the development, detailing, also responsible for the fabrication of to the 76,125 seat capacity, the stadi- and constructibility of the project. more than 4,500 tons of seating sub- um also features approximately 8,500 Because of the design-build process, framing and raker frames, and 550 club level seats adjacent to two Schuff Steel’s early participation, like tons of tusks and light trusses. By the 38,000 sq. ft. clubs, 106 suites, seven numerous other subcontractors, time the stadium opens for the 2001 party suites, over 400 points of sale Pardon me, do you speak stadium-ese? Vomitories also are loosely described as "portals" or "tun- nels". They are the entrances into the seating bowls from the circulation concourses. Rakers are the primary sloped framing members which occur at each gridline, where the bowl geometry changes direc- tion.

View of the tusks and light truss at the northeastern corner of the upper bowl. The pipe Z-frames will support one of the two scoreboards that are hung from the tusks in the northern corners. Also apparent are the geometrical complica- tions due to the "sombrero" shape of the rear wall of the upper bowl - notice the steps in the tread supports along the wall. Owner: City of Denver for concessions, thirteen elevators, worked as the firm’s structural princi- Structural Engineer: Walter P. more than eight escalators, two video pal-in-charge for the project. Moore boards (including one that measures Lanson B. Nichols, an Associate Vice 96’ by 27’), 550 televisions and Steel Detailing: BDS Detailers, President with HNTB, served as cupholders in every seat. The new Coast Detailers and Steel Draft senior project manager for the new stadium also has more appropriately Denver NFL Stadium. Steel Erector and Fabricator: apportioned men’s and women’s Schuff Steel restroom fixtures than Mile High Stadium. The total square footage of Software: MS Visual Basic/MS the new stadium contains over 1.7- Excel, RISA-3D, SAP-2000, million sq. ft. – more than twice the RAM S-Beam size of the existing stadium. The seats Architects: HNTB Sports are also wider (ranging from 19 to Architects in association with 21”) and have more legroom (33” Fentress Bertram Architects typical row spacing), and the main Ltd. and Bertram A. Burton and concourses are much wider (mini- Associates mum width of 45’ at the lower con- course and 30’ at the upper con- Construction: HNTB course, versus eighteen and twelve Design/Build and Turner feet at the existing stadium). The sta- Construction dium, ADA compliant, has 730 pairs of ADA-accessible spaces and com- panion seats, compared to only 26 pairs at Mile High Stadium. With such fan-friendly and family-friendly amenities available in a new state-of- the-art facility, the Broncos will undoubtedly continue their thirty- year-long streak of home sellouts. Dennis R. Tow, P.E., an Associate with Walter P. Moore and Associates, Inc., served as structural project engi- neer for the bowl framing of the new Denver NFL Stadium. Michael S. Fletcher, P.E., S.E., a Vice President with Walter P. Moore,