Tackling a Stadium Project
On Jan. 9, 2000, the Seattle Seahawks football team closed its season in the Kingdome, an inverted concrete hemisphere they had for years shared with the Seattle Mariners baseball team, home and garden shows, boat shows, monster truck events and the occasional rock concert. Known for its potato cellar-like acoustics and leaky roof, it was imploded in a 2.3 Richter-scale event on March 26 that same year, making way for construction of the Washington State Football and Soccer Stadium (WSFSS), the new home of the Seattle Seahawks.
From start to finish, surveying figures prominently in the construction of major sports stadiums. A different sort of layout than what most in the profession are accustomed to, large stadium complexes are for the most part devoid of right angles. The bowl and roof portions of the WSFSS are comprised of a total of 14 radius points, 112 radial grid lines, 50 lateral grid lines and 10 miscellaneous grid lines.
The Team’s EquipmentBaldwin & Associates Inc. (BAI), South Bend, Wash., uses Leica (Leica Geosystems, Norcross, Ga.) equipment exclusively. Total stations employed during construction of the Seahawks Stadium included the TC1100 and the reflectorless TCR1103. A T16 theodolite was used for tasks that involved vertically “plumbing” or aligning steel. An NA2 level was supplemented with an NA720 for those setups where vibrations from wind, train traffic, other workers or ground-level compactors upset the compensators in the more sophisticated NA2.
Data collection was by pencil and book. A collection of 30 hardbound field books were used on the Seahawks project. Occasionally an HP 48 running TDS software was used for layout, but this was rare. R.W. “Buzz” Baldwin, PLS, president and owner of BAI, prefers the use of computer printouts or “data sheets,” which are kept organized in ring binders by project task, sector and level. It is Buzz’ opinion that such tools as data collection hardware and related software are fine when used properly, however, on construction sites they introduce too many chances for error. While helping to make data collection more effortless, they also make it more “awareless.” Instrument operators have a tendency to become overconfident and complacent with the use of automated data collection according to Buzz, which is the last thing needed on a construction project of this nature. Awareness and repetition of procedure is of utmost importance in catching errors before they occur. According to Buzz, “One must learn the trade before attempting to learn the “so-called” tricks of the trade.”
The completion of this stadium demonstrates that a project of this magnitude does not require the very latest versions of hardware or software. A Hewlett-Packard Vectra series 3 (5M/RAM/100M HD) running AutoCAD Release 12 (DOS) with Survey/Civil Solutions for AutoCAD Version 5.0 (Boston Harbor Software Inc., Olympia, Wash.) made up BIA's computing center. For primary control nets and major control checks, BAI used StarNet software (Starplus Software, Oakland, Calif.) for adjustment. On secondary control and day-to-day checks, Buzz preferred to balance angles and run Compass Rule adjustments over traverses. Raw closures with error greater than 1:100,000 were rejected and re-traversed. Buzz maintains that the equipment is capable of closures higher than that (he regularly sees 1:125,000 and above) and anything less than 1:100,000 indicates a problem.
Custom equipment manufactured specifically for the stadium project included an aluminum bracket designed to support a survey instrument while C-clamped to a vertical steel column, fabricated by The Erection Company (TEC), the Seattle steel erection firm contracted to construct the structural steel portion of the project. Other custom equipment included a 50-foot “story pole” made by gluing a fiberglass tape into position (calibrated with a steel tape) along the face of fiberglass range pole sections, used in determining elevation of overhead beams and seating stools. Plastic targets affixed to an adjustable flange on 24" steel rulers attached to 2" x 2-1⁄4" x 1/2" magnets were also made by TEC for ironworkers to use. The magnet end was affixed to the face of steel members being “flown” (by crane) into position as BAI sighted the target end as a guide for correct placement.
Safety equipment consisted of the usual hardhat, boots and safety glasses, as well as special vests integrated with fall-protection harnessing. Though TEC’s ironworkers routinely handled the “hanging out in thin air” chores, BAI personnel frequently found themselves on the open edge of decking and other areas requiring fall protection gear.
Control IssuesEstablishing and maintaining control can be a daunting task. It often seems as though all the forces of the universe are directed at foiling the surveyor’s efforts in setting up and keeping a convenient and precise control network. From excavation to de-watering to pile-driving to footing pours and so on, onsite control is constantly in a state of flux regarding its position and usefulness. The emphasis therefore must be on stable and secure offsite control, a network of monumentation, which serves as a baseline from which onsite control can be established, re-established and monitored throughout the construction process. Like a spider fabricating its web, primary segments are established from which numerous auxiliary webs are formed. As with spiderwebs, disruption and destruction occur, requiring the need for reconstruction or outright abandonment of certain segments.
Initial control for the Seahawks stadium had been somewhat problematic. Coordinates were published for the southernmost property corners, a series of nails along the west side of the project in Occidental Way, and a brass cap set in a huge slab of concrete representing the north/south center of the field at the 50 yardline, center/center as it were. During recovery and analysis of the control, anomalies were discovered. The Prime Management Contractor (PMC) insisted that the south property corners and their relationship to the center/center mon should hold over the positions of the PK nails, set at construction grid intersections in Occidental Way and adjusted after pile-driving was completed. What had not been considered was the fact that the south property corners were lost and then reset during construction of the Stadium Exhibition Center (the building separating the new Safeco Field and the WSFSS) and no longer had the same relationship with the other control, which had remained safeguarded. Buzz decided to hold the center/center mon’s x,y,z values while also maintaining the due north azimuth of the Occidental nails. New values were established for the south property corners. This done, onsite control could begin.
The establishment of onsite control was planned out much like a series of chess moves. Construction sequencing was considered, as well as potential loss due to material stockpiling and the eventual erection of material. An almost redundant array of control points were set and traversed through so that during monitoring or critical positioning of steel components, there was always a second (or third or fourth) point to occupy if something was in the way.
Once above ground, concrete became the base for control. Steel by itself is subject to vibration as well as expansion/contraction caused by temperature fluctuations. Especially in late spring and early fall, the Puget Sound region can experience morning low temperatures in the mid-30 degree range, rising in the afternoon to the mid-70 degree range. Likewise, the effect of radiant heat upon the easterly face of steel members as the morning sun rises, occuring while westerly faces of the very same members are subject to ambient temperature changes only, underscores the futility of establishing control on steel. Though concrete has roughly the same coefficient of expansion/contraction, it’s larger mass and weight make it much more desirable for control placement.
Convenience was also considered while setting control. Points had to be easy to access, yet out of well-traveled areas. Where possible, reflective sticky-back targets were affixed to concrete walls for backsighting, allowing the head chainman to immediately move into the layout area and affording the instrument operator the ability to instantly monitor the stability of his instrument and its precise location. By routinely checking extraneous points during initialization of the instrument, errors were avoided before they occurred.
Maintaining the integrity of control is a matter that can’t be ignored on any construction site. Excavation, compaction, heavy equipment and weather all play roles in “adjusting” a control net. Throughout the stadium project, survey crews tied each other’s control as they performed their individual tasks, comparing values with each other as time permitted.
The Game PlanThroughout the control, layout and monitoring phases, procedures were established to guarantee a minimum of error while maximizing efficiency. Efficiency necessarily took a back seat to accuracy when the two conflicted. Redundancy became an ally rather than an inconvenience. Tri-brachs and tripods were checked regularly for tightness. Optical plummets were adjusted when necessary. Throughout the workday instruments were calibrated. Vertical circle calibration took place constantly. Much stadium work is accomplished while sighting with considerable deviation from the horizontal plane. The z-value is just as important as the x and y on such a project. Although vertical control is often set and always checked by chaining along columns in the early morning hours, layout is almost exclusively performed by radial stakeout. If the vertical circle is off, the elevation of the point being set will be off. Fortunately, Leica, as well as most modern equipment manufacturers, make calibration a relatively painless task.
Temperature, humidity and barometric changes were monitored and the instrument re-initialized for such changes. Despite the fluctuations in expansion and contraction of steel once it’s set, layout had to be be exact. Ironworkers routinely work to tolerances of 1/16" (0.005').
The design of a stadium can be compared to that of a tree. The WSFSS is supported by 1,737 individual, 45'-85' long, 18" - 24" diameter, concrete filled, closed-end piles, comprising the root system. Each pile is designed to support 200,000 to 500,000 lbs of concrete, steel, sheetrock, glass, brass and “liveload” (spectators). Pile caps, piers and grade beams make up the base of the tree, structural steel the trunk and branches. Upon the branches, leaves of concrete seating plank, stairways and canopy reside.
Whereas Mother Nature has a tendency to stick with the same design during her construction efforts, man (or at least architects) are not always so consistent. The WSFSS has a variety of stair, bent, wall, seating and grade-beam components. Some were cast in place, some were pre-cast and some were a combination of both. Each required a different type of layout; some simple, some more complex.
Camber values also had to be considered throughout the construction process. In some cases, such as the cantilevers for the upper deck seating, camber values were “straight-line,” with a value of 0" at one supported end, 2" at the far support and a straight proration from there out to the terminus. The beams interconnecting these same cantilevers, however, had “rolled” camber values, with 0" at each end and an elevated value at center. In addition, as the raker beams and cantilevers rise to the higher levels of seating, the incline increased, producing a sort of vertical arc (by chord) in the bowl. As the incline increased, camber values changed to accommodate the shift in load centering.
The most repetitive and therefore rote function performed on the new Seahawks Stadium was the shimming of the z-plank and the calculations made to determine their (the shims) values. Used in the vertical adjustment of precast raker beams, vomitory walls, columns, cantilevers, bents, raker beams and z-plank, shims were everywhere. Tens of thousands of them. BAI had taken the time to set vertical control throughout the project to aid in shim layout. At any time BAI personnel were able to find a red and white sticky-back target set at an even foot of elevation, “buck-in” on it and commence to setting shims.
Layout for the WSFSS began at an expansion joint at the southwest corner of the bowl and moved easterly and northerly. Once steel began to rise, q-deck was set into position, with bent plate and edge form installed immediately thereafter. As concrete flooring was poured and cured, new control was laid out from which to set vomitory walls and the support columns for said walls. As the steel climbed ever higher, raker beams, cantilevers, and the z-plank supported by them were laid out, bolted up and locked down. Construction progressed northerly along the east side of the stadium and eventually led into erection and placement of the east roof structure.
A 720-foot-long tri-cord truss was flown up in sections, supported temporarily by a line of three 200-foot-tall false towers. The tri-cord supports a series of A-frames, which in turn support a box arch made up of individual sections or “boxes” of 2"-thick steel plate weighing from 30 to 80 tons each. Interlaced with five tendons, themselves threaded with 26 post-tensioning cables rated at almost a million pounds each, the entire roof assembly was eventually brought into free suspension, each end resting upon a large, arc-shaped Teflon bearing pad allowing it (the assembly) to slide in a north/south direction in the event of an earthquake, or the more frequent (and less dramatic) movements occurring from wind, rain, snow and temperature fluctuations. Known as seismic isolators, these contact points allow the “play” necessary to keep both roof and bowl independent of each other during nature’s outbursts.
Layout for the roof was conducted on the ground where various steel components were assembled. While earthbound, targets were affixed to the assemblies for use in monitoring their positions during and after installation by crane. Once fully interconnected, the tri-cord was continually monitored as A-frames and box arches were added to assess the effects of increasing loads. The keystone was finally set, locked down and fully welded, in preparation of the post-tensioning process.
As the roof assembly for the east side was being constructed, the west side steel, raker beam and cantilever construction demanded BAI’s attention, also. Beginning once again at the expansion joint in the southwest corner, work progressed northbound toward South King Street. Meanwhile the tube steel for the eastside concession structures required layout as well. BAI personnel worked 60 to 70 hour workweeks during this period. Coffee, aspirin and antacids became primary dietary supplements. Personal lives were moved to the far rear burners and “marriage” to the “iron maiden” began taking its toll. Like baseball players pounding through the long months of summer, surveyors began to take on a grizzled and sleep-deprived appearance.
Point MonitoringFrom the placement of the first tri-cord sections upon the false towers, to the post-tensioning process as the roof settled into free suspension, BAI monitored each stage of roof construction. As each assembly was completed, a full monitor was conducted and the data supplied to structural engineers.
The post-tensioning process began with the cables at each end being stretched to 20 percent of design tension. At that point, jacks atop the false towers were carefully lowered. Monitoring data helped engineers determine which jack to lower and by how much. As data was made available, engineers analyzed the monitoring data from each phase of post-tensioning and made adjustments accordingly. This process continued until the full load of the structure was entirely transferred from the false tower jacks to the post-tensioning cables.
Monitoring was conducted much like layout. Control was set, backsight targets placed, as well as targets for the object being monitored. In the case of roof structures and other high steel, targeting was done on the ground where possible, eliminating the need for personnel to scramble about while tied off 260' in the air. Thanks to detailed construction plans, any point along a structure could be assigned an x,y,z value and monitored for location throughout the erection process, as well as during the monitoring phases.
Careful consideration had to be exercised prior to targeting to avoid areas that might either become obscured from view or damaged in the course of construction. Reflective targets attached with adhesive anywhere near a welding joint would become melted vinyl goo once a torch was applied. Where adhesive targets couldn’t be applied directly to a specific surface, targets were affixed to a 2" x 2", 1/2"-thick magnet, which was placed upon the structure being monitored.
Goal! A Stadium to BeholdDespite a brief interuption in construction caused by the Seattle area’s 6.8 Richter-scale earthquake on Feb. 28, 2001, the WSFSS project is on schedule for completion by August 2002, when the Seattle Seahawks will take the field for the stadium’s inaugural game. The structure itself completed, all that’s left is the installation of the “brass and glass.” FYI: The easterly stadium roof assembly and the bowl beneath it performed exactly as designed during the quake, a testament to all involved in its design and construction.
Definitionsbent a framework positioned transverse to the length of a structure, designed to support lateral and vertical loads.
bent plate relatively heavy (1/4" or so) gauge steel plate which attaches to structural members, often extending outward unsupported. Bent plate becomes the backstop for poured concrete flooring.
cantilever a structural member which extends out (unsupported) from its attachment points. An example would be the extension of steel raker beams that support seating as they hang out into the air toward the field.
clip a large chunk of angle iron set into concrete walls or on steel columns to support horizontal steel members.
corbel the lip which protrudes out from a structural member (usually a column) upon which the end of an interconnecting member is set.
edge form a lighter (than bent plate) gauge steel backstop for concrete floor pours. Same principle as bent plate but not in itself designed to be load bearing while unsupported.
keystone the last piece of structural steel to be set in place (sometimes referred to as a "capstone") along a section of steel.
lockdown the process of tightening bolts down hard and welding steel into final position.
q-deck the corrugated, relatively light-gauge steel upon which concrete flooring is poured.
rack & roll the term used for the twisting, tweaking and adjusting of steel members as they are bolted into position.
rakers in stadium construction, the lateral beams which are set in an inclined (or "raked") position, generally used to support seating.
reveal the prescribed gap between two pieces of material, for instance the calculated space necessary between two pieces of structural steel about to be connected by deep-penetration welds.
shim pack a stack of shims used to bring the base of a column into its correct vertical position prior to lockdown.
vomitory a term used since the time of Roman amphitheater construction to describe the openings through which spectators enter or exit seating areas. Often referred to as "voms."
z-plank pre-cast seating treads that are shimmed into vertical position and upon which the actual seats will be installed.
The author would like to thank Martin Page, PE, of Shannon & Wilson Inc., Seattle, Wash.; Margaret Stout of Ellerbe-Beckett, Kansas City, Mo.; Buzz Baldwin and Steph Jones of Baldwin & Associates Inc., South Bend, Wash.; and The Erection Company of Seattle, Wash., for their assistance in compiling this article.