Safeco Field Seattle, Washington

n 1995, Seattle’s profes- sionalI baseball team, the Mariners, decided they wanted a new . Across the country, great baseball venues were being created, echoing the early days of old-style , fresh grass fields, and the great outdoors. After 21 years of playing in the fully enclosed , the Mariners, too, wanted out in the sun, both for the joy of playing outside and the financial boost it would bring the team. But Seattle’s rainy climate dic- tated that the stadium be equipped with an “umbrella” to shield fans on days of inclement weather. And, thus, the demand was made: Build us a new stadi- um, open to the sky, with real grass, but make sure we can cover the field and the fans when it rains. Plus, do it by opening day 1999. With those ground rules, the design team set to work. The result is Safeco Field, a 47,000 seat, state-of-the-art, retractable roof ballpark. This one-of-a-kind project offers a landmark public amenity that will keep in the region for years to come. It pro- vides good family entertainment, while stimulating economic growth and redevelopment in the area. Proactive management and innovative design solutions were required to meet the aggressive

Modern Steel Construction / March 2000 project schedule and design chal- lenges. The retractable roof was designed for speedy erection and to minimize the impact on the construction of the seating bowl. The close proximity to the Seattle Fault required special seismic considerations, such as the use of an innovative viscous damping system in the roof that reduces the seismic forces by 50%. The exposed steel structure was designed to be functional as well as aesthetically interesting. The complex dynamic interaction between the three roof panels and the supporting runway required the use of very sophisticated lead- ing-edge analytical techniques. Large, three-dimensional, non- linear, time history analytical models were used to simulate dif- ferent earthquakes and develop the criteria for designing the damping system. The roof has over 12,800 indi- vidual pieces, weighing a total of 10,800 tons. It covers 8.8 acres and is supported by eight 655’ long tri-chord trusses. The roof rests on eight 90’ tall steel lattice legs. The trusses are pinned at one end to allow for lateral deflections due to temperature expansion and snow loads with- out imposing large stress in the structure. The legs are supported

Modern Steel Construction / March 2000 Washington, on a site constrict- ed on three sides by busy streets and the Kingdome on the fourth. • Provide an architectural con- nection to the adjacent Pioneer Square historical district • Complete the stadium in time for opening day 1999 Application of New or Innovative Technologies The use of steel was the key to the stadium’s success. Apart from the precast seating bowl, virtually all the project elements incorpo- rated steel. Below is an explana- tion of some of the more out- on large travel trucks, which suites and 5 to 7 party suites standing and innovative ideas and move along two elevated runway • Include administrative offices, technologies applied. Several structures on the north and south a stadium club, restaurant(s), new and innovative applications sides of the stadium. Through state-of-the-art clubhouses, were incorporated into the roof this system, the stadium roof and parking facilities design: moves at the rate of 1’ per sec- • Incorporate the retractable ond, taking 10 minutes to fully roof as an “integral part of the First-EEver Fully Retractable open or close in moderate winds design” Roof Utilizing Linear up to 20 mph. Tracking Movement With The stadium was designed in Jurors’ Comments: Three Independent Roof an amazing 9 months and built in just 27 months, 16 months less A one-of-a-kind project Panels than a normal design and con- taking a moveable roof Each of Safeco Field’s three struction schedule. The project is roof panels is completely inde- definitely a home run experience, stadium to a new level in pendent, as opposed to the inter- revitalizing the team, the fans, dependent panels of the ballpark and the city. a seismic design. in Phoenix, where the edge of Designed in just nine each panel is supported on the Satisfying the Building adjacent panel. By making the Program months and constructed panels independent, less steel was needed, and roof construction The demands of the building in just 27 months, the could be completed over the rail- program, as detailed in the con- engineers met a long road tracks without interfering tract between the Washington with construction of the seating State Public Facilities District series of complex chal- bowl. (appointed by Governor Gary Locke to oversee construction of lenges. The linear tracking and inde- the stadium) and the design team, pendent panel design of Safeco were few and straightforward, yet Field allowed the roof to be fully also incredibly complex and chal- stacked and provided a simpler lenging: • Construct the roof to expose as method of dealing with tempera- many fans as possible to the ture expansion and seismic dis- • Build a new retractable roof, outdoors when it is open placement. In the retracted posi- world-class major league base- • Provide a natural grass base- tion, the three panels are stacked, ball stadium and entertainment ball field which allows the roof to be com- complex • Locate the stadium near the pletely retracted off the stadium • Accommodate 47,000 fans, Kingdome in Seattle, and stored over the adjacent rail- including 70 to 75 standard

Modern Steel Construction / March 2000 road tracks under a long-term air- rights agreement with Burlington Northern. By completely retract- ing off the field, fans can enjoy a totally open ballpark, and not just a “peek-a-boo” view to the sky. Only Retractable Roof Ballpark In UBC Seismic Zones 3 or 4 Not only is the stadium located in Seismic Zone 3, it is built near the Seattle fault! The use of vis- cous dampers mounted on the roof panels play an important role in dissipating seismic energy and reducing overall lateral forces. mass of the structure constantly Construction This provided a very economical changes. Essentially, the stadium Once constructed, the dramatic design and saved over $5,000,000 becomes many different struc- tri-chord trusses are self-support- in construction costs. tures, depending on the roof loca- ing. By taking advantage of this tion. Gravity, wind, and seismic The original roof design con- feature, a greatly simplified and loads were evaluated at incre- sisted of five panels. In an effort shortened construction sequence mental stages along the runway so to reduce the overall cost of the was determined. A single erec- account for all the possible load- stadium, the original restrictive tion platform was built directly ing conditions. An analysis was stacking requirements were outside the stadium footprint performed using 1,500 different relaxed, which allowed the num- towards the railroad tracks. Once load combinations. ber of roof panels to be reduced each truss was complete, it was from five to three. A 70’ curved In an application never before released to be self-supporting, “brow” cantilevers off the attempted, variable-depth, vari- then rolled along the runway retractable roof and fills the gap able-width tri-chord trusses were trestle to be temporarily stored in between the roof truss and the selected to support the stadium’s the “air-rights” area over the rail- sun canopy. retractable roof. The selection of road tracks. This allowed con- The final solution also skewed the tri-chord trusses was arrived struction of the next truss to pro- the roof 8 degrees, to more close- at after considering a multitude of ceed on the same erection ly align the panels with the field. different structural, architectural, platform. and constructability criteria. The Both of these ideas resulted in After all the lower trusses (for sleek upturned tri-chord trusses complete field coverage while the end panels) were erected, the are one of the primary defining eliminating the expense of the platform was extended 50’ to con- architectural features of the ball- two additional roof panels, reduc- tinue with erection of the taller park. The very stable tri-chord ing the overall area by 30%, and trusses. By removing the roof configuration allows the trusses to cutting 100’ off the length of each erection from the critical path for be erected on a stationary work runway structure. The redesign the stadium, construction of the platform, then rolled aside to reduced the original $100 million stadium bowl could proceed make way for the next truss. The roof estimate by nearly $30 mil- unimpeded. Additionally, this tri-chord truss is the most effi- lion. solution minimized disruption to cient way to span long distances the West Coast’s main Most structures don’t move and when using an upturned truss. can be analyzed using conven- north/south Burlington Northern tional tools with approximately “Not only were they more sta- route. ble, they were more beautiful,” 100 load cases. The moving roof As each truss was built, it was said NBBJ principal and design of Safeco Field presented a totally temporarily supported on the team member Richard L. Zieve. different challenge. As the roof erection platform. A jacking sys- moves along the runways, the tem on the platform allowed each stiffness and distribution of the Trusses Enhance truss to “drop” into its self-sup-

Modern Steel Construction / March 2000 Additionally, a computerized monitoring system utilizing 50 accelerometers was put in place to verify damper performance in the event of an earthquake or high-wind event. The monitors will capture data for review of displacement, and determine if the Mariners can play ball imme- diately after a seismic event. Wind Won’t Blow The Game One of the most critical design requirements for the Safeco Field roof was providing wind resis- tance during storms. In many ways, the roof is more like a long- span bridge than a building. To fully understand the effect of storms on the structure, a series of tests were conducted on detailed scale models at a wind porting position. The trusses also north side, 18-inch-diameter, 22- tunnel in , Canada. The changed position as the secondary foot-long viscous dampers lateral- tests simulated storms coming framing and roof panels were ly secure the roof to the legs. from all directions and used prob- added. A sophisticated stability Like shock absorbers on a car, the ability theory to predict the check was performed on all the 800-kip dampers absorb earth- appropriate levels of stress in the trusses to ensure they would span quake and windstorm energy and structure. in both their temporary and final dissipate forces from a potential conditions. The check predicted seismic event. When in the extended or that roof movements would be retracted positions, the roof sec- During the design process, the 18” vertical at center and 9” hor- tions have “lock-down” devices stadium was subject to 30 major izontally at the ends. Actual that tie them to the support earthquakes…not real earth- deflection was within an amazing below, to provide additional wind quakes, but computer simula- 3/8” of that predicted! resistance. If there is a forecast of tions. The dampers allow the storm winds, the roof will not be The tri-chord truss designs roof to deflect up to 6” through a moved between the lock-down involved incredibly complex con- hinge located between each hori- positions. nections and geometry. To facili- zontal truss and its leg. In tate construction, the entire essence, this makes the structure analysis database was provided to transparent to temperature and Lattice Steel Legs Function the builders, who incorporated snow horizontal thrust force. as Moving Support System the information to make their A new 3-D modeling program Supporting the roof on steel process more effective. was used to evaluate the dampers “legs” allowed lowering of the and predict how they would per- north runway and also further New and Innovative Use of form through 12 different time reduced construction costs, since Viscous Dampers histories (with 10,000 elements). these support legs move with the roof, rather than support it as it Safeco Field incorporates The program digitized ground moves. To briefly explain, as dampers in a first-ever use of its motion to 1/50th of a second. moving weight travels across a type. The dampers are also the Use of the dampers cut the seis- supporting structure, each and largest viscous dampers ever used mic forces in half and reduced every piece beneath it much be in a building application. On the the size and stiffness of the run- designed to support the weight. south side of the stadium, the ways by 50%. Although the For example, if there were 40 roof secures to its lattice steel legs dampers cost $750,000 (including fixed vertical supports, all 40 by rigid connections. On the testing), they cut $5 million from the cost of the stadium.

Modern Steel Construction / March 2000 would have to be able to carry stiff as it extends away. the full load of the roof as it pass- The incredibly complex nature es overhead. However, since the of the geometries and sequencing legs that support the roof actually required that the stadium be travel with the moving weight, detailed and built in four stages. only the 16 legs had to be The stages were determined designed to carry the roof weight. through a phased analysis, which This unique concept of moving predicted where the structure support drastically reduced the would be at the time the next construction cost. stage was built. The four stages Because of the seismic require- used were as follows: ments, plan, and size of building, • Construction of the tri-chord the stadium is actually designed roof trusses, shored on the as seven separate structures, staging platform joined only with seismic expan- • Geometry after the trusses sion joints. This design permitted were released from the plat- simultaneous field-level and form and standing alone upper deck construction, which • Geometry after the secondary pared the 18-month calendar of roof framing was installed civil, foundations, and rough elec- • Geometry when the roof “eye- trical work to 10 months. brow” was added To complicate matters even The structure was designed to further, a 15’ layer of liquifiable the 1997 Uniform Building Code soils meant that the structure had before the code was released. By to be designed so that it could sorting through the various pro- “float” in the event of an earth- posed code additions and ascer- quake. The structure was there- taining the intent of the new fore built on concrete-filled pipe code, it was possible to design the column piles driven to a depth of stadium using the latest proposed 60 to 100’. seismic provisions. This provides the safest, state-of-the-art struc- The runway structures also ture: a stadium built today that required complex analysis, to meets the codes of tomorrow. appropriately design for moving wheel loads of 230 kips each. It was necessary to design the run- ways in one single piece—without any joints—so that splices would Safeco Field not interfere with trolley travel. Seattle, Washington To accomplish this, the lateral Owner: Public Facilities District load was concentrated in the cen- Architect: NBBJ, Seattle, WA ter of the runway structure in the longitudinal direction, and the Structural Engineer:Stephen runways were cut loose from the Tipping + Associates, Berkeley, CA stadium bowl on either end. Steel Fabricator: Fought & Co., Designing the runway structures Tigard, OR, Herrick Corp., (AISC members) to be structurally independent of Pleasanton, CA the bowl was also key to facilitat- Erector: Herrick Corp., ing roof construction concurrent Pleasanton, CA (AISC member) with the bowl. Additional analy- Detailer: DOWCO, Burnaby, sis had to address the fact that the BC, CANADA (AISC & NISD members) behavior of the runway structure General Contractor: Huber, varied, depending on the location Hunt, Nichols/Kiewit, AJV of the roof. The structure is (CM/CG), Seattle, WA stiffer over the bowl, and not as