Houston Astrodome, The Eighth Wonder of the World

Since the inception of the sports, football and baseball fans have been subjected to a wide range of weather conditions ranging from rain, sleet, and snow to hot and humid weather in an open-air environment to watch their favorite teams compete.

That was until the Houston Astrodome was opened in 1965. Touted as the Eighth Wonder of the World, the Astrodome was a catalyst for today’s modern billion-dollar indoor stadiums that dot the worldwide sports landscape.

Early stage of erection of dome roof structure using steel erection towers

The dream of a fully air-conditioned domed stadium was that of Roy M. Hofheinz, a Houston politician who served as Houston’s mayor and a Harris County judge during his career. Together with his business partner Bob Smith, they created the Houston Sports Association with the goal of getting a major league baseball franchise in Houston. The Houston Sports Association needed the help of Harris County voters to approve a public bond to build the Astrodome. To pass the bond, the support of African American voters was critical, so Hofheinz and Smith elicited the help of Quentin Mease, one of Houston’s most respected and influential African Americans. Mease and other African American leaders agreed to campaign for the bond on the condition that the stadium was opened as an integrated facility.

In 1961, Harris County voters approved a general bond issue of $42 million for the construction of the Astrodome. Ground was broken on the project in 1962, and construction formally began on the Astrodome in 1963.

Originally called the Harris County (Texas) Domed Stadium, it served as the home to the Houston Colt .45s (Major League Baseball, now known as the Houston Astros) and the Houston Oilers (National Football League, now known as the Tennessee Titans). It was the first time a stadium was built for baseball and football that was totally enclosed and air-conditioned.

Still standing today but currently unused, the Astrodome’s circular shape has an outer diameter of 710 feet, covers 9.14 acres, and the clear span of the domed roof is 642 feet. The diameter of the playing field is 516 feet, and the maximum height of the roof above the playing field is 213 feet. Original seating capacities for the nine-level multi-purpose stadium ranged from 45,772 for baseball, 52,382 for football, 55,000 for conventions, and 66,000 for boxing. In 1989, 10,000 new seats were added to the facility to increase the capacity for baseball and football games. The stadium was constructed for just over $45 million (1960s dollars), and the structural costs totaled $5.3 million for the stadium and $1.5 million for the domed roof.

Ring and lamella steel trusses of partially erected dome roof structure

The Astrodome is a domed circular concrete and steel-framed building. One of the foremost challenges of the project was the dome roof structure. The dome roof had to be affordable and aesthetically pleasing. Structurally, the roof also had to withstand hurricane wind speeds and minimize wind sail. The shape and construction of the roof also had to consider heating/cooling demands to minimize air volume. Even still, it required using equipment with approximately 6,000 tons of cooling capacity circulating approximately 2,000,000 cubic feet of air per minute to cool and heat the stadium. The fresh air intake was required to be approximately 200,000 cubic feet per minute, while any smoke or hot air was to be expelled from the top of the dome.

Competitive design proposals were submitted by interested firms with experience and expertise in long-span roof structures. Each firm submitted designs that conformed to the required specifications, which included the following:

  • Roof live load 15 psf
  • Sonic boom loading 2 psf
  • Wind load 40 psf(or load from wind tunnel testing using sustained wind velocity of 135 mph with gusts of 165 mph)
  • Dead load: Self-weight of structure
  • Superimposed dead load: Three-inch thick Tectum deck on bulb tees with plastic skylights

Unique to the Astrodome was sonic boom loading. In a simple definition, a sonic boom is a manmade thunder caused by shock waves from aircraft flying at speeds faster than sound. Shock waves radiate from the plane in a conical shape exerting pressure. When the lower edge of the pressure cone impinges on the ground, the shock wave is heard as a boom. The speed of sound is 1,125 feet/second or 767 miles per hour. This speed is commonly referred to as Mach 1. After Charles Elwood Yaeger, a United States Air Force officer, broke the sound barrier in 1947 by flying the Bell X-1 aircraft, there was much excitement and discussion about commercial flights flying at speeds greater than sound. The study on supersonic flights started in earnest in 1955. Because the site selected for the Astrodome was only about 8 aerial miles from William P. Hobby Airport, it was considered prudent to consider this sonic boom loading for designing the long-span roof structure of the Astrodome. Indeed, supersonic commercial flights operated in the United States for about 30 years, starting in 1973.

Specifications also required a 1/8-scale model to be tested in a wind tunnel to verify wind forces on the domed structure.

Nine proposals were received for the dome roof design, and Roof Structures, Inc. was awarded the contract. They proposed to use a steel lamella framing for the dome structure.

McDonnell Aircraft Corporation conducted the Astrodome model wind tunnel test in their aeronautical wind tunnel facility in St. Louis, Missouri. Such aeronautical wind tunnels focused on uniform air flow and did not account for the dynamic and turbulent characteristics of wind in the earth’s boundary layer, which is a more realistic simulation of the wind interaction with on-land structures. At the time, unlike today, there were no boundary layer wind tunnel facilities for testing on-land civil engineering structures. The first boundary layer wind tunnel test facility was built in 1965 at the University of Western Ontario in London, Canada. As such, once the aeronautical wind tunnel Astrodome model tests were completed, Dr. Herbert Beckman, Aerodynamicist and Professor of Mechanical Engineering at Rice University, was retained to evaluate the test results independently. In his 1961 report, he wrote that the models were subjected to a steady air stream while hurricane winds consisted of small grain turbulence with a gust diameter of usually not more than 100 or 200 feet. The gusts will impose only partial loading of the building and, consequently, would be less effective than a steady wind. The wind tunnel data was thus considered to give conservative loads compared with corresponding flow conditions in hurricanes. The Astrodome has successfully withstood four hurricanes with no significant damage since its construction.

The dome roof steel skeleton

Reactions on the dome support columns using the wind tunnel test results were very close to the reactions computed manually by Roof Structures. To verify the analytical procedure using shell analogy for designing the dome, Roof Structures built an extensively instrumented test model and subjected this to various loadings. In addition, the design was extensively peer-reviewed by various experts.

The lamella dome structure has a diamond-shaped pattern on the spherical surface. The arch ribs or ring members are steel trusses with an overall depth of 5.5 feet. The top and bottom chord sizes vary from wide flange WF 16 x 58 to WF 16 x 78. The web members are two angles, 31/2 inches x 31/2 inches x 1⁄4 inch. The short lamellas between ring members are also steel trusses 5.5 feet deep. The top and bottom chords of these trusses vary from WF 14 x 30 to WF 14 x 53. For these trusses, the web members are also two angles, each 31/2 inches x 31/2 inches x 1⁄4 inch. The lamella dome framing is supported on a tension ring, a truss 5 feet, 6 inches deep. The top chord of this highly stressed truss is WF 14 x 370, and the bottom chord is WF 14 x 314. Once again, two angles, 31/2 inches x 31/2 inches x 1⁄4 inch, were also used as web members in the tension ring.

All structural steel used in the lamella dome structure was ASTM A36 steel. Connections between the various elements of the lamella framing were made using ASTM A325 bolts. All welding was done using AWS E7018 electrodes. Continuity in the tension ring’s top and bottom chord members was provided by using full penetration butt weld splices.

Perimeter concrete retaining wall

The dome structure is supported on steel columns WF 12 x 65 located at every five degrees around the dome’s perimeter. These columns were designed to permit the movement of the dome structure toward or away from the centroid but not to allow movement from the tangential shear forces resulting from lateral wind loading. This was accomplished using a knuckled column design conceived by Ken Zimmerman, the lead structural engineer on the Astrodome at Walter P Moore. The knuckled columns have four-inch diameter high-strength steel pins at each end of the column. The lower bearing of the pin was welded to its plate support, and the top side was free to rotate in a close-fitted plate with a milled surface. Anchorage was provided at the top against uplift with U-bolts.

The lateral wind loads were resisted by X-braced bents extending the stadium to the foundation. However, providing the X-braced system in certain areas was not feasible, so moment frames were used instead. In addition, because there are seven expansion joints around the perimeter of the dome structure, each isolated sector had to have its own system of lateral load-resisting frames.

The dome framing required the fabrication and erection of 37 steel shoring towers. The erector placed the dome framing in pie sectors in opposing pairs, with 12 sectors of 30 degrees each. The erection of the steel presented problems because the criteria required that the tension ring stay vertical when dead loads were applied. Jacks were placed at the top of the erection towers to make the adjustments as erection progressed.

After the alignment was confirmed and all connections were made, the jacks were gradually retracted over all the towers. Again, there was significant interest in the dome’s deflection. At each lowering of the jacks, tension ring alignment and supporting column plumbness were checked. However, the results of the plumbness of the columns varied daily. This was initially a concern for Roof Structures’ and Walter P Moore’s engineers, and the local elected officials. However, after checking and rechecking the monitoring data carefully and concluding that there was nothing amiss with the design of the supporting columns, the decision was made to retract the jacks and set the frame free.

Astrodome – interior view during football game

The monitoring of column plumbness continued, and surprisingly, it was observed that the columns did not stay consistently plumb but varied daily. After several days Ken Zimmerman figured out that the variation was due to the temperature effects not being considered in the monitoring. The columns needed to be checked at the same time on successive days to ensure there was minimal temperature variation. Once the monitoring procedure was adjusted, the columns were noted to perform as predicted. The dead load deflection was calculated to be 1.88 inches. When the jacks were released, and the dome was free from all erection towers, the measured deflection was within 0.25 inches of that prediction. This was remarkably accurate given the limitations of design processes available at the time.

Considering the dome was going to be air-conditioned, a temperature differential of 70 degrees Fahrenheit was used above or below the base temperature of 60 degrees Fahrenheit for temperature stresses and movements. The horizontal movement of the roof for temperature was determined to be +/- 1.80 inches. For the design wind load, the horizontal movement was 5.5 inches. This posed a challenge for architects and engineers to design the expansion joint at the edge of the dome roof structure adjoining the flat roof of the stadium. The joint needed to be designed for a total movement of 11 inches. The design team produced a virtually maintenance-free solution. The solution consisted of a screen attached to the tension ring that extended beyond a concrete curb on the edge of the stadium roof just below the dome. The screen and curb lapped sufficiently to prevent the rain from blowing into the interior, and the curb height was designed to not allow rainwater from spilling down the roof edge.

The foundation structure for the Astrodome was simple and based on the geotechnical recommendations by National Soils Services, Inc. Because of the sandy characteristics of the underlying strata, the differential settlements were negligible. Interestingly, 50 percent of the footings were founded on pure sand located five feet below the playing field. It was only in the 10-foot-deep combined footings at the expansion joints that some wet conditions were encountered. The original water table was at an elevation of 44 feet, the playing field elevation was 33 feet, and the bottom of the deepest excavation was 25 feet. The water table was lowered using a well-point system designed by Lockwood, Andrews & Newnam to accommodate this. Lowering the water table was essential during construction to maintain dry conditions and permanently eliminate the pressures associated with the hydrostatic head of water. Below-grade perforated drainage pipes were installed throughout the dome’s interior and circling the exterior of the perimeter basement wall stoprovide permanent dewatering. Two dewatering lift stations were provided within the perimeter of the Astrodome – one in the southeast quadrant and one in the northeast quadrant – which are still operational.

For about 60 percent of the perimeter, the retaining wall extends from the first level to the fourth level of the stadium for a height of 33 feet. The other 40 percent of the perimeter wall extended from the first to the third level for a height of 25 feet. Three concepts were developed to design the walls:

  1. Counterfort system
  2. Wall braced to interior column footings by diagonal struts and horizontal struts between footings
  3. Tie-backs using pre-stressing strands to dead-man anchors around the perimeter of the dome structure.

A cost comparison of the three schemes indicated that the system using tie-backs and dead-man anchors was the most economical.
A drained sand backfill was used to reduce the lateral earth pressure against the retaining walls. The geotechnical engineer computed the lateral equivalent fluid pressure to be 30 pcf. All walls were designed to span horizontally, with tie-backs placed at 2.5 degrees around the perimeter. Two levels of tie-backs were provided such that the positive and negative wall moments were equal. The lower tie-back was placed close to the footing, and the second tie-back was placed close to the mid-height of the wall.

Strands with a diameter of 0.25 inches were used. The distance from the wall to the dead-man anchors was approximately 80 feet. Because the strands needed to be buried in the soil, there was a serious concern about the possibility of corrosion over the years and the resulting loss in the cross-section of the strands. As such, it was decided to use the cathodic protection system to protect the strands from the corrosive effects of the soil.

When the dome was completed, the United States had entered into the Space Age with the NASA facility located in Houston. The prefix “Astro” became very popular and synonymous with “gigantic.” The owner of the MLB team renamed them from the Colt .45s to the Astros, and the Harris County Domed Stadium became the Astrodome and was branded as the “Eighth Wonder of the World.”

The soffit of the exposed steel lamella dome roof structure

In the late 1990s, the MLB and NFL teams moved on to new stadiums – and even a new city in the case of the Oilers. The Astrodome sat idle for most of the time aside from the annual Houston Livestock Show and Rodeo, which was last held at the stadium in 2002. In 2005, the Astrodome was used as a shelter for the residents of New Orleans impacted by Hurricane Katrina. By 2008, the Astrodome was officially shut down after inspectors deemed it unsafe for occupancy. In 2014, the Astrodome was listed in the National Register of Historic Places and received a Recorded Texas Historic Landmark Marker in 2018. This is the highest honor the state can bestow on a historic structure. It also adds another level of protection to ensure the Astrodome will exist for the foreseeable future.■

About the author  ⁄ Narendra K. Gosain, Ph.D., P.E.

Narendra K. Gosain, Ph.D., P.E., is a senior principal and the Structural Diagnostics Services Group's executive director at Walter P Moore. Dr. Gosain can be reached at NGosain@walterpmoore.com.

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