After Hurricane Katrina hit in 2005, the area of Covington, La., began to experience an industry boom as a result of many companies seeking a safer, more elevated area. Chevron Corp. needed a new headquarters for its Gulf of Mexico operations as a result of the hurricane. Covington's safe elevation and remote setting fit the needs of this building.
The Chevron project—a $78 million, 300,000-square-foot, design-build office building—was constructed in Covington in a record eight months. The building design incorporates low-rise three- to four-story concrete tilt-up panels and glass. Constructing a building of this type in a hurricane region, during hurricane season, required extra bracing to protect the panels. The contractor and owner made additional bracing considerations to ensure the building was well reinforced and protected in an area where wind speeds suddenly can become ferocious.
Bracing design guidelines developed by the Tilt-Up Concrete Association (TCA), Mt. Vernon, Iowa, recommend panels be braced for period wind conditions that might influence life safety, per ASCE 7. Therefore standard bracing designs are not expected to withstand the intense wind loads of storms that would have cleared the jobsite well in advance of the incident.
However, bracing the panels for hurricane wind speeds was important for the builder risk insurance. If high winds occurred after the panels were erected, there was a good chance some of the panels could blow down, causing major construction delays. To avoid this, the tilt-up panel bracing capacities were increased to withstand hurricane force winds.
At the start of the project a few challenges immediately arose:
- Could the project schedule be met?
- Could the panels be cast onsite with limited space?
- Could a cost-effective 300-ton crane be found to erect the panels?
- Could the panels be braced to the outside of the building to expedite structural steel erection?
- The owner's insurance company requested the panels be braced for 120-mph wind speeds because the tilt-up panels would stand without the structural steel completed during the hurricane season. They were willing to pay a premium for the bracing. Could this be done?
- How can 2x68-foot link panels be braced?
Handling construction challenges
The Chevron project consisted of 116,969 square feet of tilt-up panels, a 65,000-square-foot casting bed, two months of 300-ton crawler crane rental, and two months of brace and anchor system rental. The schedule for the project required a 16-week time frame for the foundations and an 18-week interval for the tilt-up panels.
The Chevron panel bracing design involved a special bracing analysis to accommodate the higher wind speeds than normal. A hurricane analysis was modeled to ensure the best bracing solutions to use. Due to the severe forces being transferred to the braces and the relative height of the panels in relation to the floor slab, the bracing scheme incorporated auger-like brace anchors that are driven into the ground outside the perimeter of the building and attached to the massive braces to substantially increase the load transfer capacity of the brace to the ground. The biggest challenge of all was a 194-kip panel (weighing 194,000 pounds) being pushed by the severe 120-mph wind load at a height of 70 feet.
The overall construction schedule was set already at an eight-month duration spanning from the start of earthwork through to the issuance of the temporary certificate of occupancy. The tilt-up panels were on the critical path of this schedule and accommodating the increased bracing scheme only heightened the intensity during this section. Once the tilt-up panels were lifted and braced, structural steel could proceed, followed by the placement of concrete on the floor decks. To accelerate the tilt-up panel schedule, casting beds were installed for the panels that would not fit on the floor slab. The crew worked overtime seven days a week and the reinforcing crew worked during the night.
Due to the site's limited size, a number of constraints dictated the location of the casting beds. In addition, there was a large retention pond to the south of the jobsite and a large fence with a public street to the north. Construction around these obstacles had to be carefully sequenced.
One project challenge involved 200,000-pound tilt-up panels that required a 300-ton crawler crane to erect the panels. Such a crane was unavailable in the local market. Sunshine Specialties, a tilt-up panel erector based in Apopka, Fla., had a 300-ton crane that was available for the needed time frame, and employed a crane operator with experience in erecting large, four-story tilt-up panels.
The brace design proved tricky, because the steel erection had to be expedited. An inside and outside brace design was used. The panels were initially braced to the inside, which allowed a faster panel erection schedule, and then braces were reinstalled to the exterior to allow the faster steel erection. The location of the retention pond relative to the jobsite did not allow room to brace to the exterior, leaving one corner of the three-story building with interior braces.
A big concern early on in the planning stages was how to erect and brace the narrow 6x68-foot panels that joined the four- and three-story building together. These panels featured a full height glass curtain wall between them. It was important to brace the panels so they would stay plumb in both directions until the structural steel was erected. The solution included a horizontal tube brace installed between the panels at the same elevation as the brace point on the panels; an addition to the typical panel brace design. The horizontal tube brace held the panels in the correct position (left to right) while the structural steel was erected. See photo on the top of page 28.
Panel bracing design
The panel bracing design for the Chevron building started out with a standard 72-mph wind design with standard equipment. It required 14 braces, each with two 10-foot extensions. They were too many braces to use with the exterior helical ground anchor system.
The standard wind load design uses a force coefficient factor Cf = 1.2, which is intended for solid panels. However, the American Society of Civil Engineers, Reston, Va., specify in ASCE 7-95 to allow the removal of openings from the projected area of the panel with an increase in the Cf factor. (Note: The latest version of ASCE 7 is ASCE 7-05, but the section referred to in the TCA document is unchanged from ASCE 7-95. For the sake of consistency with TCA, ASCE 7-95 is used here.) It provides a chart with various force coefficients dependent on the ratio 3 of solid area to gross area. For example, in panel 8 the gross area of the panel is 2305 square feet and the total area of the openings in the panel is 938 square feet. Therefore, ratio 3 = (2305-938) / 2305 = 0.59. With this area ratio, Cf is raised to 1.6, which results in a 33% increase in wind pressures, but the 41% decrease in surface area yields a total resultant wind force that is reduced by 21% and the number the number of braces plus extensions to 11.
Substituting the braces with stronger braces with no extensions and a 10,700-pound safe working load, the number of stronger braces was reduced to four. This made the anchors much more economical with installation spacings at approximately 8 feet.