San Francisco International Airport

Exterior maintenance plays a key role in preserving the structural integrity, safety, and aesthetics of buildings and other permanent structures and The San Francisco International Airport is no exception. SFO is actively implementing a 4-phased approach to provide necessary repairs and repainting at the International Terminal along the entire West face, roof soffit, roof truss members, and columns. Our team at D.H. Charles Engineering, Inc. has played a critical support role in this process and is proud to that our scaffold designs are assisting in the continuous operations of the airport, serving our local community and visitors for many years to come.

DHC partnered with Performance Contracting Inc., to provide a supported scaffold to give contractors convenient access while also prioritizing the pedestrian experience. The SFO Airport serves about two hundred thousand passengers each day and normal operations need to remain seamless even during these large construction and maintenance efforts. We quickly determined that our work was cut out for us. We embarked on our scope with a preliminary site walk to review site constraints, existing structure nuances, and pedestrian flow impacts related to our scaffold. During this job walk, we established a common vision with the PCI and Webcor team.

To meet demands of a busy airport with active construction interfacing with the public, we prioritized a scaffold design that minimized impact to pedestrian & vehicle traffic flow and we implemented pedestrian overhead protection tunnels to keep people safe from any potential falling objects or debris. This challenge was coupled with the fact that our 710’-long x 41’-wide x 90’-tall scaffold was supported on a raised concrete slab, rather than slab on grade.

With garage levels below that needed to remain open, our scaffold leg loads quickly became a critical design component. We brought the existing structure’s engineer of record into the conversation to comment on the capacity of the existing structural slab and support beams at the scaffold post locations. Unfortunately, the capacity of existing structural slab was not adequate to support our scaffold legs. The structural engineer required we modify our design for one of the following options:

1. Provide a scaffold layout change, redesigning to move the scaffold posts directly in line with existing beams.
2. Provide sill beams of adequate stiffness to span post loads to existing steel beams below, skipping the concrete slab altogether.

While both options appear to be simple solutions on paper, our site constraints did not allow for either option to be seamless. This difficulty was due to obstructions with existing structural elements and impacts to walkway clearance for workers and pedestrians. Aligning scaffold posts with the existing steel beams below solved the issues for the existing structure, but it meant that our scaffold would interfere with existing diagonal braces and truss members. To avoid these members and line up our scaffold posts with the existing steel columns below as required but the Structural Engineer of Record, we would need to provide scaffold posts at both sides of the existing structure’s diagonal braces and roof truss bracing members.

To achieve this, we provided a scaffold base below, with posts set in line with the existing steel beams wherever possible. If we were not exactly in line with the steel beams, we coordinated to provide sill beams of adequate strength and stiffness that could transfer the load to the existing steel beams, avoiding loading the structural slab which was deemed inadequate by the SEOR. Once we set a base that transferred load to the proper locations, we needed to build up and avoid the obstructions. To do this, we provided a shorter 4’-0” long bay with posts centered around each diagonal brace of the existing low canopy structure. This required us to utilize the scaffold below as shoring for the discontinuous posts, with shoring beams stacked to support the posts above, allowing us to avoid the existing structural framing and provide adequate access to all existing steel requiring maintenance.

Because the entire scaffold was shrink-wrapped and our ability to tie to the existing structural was limited to only the bottom chord of the existing truss and the existing canopy beams, designing the scaffold to resist and transfer all lateral loading through the scaffold was another challenge. We utilized scaffold tube-and-clamp yoke ties, scaffold diagonal braces, and wire rope cable cross braces to resist lateral loading from wind and worked with SFO to determine a wind speed limitations for worker access and shrink-wrap removal.

This project was an excellent example of collaboration between the general contractor, scaffold contractor, scaffold engineer, and engineer of record. Now, each time I fly out of SFO I am proud to see our scaffold and am reminded of the important role we have as civil engineers: to provide designs that prioritize safety and efficiency while supporting the advancement of our community through infrastructure.