Collateral Forces

The building structure is the supporting frame of the building. It is designed to withstand the vertical forces imposed on the building which include the weight of the building materials, the weight of the things in the building (people, furniture, files, etc.) and weight imposed externally, like snow on the roof, equipment, etc. The building must also resist lateral forces. These are horizontal imposed forces on the building structure and are primarily wind loads. The other forces that the building structure must resist are seismic forces which are the result of ground movement. The vertical forces are relatively static loads, however, when lateral forces are applied to the structure, they try to push it over and in the process try to twist and rack the structural frame.

The ground is pushing up as the building gravity forces push down. But it’s the connections of the structural frame to the foundations and the connections from member to member that work to resist lateral forces and seismic forces. Our building structure consists of concrete footings, retaining walls, concrete slabs on grade and an elevated concrete slab over the mechanical crawl space. From the concrete upwards the structure is steel. We have steel tube columns and on top of the columns are beams of tube steel on the upper structure and joists on the lower structure. Sitting on top of the beams and joists are purlins which are smaller structural members that span between the beams and joists.

The connections between all of these members and the foundations transfer vertical loads from one to another down to the foundations and resist lateral forces. But sometimes they need help in resisting lateral forces. If you have a square frame of some type and you push on one corner, you’re likely to change it from a square to a diamond shape. However, if you screw a square piece of plywood to the frame and try pushing on the corner again, it resists being changed into a diamond shape. The plywood acts as a diaphragm.

There are diaphragms incorporated into the structural frame to help resist twisting and racking. The roof decking becomes a large horizontal acting plane when it is all fastened to the structure and does a great job as a diaphragm. The vertical steel also needs help and we incorporate sheer walls or large vertical planes of structure that help resist the building from leaning and racking sideways. We have incorporated vertical cast in place concrete walls in the northwest corner of the building and in the south wall of the building. The elevator shaft also acts as a sheer wall in both the north/south and east/west directions.

Once the frame is stabilized with all of its components, the rest of the exterior becomes a skin that wraps the building and encloses it from the exterior elements. Following the foundations during construction, the structure is the next major construction activity. It typically goes up fairly quickly and once it is up the building begins to take on a three-dimensional shape that foretells the character and massing of the building. We are getting closer to the day that construction will begin.

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