The Line of Force

by Don Friedman on March 2, 2016

As long as I’m rambling on about shoring, I’ll use this opportunity to make a point about how structure works. The picture below shows one piece of a shoring system we installed to keep the front facade, which was moving outward, from peeling off the building.

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The shoring system consists of horizontal steel braces that sandwich the wall at the window openings – one inside and one outside at the top and bottom of each window, bolted to each other to grab the wall without making holes in it – fastened to the floor joists. The diagonals run from the horizontals at the windows to horizontal steel angles lying on the floor and bolted to the joist tops. Using the angles at the floor enabled us to fasten each brace to multiple joists, spreading out the load.

The diagonals are quite slender for their length, and are effectively truss bars, able to transmit load only in the direction of their long axes. The problem is that the force in question, the tension that would be created if the wall started to (again) move outward, is horizontal and therefore not parallel to the diagonals. What happens with a mismatch like that? We create an upward force on the floor joists big enough so that, when it’s added to the horizontal force, averages to the angle of the diagonal braces. No human intervention is required for that result, it’s what statics shows us when we use vector addition. We checked, and the joists are strong enough to take that uplift force. (The horizontal force is distributed through the subfloor diaphragm.)

This problem shows up all the time in shoring, when the forces you want to counteract are not in a direct line with the solid structure or ground you want to use as support. A lot of shoring runs on diagonal lines and therefore creates incidental forces; the further the line of the shoring deviates from the line of force, the greater that incidental forces are. Past 45 degrees of deviation, it’s quite difficult to make the shoring work.

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