When you stand on a trampoline, it bends down. When wind blows on a skyscraper, it bends sideways. This movement under weight is called deflection. Deflection is a serviceability concern, which means the skyscraper won’t fall down if it bends too far but it might be uncomfortable for people inside. Building codes specify maximum deflections so that occupants are not only safe but also happy.
Sometimes deflection becomes a structural concern. PERCH recently completed a project in which I needed to support a crane on a pair of beams, through a slab and a collection of vertical posts. The beam and the floor slab are forced to deflect the same amount, and since the slab is stiffer, it needs to take more weight to deflect as far as the beams. But I didn’t want the slab to carry much load, and I really didn’t want the crane to bounce as it picked up things and swung them around.
Here are five strategies to reduce deflection in a beam.
1. Decrease the load. Obviously if you need to support a 5-ton elephant, you can’t say “well, let’s pretend the elephant only weighs 3 tons.” But maybe you can use two beams instead of one, or reduce the spacing of floor joists, to decrease the tributary load (link) on each beam.
2. Shorten the span. This isn’t always possible. In my project, the beams needed to span between existing pile cap supports buried in the ground, so the beam length was predetermined. But deflection is proportional to span cubed, so a small reduction in length can make a big difference.
3. Stiffen the beam. Both the material and the shape contribute to stiffness. Steel deflects less than wood, and LVLs deflect less than dimensional lumber. Given two beams of the same material, larger cross sections with a larger moment of inertia deflect less.
4. Add weight to the beam ends. If you can make your beam curve down over the ends then it won’t curve up as much in the middle. The problem with this strategy is that it increases the total load, which might become a structural issue for your supports.
5. Fix the supports. To an engineer, “fix” means “prevent all movement and rotation,” not “repair.” Fixed-end beams deflect much less than simply supported beams. On the other hand, they transfer flexure forces to the supports as well as compression, and the supports must be beefier (to use the technical term) so they can handle the flexure.
I used a combination of strategies 1, 3, and 5 to reduce deflection on my project to a level manageable for the tower crane. My client reports no visible movement while the crane is operating – that’s what we want!