Footings for Load-Bearing Posts – GreenBuildingAdvisor – GWC Mag

In this Q&A post, oldbungalow writes about adding a beam beneath his kitchen floor to correct a bounce. The floor is framed with TJIs that come off a 4-ply LVL beam. The 5-ft. by 7 -ft. kitchen island could be contributing to the bounce, although the TJIs below it are doubled. The new beam will be supported by two posts, and run parallel to the existing beam, directly under the area where the bounce is most pronounced. The basement slab consists of 4-in.-thick fiber-reinforced concrete, over 2 in. of XPS foam and compacted gravel.

Oldbungalow wants to know how to determine if he needs to dig footings for the posts. Typically, an engineer would determine the size of a footing, which is determined by the load being carried by the post. His builder doesn’t think it’s necessary to dig footings, because the new beam isn’t carrying the primary load.

And if he decides to dig footings, does he have to go to the frost line?

First, let’s define the terms

Walta100 says the term “bouncy” that oldbungalow used is hard to quantify in terms of engineering a fix. He wonders if the rest of the house’s floors are bouncy, and suggests oldbungalow get an engineer’s opinion as to whether the floor framing is up to the task as built, and if not, have them design the necessary changes.

Malcolm Taylor explains there’s a difference between deflection and bounce, although both are caused by an inadequate ratio of height to span on the joists. “Bouncing doesn’t occur due to excessive loading or sagging,” he says. “It can happen in a room with few loads, when someone walks through it. In fact, one of the pieces of engineering advice to cure bouncy floors is to…