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Re: Replace Beam in Basement

From: charr67@charter.net
Category: Structural
Remote Name:
Date: 26 Feb 2006
Time: 10:38 PM


Question: I am currently considering opening up the basement area of my house. The current center beam running the length of my house is 40’ and it is supported on the ends by the cinder block walls and at three intermediate steel posts equally spaced over the 40” run. The beam is composed of (3) 2”x10” boards nailed together. This beam is adequately supporting all loads that are being applied to it. What I want to do is to have the house held/lifted and then remove the existing wood beam and 3 equally spaced supports with a single (or 2 separate) steel beams with only one support at 10’ from the back of the house. What size of steel beam would be required to provide the same support as the existing wood beam and 3 supports, with one or two steel beams with a single support 10’ from one end? Please email if you need any further information.

Builders Websource® Answer: Without have a precise loading diagram of your home, there is not enough information here to size the steel beam. For example, some of the existing load is being transferred to the columns and there is no guarantee that the load across the entire beam is uniform. There may be point loads at or near the existing g columns that have little bearing on the existing beam size. Plus, there are other loads to consider, such as wind, seismic and snow loads that may impact the computation. Furthermore, by eliminating at least two of your existing columns (which are currently about 10 feet apart), you're now spanning at least 30' feet without intermediate supports, which means that the foundation under the remaining column may require reinforcement to carry the added load (or risk settlement of the column support) and the column may need to be larger to avoid buckling. In addition, you also need to be concerned about total deflection. Although the building code allows a maximum deflection of L/360, the absolute deflection will typically be less for shorter spans. That means that for a span that's 3x greater (without intermediate supports), you could actually experience a total floor deflection 3x more, yet still meet the letter of the building code. This doesn't necessarily translate into a better "quality" feel. In fact, quite the opposite. Your floor could start to feel spongy under heavy live loads unless you design it to stiffer parameters, such as L/480 or even L/1000, where L is the unsupported span.

Let's take a small example. Suppose your existing floor was designed to have a maximum deflection of L/360. For a 10' span, that would be a maximum deflection of 0.33 inches. Were you to keep the same floor deflection criteria, but allow your span to increase to 30' (unsupported), you could experience a full inch of deflection in the center of your floor, yet still meet the code requirements! Yet, experience shows that total deflections in excess of about 0.3 inches can feel excessive to people. If your floor has tile, marble, or other heavy substrate, your floor needs to be particularly stiff to minimize deflection.

In summary, these kinds of design and engineering decisions require the consultation of a licensed structural engineer who can evaluate your current loads and examine all aspects of the project, including not only the beam size, but the foundation, the bearing on the cinder blocks, reinforcement of the block wall, the column size, the anchoring mechanisms, and shear considerations, as well as impact of other hazard loads and overall floor deflection considerations. Only once you've performed this analysis should you even consider to replacing your beam. Everything is possible, but do the engineering first and do it right with a licensed professional.


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