Expert: Bruce E. Johnson - 3/28/2011

Question
I am in the process of finishing a walkout basement and would like to remove two support posts. This section of the two story house has 750 sq. ft on each floor and in currently supported by a 6x3.5" steel I beam. The ends of this beam are supported by concrete walls and has three metal posts with spans of approximately 7'3". If possible I would like to leave the middle post in and add a beam to one or both sides of I beam to have a 14'6" span on both sides of this middle post without going deeper than 10.5". Are LVL beams an option, if so how many would I need, or do you have another idea

Answer
Hi Tim, there is a little more involved than just moving posts and installing larger beams.  Without knowing what kind of foundation your existing posts are sitting on, particularly the center post, it's hard to make a value judgement.  Each post is probably sitting on a concrete pad that is under your floor slab.  Oftentimes this pad is built into the slab or poured monolithically with the slab.  The pad sizes are based on the weight designed to bear on that post.  By removing posts you are transferring that weight now to a single post sitting on a single pad whereas before you had three posts and three pads supporting the same amount of weight.  In your case, the size of the room and the room above are not huge and the weight isn't that significant so you probably can rearrange your supports to accomplish your goals.  If you have a set of old plans used in building your house you can look at the foundation plan and it will show you the size and depth of your support pads.  Most residential buildings have simple bearing weights attributed to the soil at 2500 pounds per square foot.  So if your center pad is 2 foot by 2 foot you have four square feet of area capable of sustaining ten thousand pounds (2x2x2500) of concentrated loading.  Giving a value of 60 pounds per square foot for your 750 sq. ft. two story room you are supporting 45000 pounds overall.  Now your external walls are bearing the bulk of the weight, especially the roof load so you can decrease the load attributable to your internal footings to roughly 20 pounds per sq. foot bringing your internal loading down to 15000 pounds which in turn puts 5000 pounds of load on each post. Will your one  center footing be capable of supporting that extra five thousand pounds? Probably. But it wouldn't hurt to have an engineer in your area look at the existing conditions and calculate your actual load to make sure.

As far as new beams go to support that longer span. The following information is available on line at this location:http://www.parr.com/PDFs/LP%20LVL%201.9E.pdf
According to the quick guide in order for you to support 14' spans with a double LVL beam with a thickness of 3.5" your beam would need to be 14" deep.  If you go to a triple width LVL 5.25" wide your beam will be 11 1/4" deep.

Another option would be to increase the size of your steel beam to 8-10" depending on weight of the beam.  Be careful when choosing steel beams as they can be heavy and burdensome to maneuver into position etc.  A local steel supplier can help determine the proper sized beam weight for your application. A W-10x12 looks like it would work..this is a ten inch deep beam that weighs twelve pounds a lineal foot.

Well that's about all I can say on the subject since I am not an engineer.  Before removing or altering any structural elements in your home I always recommend that you consult a P.E. (professional engineer) in your area regarding field conditions, concentrated loads, beam spans and post/column capabilities.  I hope this information helps, please feel free to write again regarding this or other matters ,sincerely bruce e johnson..bejohnsonconsulting.com