I have an 8"x5" steel I-beam in the basement spanning 30' with a
single post in the center supporting one floor and a roof. I would
like to move one post to 8' and would like to know how far in from the
other end I would have to add an additional post. I'm trying to open
up the space for a recroom.
On 17 May 2007 08:03:35 -0700, email@example.com wrote:
saying 5 x 8 steel I beam doesn't tell us enough to give you a
meaningful answer. The thickness of the steel in the web, and plates
is an important factor.
That said, steel is not as strong as many people think it is. Right
now you have two 15' spans if there is a single support in the middle.
Were you keep that 15' maximum span (not a good assumption, only an
engineer can pinpoint the load points), then were you to move the
support to 8 ft from the end, then then next support would need to be
at 8 + 15 ft, or 23 ft. Safer might be just to use two: one at 8 ft
from each end.
But: get an engineer (as the other poster suggested) to evaluate the
If I were the engineer I would specify that a new footing must be placed
under any new columns. It generally means cutting a hole in the floor
slab, excavating 10-12 inches deep, adding a rebar mat and pouring back
flush with the floor. A variation can include hanging the new column from
the beam before pouring concrete so that the base plate is buried in the
new pour -- no exposed anchor bolts to trip over.
Bob Morrison, PE, SE
R L Morrison Engineering Co
Good point. Most basement slabs are a flat pour, which should be OK
here. But in reality it could be anything, couldn't it? One could do
some cores and determine the composition of the slab at each proposed
If the load was uniform then you might expect the beam to be ok as long as
you don't create an unsupported section greater than 15' long.
30-8 = 22 which is greater than 15' so you do need another post say 7 or 8
ft in from the other wall. eg A spacing of 8,14,8 would seem possible.
But it's likely the load isn't uniform and there are other factors that must
be considered. You absolutly must get this designed by a professional (which
I am not).
As you can see by the answers you have received, it's a costly procedure at
best. There are many structural considerations to take into account, not
just adding additional steel columns (or lallys).
Your best over all approach might be to reconsider how you could lay out the
basement with the existing column in place; perhaps over near a wall or
decorated to partially hide it.
If you still want to remove the existing column and add additional ones, you
have no choice but to hire a local licensed structural or civil engineer and
have him analyze your specific situation. (Remember, even if you sell the
home in the future, you would continue to be responsible for this basement.
Having an engineer sign off on it places the responsibility onto him as far
as the insurance and future lawsuits go.
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