The OP could also install knee brackets running from the corner posts
to points more interior on the beam to reduce the span. I didn't do a
calculation as I haven't had lunch yet and I learned my lesson from my
first post in this thread, but the OP might be able to get away with
sistering on a single 2x10 if the knee brackets reduce the span
It used to have a center post, which probably explains why what was there
before worked okay. But, I want to end up with just the two posts (which
are actually steel fluted columns) because it makes the whole house look
better. So, I'd rather do whatever needs to be done with the beam to make
Thanks for doing all the calculations and posting your suggestions and
Another option might be to move the posts in towards each other a little to
reduce the span? But, I haven't done any calculations to know how that
would affect the beam sizing.
The good news, is it probably wouldn't be that difficult to jack up the
roof and replace the beam with a new one. The only issue I can think of is
the height difference of the beam. With wooden posts you could just cut a
couple of inches off. I'm not sure how easy that would be with your steel
For a beam carrying a distributed load, you can reduce the maximum
moment in the beam (and hence the maximum bending stresses) by moving
the supports inward so that the ends are cantilevered. This increases
(in magnitude) the moment at the supports and decreases (in magnitude)
the moment in the middle of the beam. To minimize the maximum moment
(in magnitude), you want the moment at the supports to be equal (and
of opposite sign) to the moment in the middle. This is achieved when
the central span is (2 - sqrt(2)) of the total span, or 58.6%.
Of course, depending on the situation, you may wish to minimize
something else, such as the maximum deflection (not sure when that
occurs) or the maximum shear (which occurs when the central span is
50% of the total span). Also, for an application like locating stair
stringers under a stairway, the effects of a point load may be more
critical than the effects of a distributed load.
For the OP, this is probably all academic. You need a deeper beam,
and the main problem will be to get the detail right at the support.
Do not notch the deeper wooden beam at the support to make it fit;
this will create a stress concentration and may create a weaker beam
than what you have now.
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