Two Steel Beams placed side-by-side

Question

Two Steel Beams placed side-by-side

micana (Structural)

(OP)

14 Feb 14 13:30

Hi all,

I'm new to this forum and was hoping someone out there could help me out with a query I have.

Consider an existing loadbearing wall which needs to be opened up. A steel beam will be installed in order to make this opening possible.

As there is a slight issue with headroom, I intend placing two steel beams side by side instead of one larger one in order to reduce the overall depth. In doing so, I think it would be correct to assume that half the load will be taken by each of the two beams, correct? This will obviously allow me to have shallower beams.

My main concern is however related to deflection. A few of my peers have suggested that even though the two beams will effectively double the flexural and shear capacity of a single beam with the same size, the same cannot be said with regards to deflection. Is this correct?

They didn't really given a reason for their statement, hence why I have turned to this forum. Could anyone kindly provide me with feedback as to whether the statement regarding deflection is correct or not? ..in my opinion it is not correct!

Thanks

JAE (Structural) · Answer 1 · 2014-02-14 14:02:45

Two shallower beams will EACH have less strength than the deeper beam, generally (depends also on lateral torsional buckling effects).

But deflection is much more sensitive to depth than strength is. Just look at the equations for beam moment strength and beam stiffness and you can see that.
Also, the bending moment applied to the beam is based on span squared while the deflection is span raised to the 4th power.

Haven't you had basic steel design before?

desertfox (Mechanical) · Answer 2 · 2014-02-14 14:10:20

hi
Have a look at the beam deflection formula, the I value follows a cubed law, so if you calculate the I value for a beam and then calculate the I value for a beam half the height and use them in the deflection formula, you will get the answer.

micana (Structural) · Answer 3 · 2014-02-14 15:38:23

Thanks for your replies.

Perhaps my question made me look less knowledgable on the subject than I actually am. I am aware of the relation that both the length and stiffness of a beam have on its deflection, flexural and shear capacity.

My concern is really quite trivial but I am asking it due to other people casting doubts in my mind. Also, perhaps I went slightly off-topic when i spoke about deflection, since the answer to my question below will be enough to clear up my concern. My query is related to the assumption of how load gets distributed to the two beams when they are placed side by side. Would you agree that if we place two shallower beams in place of one deep beam, then the load per meter run on each of the shallower beams would be half the load transferred to a single beam? Note that I will ensure that the contractor places a levelling grout above the beams such that the load from above is spread evenly.

If this is the case, then the overall capacity of two shallower beams will obviously be double that of just one SHALLOW beam, since all parameters are linearly proportional to the load. Please note that I am not comparing the capacity of the two shallower beams with one deep beam.

Thanks again

desertfox (Mechanical) · Answer 4 · 2014-02-14 16:07:08

Hi micana

Yes I think I agree with your last post.

JAE (Structural) · Answer 5 · 2014-02-14 16:26:20

Quote:
Would you agree that if we place two shallower beams in place of one deep beam, then the load per meter run on each of the shallower beams would be half the load transferred to a single beam?

Well - if you had twice the floor span coming in on one side and half the floor span coming in on the other (i.e. the load bearing wall wasn't centered on the floors on either side of it) then no - the two beams might not take equal load - depends on if they are connected together somehow - by the floor framing itself or some other means.

If the two beam are equal, and deflect the same amount - then they are taking equal shares of the load.

ameyerrenke (Structural) · Answer 6 · 2014-02-14 16:46:19

I think its to be assumed, but I'd make sure both beams are the same size.. ie W8x28 + W8x28 not W8x18 + W8x40. And yes, make sure trib is the same and loads apply evenly.

Why not just use a heavier beam? Like a W8x58 instead of W8x28 + W8x28??

micana (Structural) · Answer 7 · 2014-02-15 05:43:27

Thanks to everyone for your feedback - I think my question has been answered sufficiently.

Ameyerrenke - Cannot use a heavier beam due to other constraints. I would have gone down that route if I could.

EngineeringEric (Structural) · Answer 8 · 2014-02-19 08:41:14

I am guessing that the earlier comments about deflection relates to this: The two beams will undergo equal deflection, so each beam deflects say 0.25" (or a 1cm). The load to each beam is related to the imposed deflection. Now when the beams have equal stiffness (EI) then they have equal load. but if one member has 2EI vs EI then the stiffer member will pick up 2/3 of the overall load.

Think is one beam was glass and one was rubber... the stiffness of the glass would make it take all the load; and therefore not half the load.

(?)

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