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Reinforcing existing wood beams

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Despy

Despy

Structural
Feb 9, 2007
14
I am looking for some help on reinforcing existing wood beams. This particular problem crops up from time to time and I need a good way to address it. If you have a wood beam in an existing structure that needs to be reinforced to carry increased loads it is relatively easy to add a steel plate (or channel) to the side of the existing member. Determining the new capacity of the composite beam is a matter of relative stiffness and has been addressed elsewhere in these forums.

My question is about the mechanical connection requirements between the two pieces. To me it seems there are two cases:

1. The side plate extends over the supports.

2. The side plates stop short of the supports. This is more common since the supports are frequently inaccessable.

In the first case the load sharing is complete and the bolts/nails/screws holding the two pieces together only have to transfer the appropriate percentage of vertical applied load to the side plate (basically a flitch beam).

In the second case it seems that the bolts/nails/screws have to resist an additional component since the side plate does not reach the supports. Is it correct to calculate this additional force as the tension/compression resulting from the moment in the side plate?

Ex: A side plate carries 6000-lb*in from the moment at the center span of a uniformly loaded beam. Bolts connecting the two members are in two rows 4" apart. Thus both the upper bolt row and the lower bolt row must each carry an additional 6000/4 = 1500# in a direction parallel to the length of the beam.

Thoughts?
 
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If the steel doesn't reach the supports, the first thing I would worry about is shear in the wood at the support.

If you have enough connectors between the wood and steel to transfer bending stresses, I don't think it matters if the steel reaches the support or not, provided that the wood can handle the shear and bending that are left for the portion that was not reinforced.

I don't much buy into that composite interaction with steel and wood anyway, though. Unless I have no choice, I make the steel strong enough to carry the load without the wood. I don't trust contractors to install it the way I draw it.
 
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Usually shear in the existing beam at the supports is not a problem. Cases where this crops up are usually longer spans where bending or deflection controls.
 
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Despy:

I agree with StructuralAggie. I also design the steel to take the entire laod. The steel is usually so stiff compared to the wood, that after you perform a relative stiffness analysis, you tend to find the wood's contribution is almost negible negligible. So why not just assume the steel takes all of the load.

If I cannot run the steel over the orginal support (column, wall, etc), I usually buld some kind of bracket or seat off the face of the column or wall to handle the reaction from the new steel beam(s). I do not like to transfer the vertical shear from the steel back into the wood to then carry it the last bit to the support. Kind of a convoluted laod path that I have some conerns over.
 
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I agree that if you can support the load on what amounts to a new member it is better, but sometimes I only need to increase the capacity of the existing beam a small amount (original is loaded to 110% -120%). In such cases, if I can resolve the plate-to-beam connection issues by calculation it makes field installation considerably cheaper and easier.

Similarly I could add wood side plates if the required capacity increase was small enough (big time cost savings).
 
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With side plates stopping short of supports I see no difference between this and the flitch beam analogy. Its usual for a flitch beam to have the steel plate a little narrower than the timber beam anyway, so the end reaction is carried by the timber alone, as would be in your case. Bolts throughout the span should be designed to transfer the proportion of loads carried by the steel plate. At the ends, bolts should be designed to transfer the full load from the plate(half the total load on the steel to each end reaction) back to the timber. The timber should ofcourse be checked at ends for adequacy.
 
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