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Designing Base Plate for Eccentric Loading

Designing Base Plate for Eccentric Loading

Designing Base Plate for Eccentric Loading

(OP)
Morning guys,

I am in the process of designing a base plate in LRFD for a guide rail that is bearing against an unknown timber material and am having trouble with compression in the timber on the bottom of the base plate when impacted by a vehicle. The trouble is my compression stress from the moment is great than my allowable compressive strength which is causing it to fail. Additionally, since I am assuming the least strength in compression from the timber this is also contributing to why I can not get it to pass.

Note my bolt configuration consists of four bolts, two above the neutral axis and two below. See attached photo taken from the AISC for the basis of my calculation. While the equation accounts for the force in the bolt from tension, you can use the same basis to calculate the compression below the neutral axis.

The guide rail I am designing is for an extreme event and therefore, I am considering replacing the section modulus (I / c) term with the plastic section modulus (f*I / c). Note the shape factor 'f' equals 1.5 for rectangular cross sections which applies to my case and therefore am considering to use it to increase my compressive capacity.

Any other recommendations on how to approach this problem or issues considering my assumption would be much appreciated.

Thanks!

RSB

RE: Designing Base Plate for Eccentric Loading

I recommend posting a sketch of your situation and the loads imposed. I'm finding this difficult to visualize and suspect that other's may be in the same boat. Hence the limited response to your question so far. Some starter thoughts:

1) I'm not sure that it will be appropriate to use that method from the steel manual for your situation with wood as the substrate:

2) The wood crushing limit state is generally about keeping deformation under control. You may be able to take advantage of that fact in a situation where deformation is not critical and/or the loads are rare and extreme.

RE: Designing Base Plate for Eccentric Loading

Have you accounted for the different stiffness of timber to steel? I think the method in your attachment assumes E is the same for the bolts and the compression block. The calcs would be similar to reinforced concrete at service loads. Alternatively, reinf concrete at ULS after the timber crushes a bit.

RE: Designing Base Plate for Eccentric Loading

(OP)
Steveh49,

I did not account for the different stiffness of timber to steel. I am going to consider that and try to figure out that equation, I am having a hard time grasping the reinforced concrete idea...

Thanks for the help,

RSB

RE: Designing Base Plate for Eccentric Loading

(OP)
Steveh49,

I am now approaching the problem like this:

(stress)comp = (stress)tension
(E*strain)compression = (E*strain)tension

Can I assume a strain for my steel to find the strain in the timber in order to calculate the compression stress?

Thanks!

RSB

RE: Designing Base Plate for Eccentric Loading

I'd need a sketch to answer specifics, or at least know what is providing the reaction to the bolts (are they bearing on the underside of the timber, or anchored into concrete or...); and whether the timber is in compression parallel or perpendicular to the grain (I assume perpendicular but it makes a big difference so need to be sure).

A sketch is best though.

To answer your previous post, you need to find the strain distribution where the tensile bolt force (not stress) is equal to the compressive timber force, and the moment from this force couple is equal to the moment applied by the post.

RE: Designing Base Plate for Eccentric Loading

(OP)
SteveH,

Thank you - that will answer my questions, appreciate your help!

RSB

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