Steel 'strut-and-tie' design

[bugbus]

I have a situation where we are using a grillage of steel beams to temporarily support and jack up another structure. There are separate jacking points and packers along the cross beams; when the jack is at the end of its stroke length, the load will be transferred to the packers and the jacks will be reset, then repeat.

When the load is supported on the jacking points, there is a direct vertical load path to the support beams beneath, and the bearing failure is handled by adding a pair of vertical stiffeners - pretty straightforward stuff.

The problem is when the load is transferred to the packers. We can provide vertical stiffeners to handle the bearing issue. But the problem I am having now is that the proposed section does not have enough shear capacity (web capacity alone) - refer to below sketch.



Without necessarily increasing the section size (because the bending capacity is more than enough), I'm wondering if there is a simpler solution in just adding a pair of diagonal stiffeners that will provide a kind of strut-and-tie system as below:



A few questions:
*In general, is the overall idea sound? Am I missing something obvious here?
*I would intend to check the diagonal struts in a similar manner as is normally done for the vertical stiffeners, i.e., separate yielding and buckling checks based on an equivalent column section. Any issues with that approach?
*If we have diagonal stiffeners providing a direct load path, do we need the vertical stiffeners below the packer locations? (I assume yes because there is still the possibility of yielding/buckling of the web, but would like to hear a few others' opinions).
*Would there be any particular buckling check required for the top flange (assuming it is a compact section)?

Would greatly appreciate any help, thanks

 

[Eng16080]

Without necessarily increasing the section size (because the bending capacity is more than enough), I'm wondering if there is a simpler solution in just provide a pair of diagonal stiffeners . . .

Maybe this is avoiding your question, but I would think increasing the section size may well be the overall simpler solution, and more economical than welding diagonal stiffeners.

As to the specific questions, I think it's not possible to provide any strong guidance without knowing the load magnitude and getting into the analysis. In general, though, your ideas seem reasonable to me. For some reason I just feel better with the vertical stiffeners below the packers. I like that they will help distribute the load into the beam web, offering more redundancy than if they were not there. With that said, the option without may well be ok too. With the top flange and diagonal stiffeners in compression, and the bottom flange in tension, as long as those elements can resist the required load, that solution seems reasonable. It seems the web would serve to brace the compression elements from buckling.

 

[bugbus]

@Eng16080, will definitely consider just increasing the section size, agree that is probably the easiest solution in this case

 

[KootK]

I like it. I view this as basically transitioning from a beam to a truss at the ends and I would let that inform your design.

I like the inner stiffeners because they will smooth that transition.

It's the outer stiffeners that you could jettison because , based on the truss model, they wouldn't do much.

A more.common version of the same approach is to use tension field shear design. There, you reverse the diagonal and use just the beam web for it

Web doubler plates would be another option.

 

[bugbus]

Thanks @KootK, we will still need the outer stiffeners during the jacking operation, but agree with the rest of your comment.

I'll investigate these tension field shear design and web doubler plate options too.

 

[TLHS]

With a diagonal stiffener you're basically using the same detailing that's often used for load continuity / shear reinforcing at welded right angle moment connections. Along the same line, you could also use a web doubler plate or similar reinforcement.