welding checker plate floor to stiffener beam (steel platform)

[jmurph36]

I have been trying to find information on a problem for a while. If I am designing a platform made from steel, how would one design the welds that attach the checker plate flooring to the members used as stiffeners? The stiffeners could be any type of member that could be used as a floor beam which would frame into the sides which would be consider girders. Would the checker plate and beam be considered a composite beam? How do I calculate the forces that the weld would be designed to? I am inclined to believe that the welds which attach the checker plate to the beam would resist the bending stresses induced by the maximum moment at midspan? I am very unsure and hope that someone will be able to clear this up for me.

 

[MotorCity]

It depends.

If you are considering the plate to act composite with the supporting beam such that you are analyzing a "Tee" section, design for shear flow VQ/I.

If the members supporting the plate are strong enough such that you do not need to rely on composite action, just provide a nominally sized and spaced stitch weld.

 

[jmurph36]

That makes sense. What would be the criteria to determine if composite action is required? In my case, I am sure it isn't required but I'm curious to know what would determine if it is necessary.

 

[mmarlow]

You'll know if composite action is required if your beam isnt stiff enough or strong enough alone. I've never done any composite design with steel beams and checkered plate. I'd have concerns of how well thin plate will act in compression over wide segments. In other words, determining how wide your compression flange is may be tough.

If you're using the plate to brace the beam for LTB, I believe aisc says the support or brace must equal 2percent of the compression force in the flexuaral element... or something to that effect. I dont have my book in front of me. So a short fillet weld should do the trick st each brace point.

At a minimum, each plate should have 4 connections (in my oppinion)...each corner.

-MMARLOW EIT

 

[jmurph36]

This is for a small steel platform. The 1/4" checker plate is support by 4" channels that only span 6' and are spaced about 6' apart with a live load of 20 psf. I know that the beam is stiff enough and composite action isn't really required. I was just curious how to calculate forces in the welds along the top flange of the channel. But, if I'm understanding correctly, since the beam by itself meets the deflection requirements, then essentially there isn't any shear flow along the plate/channel interface so just use the minimum weld size nominally spaced.

 

[dhengr]

Jmurph36:
Actually, the structure is smarter than a lot of engineers, it knows how it will act (be forced to act) by the way you detail it, even if you don’t really recognize that. Once the channels and the .25” pl. are stitch welded together, they will act as a combined unit (composit?), maybe buckling a little, for lack of sufficient welds, etc. And, it is up to you to at least give that some tacit recognition, if you are going to be a good engineer. They may over stress a few welds, but they have a fairly good FoS, so only a few deform significantly or crack. Most good welders doing that kind of work, hate being called back for too small a weld or too short a weld, or bad spacing, assuming some modicum of inspection, so you likely have larger, longer welds than min. size shown on the plans. 20lbs./sq.ft. sounds pretty low for a platform (industrial?) loading, at least check the pl. and channels for the 350lb. worker (or two) and bucket of tools on a couple sq.ft. area, and at mid-span on one channel. Use 10, 15 or 20t for the effective pl. width (not full pl. width), find the combined section props. for that new section and design the welds for the shear flow at their weld joint. Over and above your original question, you would do well to always ask yourself, ‘how will this structure or detail really act and deform, based on the way I have detailed it?’ The max. stress areas are almost always related to the areas with severe rigidity or fixity, or with max. deformation (strain), and you should see, think of these, on every detail, even when our simplified analysis methods don’t give us direct or exact answers. How does the structure move, given the way I’ve detailed it? That should lead you to general areas of max. stress + or -.