C&C vs. MWFRS

[cmbyrd77]

All, I would like your opinion on a matter at hand. I work for a post-frame builder and we have a lot of porches attached to our buildings. What is your opinion on the wind load induced on the porch frames. Would it be components & cladding or MWFRS? Thank you for your help.

 

[Stenbrook]

That would depend on what you are looking at specifically when it comes to the porch. If you are looking at the roof of the porch, I would say look at components and cladding. If you are looking at the posts that are resisting both vertical and lateral wind load, I would look at MWFRS. The decking would also be components and cladding.

In general components and cladding is for exactly what is says "components" so if you are designing a piece, an attachment, etc, use components and cladding. But with MWFRS you are looking at the overall structure and its resistance to the lateral wind on the entire structure. So for the porch if you use the posts as being fixed in the ground (I am just speculating here on that) then they are part of your MWFRS that transfers the lateral load to the ground and thus you can use MWFRS loads.

 

[cmbyrd77]

Thank you for the insight. So would design the porch truss/frame and the columns and/or headers supporting it for MWFRS, and basically the rest for C&C? The code is very vague with it's definition of MWFRS. The definition states "contributes to the overall stability of the structure." Well, if the porch fails, the building is still going to stand, unless maybe you were using the porch frame as a "third" leg to a frame.

 

[Stenbrook]

Since I can't see what your porch looks like, I am going to assume it is next to a building but not attached. Correct me if I am wrong on that. When looking at the porch, it is the "structure" in question. So for the overall stability of the structure, you need to ask what are you using to take the lateral load to the ground? If you are using moment frames, then the beam and columns within that frame will be MWFRS. If you are using cantilever columns, then just the columns would need to be designed with MWFRS. Components and cladding is referring to a more localized pressure that can get much higher than what the expected average pressure would be on the entire structure. So if you look at a 1ft square area, you would expect that the average pressure on that one area would be much larger than the average pressure on say a 10ft by 10ft area. And that is reflected in the code. In general when I do a porch I use components and cladding for the beams, decking, and column vertical loading and then put MWFRS lateral loads on the whole thing. But I know there are many people who interpret the code a little differently for these type of structures.

 

[cmbyrd77]

The porch would be attached at the main building as well as have it's own columns a certain distance away from the building.

 

[Stenbrook]

In that case, you could consider transferring some of that MWFRS load to the main building or you could consider the columns to take all of the load from the porch.

 

[P1ENG]

Anything that is going through or has gone through the diaphragm (horizontal or vertical) becomes MWFRS. Until it gets to that point, it is C&C. Actually, that is over simplistic. It is actually anything that goes through a Lateral Force Resisting System (LFRS) is MWFRS. So it doesn't need to be a diaphragm or shearwall. A braced frame or cantilever column would also use MWFRS.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant