Wood Infill exterior wall supporting tie beam and roof?

[kmart30]

I have a 1 story cmu structure and the contractor wants to remove an 18’ long section of exterior cmu wall,keep the tie beam above(8”x12”), and infill with a wood wall below. The new wall would be supporting the existing tie beam and about 15’ trib of roof loads above. Wall is about 8’ high...What is the best solution here? I was thinking it would be a lot cleaner to cut the tie beam out and just have a new wood wall to support the existing trusses. However keeping the tie beam would be much better for the lateral load path...what gives?

 

[Ron]

No!
Wood can easily deteriorate with waterproofing failure and you lose your structural integrity. Not so with masonry.

It is also a violation of the Florida Building Code, assuming your project is in Florida where I believe you practice. Would also violate the IBC.


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[kmart30]

Thanks for the response Ron....I told him we were more than likely going to have to cut the whole thing out but he said he has done it that way before. Obviously a contractor will tell you anything but this seemed a bit odd hence why I asked. FBC 2304.13 says you can do it but you have to check for long term loading....is there another code reference that is more blunt just saying no?

 

[KootK]

What's the percieved benefit to the contractor here? Ease of temporary shoring?

Why is it desired to replace the CMU wall with a wood wall? Ease of new opening creating?

 

[kmart30]

He has to modify some existing openings in this portion of the wall and its an older coquina exterior finish that he doesn't want to match. So we wants to remove this whole section and put lap siding on it instead....All this for making a exterior door and window about 8" to 1' wider.

 

[Ron]

@kmart30....Chapter 21 of the FBC says no, except for the provisions of 2304 that you cited. The difficulty is "what is long term loading" when you consider that you are in a climate that has high humidity, is prone to waterproofing failures from poor construction and the weather effects are cyclic which affects wood to a greater extent than other construction materials. If you're considering creep in the wall under these loads, it will affect the deflection of the tie beam. Tie beam is a good misnomer as well. Generally they are lightly reinforced and require full support of the underlying masonry wall. Having an 8" wide by 8" deep beam with roof loads spanning 18' might be a bit difficult. Run the numbers. If the deflection of the wood for long term loading as defined in AWC-NDS, which would require a factor of 2 on the calculated immediate deflection, exceeds L/360, you would be outside the code provisions. Also, I would not take any serviceability reductions in loading since this, at some point, could/would become a primary structural concern, not a serviceability issue.


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[KootK]

I'm a bit confused here. Were the tie beam to remain in place, would things not unfold like this:

1) The contractor would use the tie beam as, effectively, part of the temporary shoring scheme for the roof. Hopefully with some shore posts under it as well.

2) In the finished condition, the tie beam would not be a meaningful spanning element as it would be continuously supported by the stud wall below for gravity loads. Rather, it would simply be a diaphragm chord and part of the roof uplift load path (perhaps an awkward part).

Am I seeing this incorrectly? This seems odd but likely doable to me.

Does the existing CMU have a sill plate atop it currently? If so, a solution to the diaphragm chord business might be to scrap the tie beam, build the wall full height, and use Simpson CST etc to splice the new top plate to the existing sill. I guess that would kind of depend on how far away the nearest, existing sill splice is. For a one story, predominantly CMU building, I suspect that some workable diaphragm chord solution can be found that wouldn't break the bank. I'd be surprised if the tie beam needed to stay for that reason alone.