porch roof cantilever diaphragm connection?

[jgeng]

I have a covered porch open on three sides on the coast (high wind area) but no seismic. The open two corners are supported by brick columns. I am tasked with designing a new roof at a higher height (2-3ft) than the exising roof. I plan to design the porch roof as a cantilever diaphragm to resist lateral wind loads. I will be extending the ex. brick columns w/ wood posts and using them for gravity loads only. the side adjacent to the ex. roof will likely sit on a cripple wall and/or lvl. I plan to detail 45°bracing down to the existing roof height to transfer the diaphragm forces into the existing diaphragm on the one side of the porch only. Does anyone if this approach is permissible by IBC/IRC? and or know of any special considerations I should look it? thanks

 

[BAretired]

I don't know anything about IBC/IRC but cannot see anything wrong with your approach provided you account for all forces acting on the structure. A sketch might help.

BA

 

[DaveAtkins]

Your approach will handle the shear on the supported edge of the diaphragm, but you must handle the diaphragm moment in some other way. There will be tension and compression chords perpendicular to the supported edge, and you must resist the tension and compression forces.

DaveAtkins

 

[msquared48]

Yes. You must tie these forces back to thge structure with straps.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

If you had to guess at how many porch roofs have been properly engineered in the US for uplift, cantilever effects (especially chord forces), etc., would it be <5%? <1%?

I'd say they all fail in uplift before they have a chance to fail because of diaphragm issues. I see porches all the time built on 4" unreinforced SOG, and then sometimes THOSE get converted to liveable spaces.

Not saying that its right, just saying that is what I suspect... After the 2004 hurricanes in Florida every contractor and his brother was out there replacing aluminum porch roofs and pool screen enclosures.

jgeng- if the porch has no walls, then there may be very little to negligible lateral force, except from your brick columns. But I suppose you have considered that.

 

[JAE]

Uplift is your greatest concern. I know it isn't your question - but "brick columns" made me think of the load path for uplift which might not be there unless the brick columns are reinforced.

 

[jgeng]

thank you all for the responses. I agree that porches are never constructed or engineered correctly which leads to more gripe from contractor's for "over engineering". The diaphragm adjacent to the porch I plan to tie into is a flat roof. I had planned on tying the 45° braces into these rafters to transfer compression and tension forces dave atkins mentions into the existing diaphram. Is there any problem with that? Also the comment about the uplift and brick columns I am concerned about that too. But they are existing to remain. In speaking with a contractor we agreed they are likely 6x6 surrounded by the brick work, I am hoping for solid concrete w/ reinf. In situations like this when you are dealing with an existing structure and maintaining the same uplift load path even if you do not know if it exists or is adequate it is how do you view this from the existing building code. I guess I reason your not making in any worse and were not talking a the more serious safety concerns of gravity load capacity...

 

[a2mfk]

jgeng- I feel your pain when it comes to existing buildings, especially residential, in a high wind area (Florida for me). The FBC has an Existing Building Code, but I don't always go to the letter of the code, many times I exceed it by upgrading existing structures. I feel once I touch a large renovation/remodel, I have "bought" much of that structure with my seal, especially with older structures (who are they going to go after?)... I try to explain to the client why we should be doing what we are doing, its to make the structure safer, and many times the structure is exposed and it makes this easier.

If you field verify the post has no uplift connections, this may constitute a "dangerous condition" or some other trigger mechanism that gives you solid code backing to show the client you must correct the situation. You can also tell the client (hopefully talk to the homeowner), if you determine it so, "Look, to the letter of the code, we do not have to do this. But why spend all this money on this new porch and have something that can blow away the next hurricane/storm when we can fix it now." Brick work can be pricey but its not gold, you are already adding a new roof..

For your project, are they removing the existing roof and then reframing it? If this is the case, some codes may require you to then bring the rest of the supporting vertical structure up to current code (ie columns, footings, connections).

Sometimes this is also based on square footage or $$ of the project, though I would not think you are triggering either of these for a porch renovation.