"engineering" residential porches

[strandengineer]

The majority of the residential work that I do is located in coastal south carolina, in a high wind and high seismic area. My questions regards the design of wood-frame residential porches to resist lateral loads. An area of specific concern is screen porches or covered patios that project 10-15ft from the rear of a residence and support a gable roof that valleys back onto the main structure.

Most engineers in this area don't really address the porches specifically, typically showing prescriptive details from SSTD10-99 or WFCM. Typically I have seen Simpson ABU post bases or something similarly rated for uplift but not overturning.

The only two options I see for resisting the lateral loads with wood columns (covered porches, screen porches) is to either embed the posts in the ground or concrete or to use steel to form a moment frame and pin the column connection at the foundation.

Further complications arise when trying to embed posts with "raised slab" foundation construction that is common in this area. If you use a masonry stem-wall that is approx 2ft. high, there is not a good way to embed the post thru the wall and into the ground. Steel on the other hand is viewed as overkill for residential structures.

I guess this is my question after all of the previous information. What are some typical ways that engineers have addressed the lateral resistance of porches for residential structures? Maybe there is something fundamental that I am missing, but I have not been able to find any publications that address this issue nor have I seen many posts dealing with this issue. Thanks.

 

[UcfSE]

Try handling the lateral by providing a roof diaphragm that cantilevers from the main house. You'll then get tie down forces at the ends of the diaphragm, but it may help the typical framing you want to work out as well.

 

[CJJS]

Could you assume the columns are pinned at the top and bottom. If your roof diaphragm is rigid enough, then I think this assumption would be valid. You just need to make sure it is anchored to the main roof adequately.
Or you design a portal frame.

 

[csd72]

Another option is to create a portal frame to the outside by providing diagonal knee braces at the eaves.

csd

 

[skier1578]

I have done:
-wood knee braces as scd72 suggested
-fabricated 'T' or 'L' brackets out of steel channels and place them at the post/beam connection to create a moment connection
-embedded steel angles or tube steel into a caisson (flag pole) and had the steel angles/tube steel only come out of the ground 2-3 feet and bolt to a wood post
-if warranted, ask/beg for a couple feet at each end and install a strong-wall

 

[Atomic25]

I don't see how anyone could ever get a residential contractor, building a few hundred homes, to provide specially fabricated steel elements or embedded posts. That's something I see them laughing at the engineer and telling him no way.

Only option I see possible is if the architect is on board with seeing knee bracing.

Would be interested to see if anyone else has any other design approaches.

 

[CJJS]

UcfSE mentioned cantilever the diaphram from the main house. This seems reasonable to me.
I am designing canopy that ties into a masonry wall. I want to try to design it this way (cantilever from main structure). The canopy roof consists of wood roof trusses bearing on (2) 2x10 beams. The beams are attached to a ledger that is fastened to the masonry wall. Does anyone know of a Simpson hanger (for fastening the 2x10 beams to the ledger) that is capable of providing tie down forces if cantilevering the roof?

 

[UcfSE]

I would consider using an LTT/MTT/HTT at the top of the beam, separate from the bucket.

 

[bylar]

I have tried to use cantilevered roofs but the design never works in 140-150 mph. Also be carefull with just calling for knee braces. It usually does not work.

 

[CJJS]

We are in 90MPH. Shouldn't be too difficult here. Key word... "shouldn't"