Lateral Design for Open Porches 1

[TRAK.Structural]

Doing a residential screened porch addition and need some input on how others would approach the lateral design for this. Generally I wouldn't be too concerned here except this particular design does have a fair amount of wall area that will receive wind pressure, see plan below, wall on left side is solid, remainder is screened. N-S direction will go through the roof diaphragm and into the main house however the E-W loading (which has more wind area) probably needs some closer analysis.

1. How would you determine MWFRS pressures for the wall/roof? ASCE 7 open structures provisions?
2. Would you try to make cantilevered posts work to resist wind loading, or some combination of cantilevered posts and new/existing walls accounting for geometry and stiffness of elements? Some other approach?

 

[XR250]

I usually do flagpoled columns on these or moment frames.
The flagpoled columns can be 8x8's sunk into concrete if the owner wants to replace them in 20 years or steel columns.

 

[jayrod12]

Or a knee braced portal frame.

 

[XR250]

Or a knee braced portal frame.

Have yet to have a customer who wants to see knee braces except below the floor framing.

 

[GaStruct]

I usually just rely on the roof diaphragm but this is large enough where you need to do something else.

I would vote for cantilevered columns as mentioned above. Moment frames would send my contractors into a coma for this.

Else you can try to convince the arch to make the mullions wide enough to fit a portal frame per APA or one of the Simpson portal frame/shear walls

 

[XR250]

I usually just rely on the roof diaphragm but this is large enough where you need to do something else.

Same here

 

[SE2607]

Or a knee braced portal frame.

If in a high seismic region, the R value will be more difficult than a cantilevered column. Also, the connection of the diagonal brace is usually difficult to do and still look attractive. I usually go with cantilevered steel tube columns in this application.

 

[TRAK.Structural]

I usually do flagpoled columns on these or moment frames.
The flagpoled columns can be 8x8's sunk into concrete if the owner wants to replace them in 20 years or steel columns.

Seems like this is the way to go. May need another column or 2 to keep ftgs and deflection reasonable.

 

[jhnblgr]

Last time I had someone wanting to do this I gave them an estimate for work and they quickly determined they couldn’t afford to build it.

 

[RipPECity]

Could you just design the new roof to be an open front diaphragm in accordance with SPDWS and then strap your chords back into the existing house? I guess you could run into issues with your existing building's diaphragm acting as a transfer diaphragm as it is a pretty large extension.

 

[JAE]

I've either added a short shear wall at the corner (maybe in the northwest corner of your plan assuming north up).
Or I've used cantilevered footings - embedding steel pipes or tubes into an embedded footing/pier capable of resisting the wind moment.

 

[TRAK.Structural]

I usually do flagpoled columns on these or moment frames.
The flagpoled columns can be 8x8's sunk into concrete if the owner wants to replace them in 20 years or steel columns.

For wood flagpoles, ever done anything like rebar drilled through the post to make sure the post doesn't rely on friction/bond between wood and concrete to engage the mass for uplift resistance?

 

[jerseyshore]

For wood flagpoles, ever done anything like rebar drilled through the post to make sure the post doesn't rely on friction/bond between wood and concrete to engage the mass for uplift resistance?

This is common at the top of timber piles to provide some uplift resistance in pile caps, but I've never seen it done for the bottom of a wood post.

I almost always use large wood posts, large wood beams, the heavy Simpson post caps and the heavy Simpson moment post bases. That makes these things about 99% stronger than all the other covered porches I've seen built.

 

[TRAK.Structural]

This is common at the top of timber piles to provide some uplift resistance in pile caps, but I've never seen it done for the bottom of a wood post.

I almost always use large wood posts, large wood beams, the heavy Simpson post caps and the heavy Simpson moment post bases. That makes these things about 99% stronger than all the other covered porches I've seen built.

For the bases I assume you use the values for moment resistance with unreinforced concrete, I'd never get a contractor to build the rebar cages from the simpson literature for these things.

You also need 6" side cover (for 8x8 posts) to use these, wondering how that works out when the slabs for these porches tend to be more or less flush with the outside of the columns? Although this detailing issue could also be a problem with posts embedded in a pole ftg.

 

[SE2607]

Could you just design the new roof to be an open front diaphragm in accordance with SPDWS and then strap your chords back into the existing house? I guess you could run into issues with your existing building's diaphragm acting as a transfer diaphragm as it is a pretty large extension.

How would the OP address the additional lateral load to the existing LFRS?

 

[phamENG]

For wood flagpoles, ever done anything like rebar drilled through the post to make sure the post doesn't rely on friction/bond between wood and concrete to engage the mass for uplift resistance?

Yes. The AWC guide for prescriptive deck design has a detail like this.

 

[jerseyshore]

For the bases I assume you use the values for moment resistance with unreinforced concrete, I'd never get a contractor to build the rebar cages from the simpson literature for these things.

You also need 6" side cover (for 8x8 posts) to use these, wondering how that works out when the slabs for these porches tend to be more or less flush with the outside of the columns? Although this detailing issue could also be a problem with posts embedded in a pole ftg.

No I don't use the values for those bases at all, but they are just stiffer than the normal ABU's so I specify them for these open porches for some extra support.

And most post bases are going directly into big sonotubes so I rarely have to worry about the pier to footing detail. Uplift controls typically on these open designs.

 

[Respawn]

I would do wood posts with knife plates. Analyze it as a cantilevered column.

 

[TRAK.Structural]

Using cantilevered wood posts the limiting factor looks like wind drift. Even using 10 yr wind pressures the numbers aren't great. What are others using for a drift limit for this type of structure?

 

[Flotsam7018]

I almost always treat it like a deck and tie the chords into the main diaphragm as long as it meets the SDPWS requirements, which this does. Is everyone analyzing all the transfer diaphragms for lateral deck ties? My guess is not. Sometimes tying back can be goofy depending on the demand and where the chords will tie into the exterior wall requiring some additional details. The cantilever columns can add some feel good factor but I'm suspect about how much load they'd actually see from the overall lack of stiffness.

As for analyzing the existing structure, my first check would be IEBC 10% allowable demand increase.