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?

 

[TRAK.Structural]

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.

In my area what I usually see is just a flat valley plate at each side of the overbuild to tie the new diaphragm to the existing.

How are you transferring chord forces into the existing diaphragm? Strapping them to existing rafters? Adding additional framing with straps under the existing roof?

 

[Flotsam7018]

Yeah, I don't know about relying on just the valley sleeper for wind perpendicular to the ridge. I typically use the beam below the rafters at the eave as the chords and tie in from there using your favorite deck tie product. Again, this gets more complicated if you're coming in away from the floor diaphragm where you need to tie into a post in the wall and then into the diaphragm above/below.

 

[TRAK.Structural]

Yeah, I don't know about relying on just the valley sleeper for wind perpendicular to the ridge. I typically use the beam below the rafters at the eave as the chords and tie in from there using your favorite deck tie product. Again, this gets more complicated if you're coming in away from the floor diaphragm where you need to tie into a post in the wall and then into the diaphragm above/below.

Not saying the valley plate numbers check out, it's just what happens a lot.

In my case I'd be connecting at the top plate elevation, but trying to get the load to the existing sloping roof diaphragm. Tension at a "kinked" joint from a flat porch beam to a sloped rafter feels like it could get weird. Would likely need additional clips to hold the rafter down to the top plate when that joint tries to straighten.

 

[JAE]

TRAK - good point and good sketch but most homes have horizontal ceiling joists just inside that could potentially eliminate the upward force on the stud.

That said, the most significant issue with tying into an existing roof is that the porch eave beams usually don't align well with any existing ceiling joists or interior beams.

The contractor would have to remove a large patch of roof sheathing to gain access and then somehow add framing to properly transfer the eave forces into the ceiling joists - and then also ensure that the ceiling joists somehow can transfer that all back to the roof diaphragm (unless the ceiling sheathing can do the trick).

This whole approach is usually difficult to design up front (before things get opened up for inspection) so early design details can only be approximated.

 

[TRAK.Structural]

TRAK - good point and good sketch but most homes have horizontal ceiling joists just inside that could potentially eliminate the upward force on the stud.

That said, the most significant issue with tying into an existing roof is that the porch eave beams usually don't align well with any existing ceiling joists or interior beams.

The contractor would have to remove a large patch of roof sheathing to gain access and then somehow add framing to properly transfer the eave forces into the ceiling joists - and then also ensure that the ceiling joists somehow can transfer that all back to the roof diaphragm (unless the ceiling sheathing can do the trick).

This whole approach is usually difficult to design up front (before things get opened up for inspection) so early design details can only be approximated.

Yea there are ceiling joists, but I probably wouldn't trust the ceiling as a diaphragm to take a concentrated chord force. I think I'd still have to drag the load from the ceiling up to the roof sheathing somehow.

Thinking through maybe using a partial "rafter" just long enough to get the load to the existing sheathing, and then header it off to the adjacent rafters. Would need to remove shingles for fastening down to this new member but I don't think that's too much to ask. Hoping/thinking they could get this member in through the attic or maybe slide it through the soffit to avoid tearing off roof sheathing, do-able?

 

[jerseyshore]

People would call me crazy if I was detailing reinforcement inside the existing roof framing for chord forces and diaphragm loading.

90% of these are built by a contractors and/or unpermitted. 9% are drawn up by architects. The remaining 1% that we get to design are leagues better than anything else out there. I make sure they are as stiff as possible towards the house. No one wants their covered porch banging into their house on a windy day.

 

[Flotsam7018]

As with any residential engineering, it is a fine balancing act of practicality, common sense and true engineering rigor. However, I do think it is our job to provide a complete load path - otherwise why the heck is anyone even paying us, they mind as well just have the designer/builder figure it out.

So I would complete the load path to the degree that my engineering judgement determines adequate (not going full Terry Malone here for the valid reasons jerseyshore brings up). This may look like Simpson DTT1Z with SDWH screw from the chord into the top plates, depending on the load I may stop there if I felt the load could be spread out from the plates into the existing roof diaphragm - or I may add connectors from the top plate to the adjacent rafters on either side. Your drag strut idea is more satisfactory from an engineering perspective but I would likely get push back in my market as well.

 

[SE2607]

Maybe I missed this, but is there any consideration for the additional lateral load at the existing rear elevation? If it's less than 10%, no problem, but, if it's over, that would mean new shear walls, hold downs and, potentially, footings at the hold downs. If this were the case, I would think a free standing patio cover might be a better idea. This would alleviate the drag tie back into the existing roof which is going to be unpopular. When I run into this situation, I usually use HSS cantilevered columns in flagpole footings, which means the inboard columns must be at least half the footing dimension away from the existing building.

 

[JAE]

As Flotsam suggests, the vast majority of residential single family homes (in the U.S. at least) have very little, if any, structural engineering involved.

But when we do get called in to help...

...we have to do it the right way or not at all and we also then have to put up with the shaking heads of contractors telling the owner we've "over-engineered" it.

 

[TRAK.Structural]

Sage advice from all, it's respected and appreciated.

For now I'm headed down the path of finding a reasonable way to resolve some amount of chord forces into the existing structure while using flag-poled columns for some extra assurance.

 

[phamENG]

Regarding the upward force due to the "kink"...unless you have a precast roof there's going to be uplift on the end of that beam. It will need to be held down one way or another.

any consideration for the additional lateral load at the existing rear elevation

It's difficult, especially since many of the houses that get these are prescriptive braced walls if they have any intentional LFRS at all. My approach is usually to check it to see if it could still qualify with the wall area it has. If not, then I add new shear walls.

 

[XR250]

IRC does not seem to care about open structures as far as the house LFRS goes.....