Porch connections to walls

[dhoward26]

I recently ran into a design that requires me to connect a wrap around porch to a wall. The architect wants a wrap around porch and part of the wall the porch wraps around has 2-story full height studs.

Per the code, I'm supposed to have an out-of-plane lateral connection to the wall. That's easy when connecting to a rim board or concrete wall. With this situation, I am using an angle for attachment as all I have are wall studs. Anyone have any better more cost effective ideas? I haven't ran the calc on the angle yet, this is just what I'm thinking of doing.

Also, where in the code does it say what you are supposed to use for an out-of-plane load for wood to wood deck connections? Concrete is an easy straight forward 200 plf...does the same apply for wood in this situation?

 

[bpstruct]

Lots of connection there. What is the purpose of the angles? I'm probably just not understanding what you are trying to achieve, but some of this seems unnecessary to me. Can you point me to a section in the code?

 

[dhoward26]

bpstruct:

It is a lot of connection. The deck screws are for in-plane shear to transfer into the ledger and from the ledger to the blocking, the lag bolts for vertical, and the angles are for out-of-plane loads. The way I interpret the 2012 IBC "Section 1604.8.3 Decks. Where supported by attachment to an exterior wall, decks shall be positively anchored to the primary structure and designed for both vertical and lateral loads as applicable. Such attachment shall not be accomplished by the use of toenails or nails subject to withdrawal." is that you have to have connection for both planes of shear...in and out of plane...similar to what you would do if connecting a ledger to a concrete or cmu wall...thus the angles @ 48" o.c. to positively anchor the porch to the house.

Could I use the lag bolts to take care of the out-of-plane loads? Yes. But, in order to do that I would have to put the ledger in a Cross-Grain bending application which all literature I have (and my previous mentors) says to avoid this.

 

[bpstruct]

If I'm understanding right, I think I would check the weak axis of that ledger for the bending you are concerned about to see if it can handle it. In terms of the out-of-plane load, I think you would just calculate the applicable wind or seismic load and apply to the structure distributing as appropriate.

 

[dhoward26]

The only thing I'm really unsure about is what the minimum value is I'm supposed to use for the out-of-plane loads. I can't find it anywhere...other than running my own calc on the deck.

 

[dhoward26]

The only way the ledger can see weak axis bending is if I have a connection that utilizes/depends Cross-Grain bending...which is supposed to be avoided. Calculating the out-of-plane load based on the material weights is easy enough, I just can't find if there is a minimum out-of-plane load that decks are to be designed for.

 

[bpstruct]

I may be wrong, but I would calculate the out of plane load. There will be out of plane loads on the wall due to seismic or wind. I don't know what you would use as a minimum. I would use ASCE 7 to calculate those. In terms of the weak axis bending, I normally don't utilize the weak axis of the member, but I have never heard that you can't. NDS even requires a "flat use factor" to be used in weak axis bending. Which tells me that you obviously can use the weak axis.

I may be misunderstanding this issue, and if so, I apologize.

 

[nac521]

Both IBC and IRC codes state "such attachment shall not be accomplished by the use of toenails or nails subject to withdrawal". Does this apply to lag screws?

The IRC R502.2.2.3 requires two hold-down devices (one each end of deck) with a design capacity of 1500 lbs, horizontally anchored. So 3000 lbs per deck.

For decks we use Simpson DTT2Z deck tie, one at each end. In your case the DTT2Z are too long for 2X6 studs. You angle solution seems sufficient, I would probably double the studs at the angle location.

The out-of plane force for structural walls is defined in ASCE 12.11.1 as 0.4*Sds*I*W or 0.10*W.

 

[RFreund]

Usually we provide hold-downs as well, similar to how Nac describes. Basically the deck is a horizontal shear wall with hold downs at the edges where your angle detail would apply. However you may have to provide a few extra studs to resist the load at this point and check the connection of the studs to the top & bottom plates.

As for loading usually you have wind and seismic or some use a person/activity lateral force which I won't get into. You then get your Tension=Compression = M/d. As for seismic I have seen an interesting approach where you actually calculate the seismic force for the building then use a simple F=ma to see what force the deck 'sees'. Basically your derive the EQ force on the building at the level of the floor the deck is at. Then knowing the floors mass, solve for acceleration. Then apply this acceleration to the deck x deck mass = EQ force on deck.

As for cross-grain bending I'm not sure what you would use for an allowable bending stress. This really is not weak-axis bending this is - looking at a cross section say 2x10 the 10" being vertical dimension you clamp/fix the bottom then apply a lateral (left/right) force to the top. Unless I'm misunderstanding.

EIT

http://www.HowToEngineer.com

 

[dhoward26]

I understand how to calculate the seismic forces and wind forces. A question I now have, is there a "MINIMUM Out-Of-Plane load" that should be considered for decks as well since there is becoming increasing visibility to this area of design?

As for Cross-Grain bending...I attached a page out of Breyer's text book. The same concept applies to what I'm doing, the only difference is that his example shows a concrete wall and mine is a wood wall. The only way to get any out of plane force for my situation into the wall is via an angle (or at least I think so) so that you avoid cross-grain bending.

It's easy when attaching to a floor system.

Maybe I'm wrong and completely over analyzing this scenario?

 

[Archie264]

While the following link deals with decks, not building additions per se, it nevertheless has some good information. To be used as a resource only, not as direct guidance as your situation is likely differnent. For what it's worth...

http://www.fairfaxcounty.gov/dpwes/publications/decks/details.pdf

 

[dhoward26]

I have seen similar...it is based on AWC (Link here

http://www.awc.org/publications/dca/dca6/dca6-09.pdf)

and IRC...however, the manual you reference in Virginia leaves out a very important part of the IRC which is section "R507.2.3 Deck lateral load connection" where it talks about using hold-down tension devices at each end of the deck.

 

[RFreund]

So we have the 'hold-down' situation covered (the angle detail) and really you're just asking if there is a minimum out of plane load for wood wall to wood diaphragm connection, similar to masonry walls?
If yes - I'm not aware of one except for the minimum created by Wind/EQ (or once again created by moving people).

It's easy when attaching to a floor system.

What's the difference?

EIT

http://www.HowToEngineer.com

 

[dhoward26]

There really isn't any difference. It goes back to my original questions of "Does anyone have any better idea than the solution I have come up with using angles?"

It then progressed to discussions about "Out-of-Plane" minimum loading requirements...which I still don't have an answer to so I will base it off of weights of materials and ASCE-7 12.11.1

 

[RFreund]

I just realized the detail shows an angle at every joist. I think the hold-downs are only need to be at the ends of the deck. Assuming there deck can act as a diaphragm.

What would you be using for you're minimum if it were attached to the floor? Why are you treating this differently? Maybe I am missing something...

EIT

http://www.HowToEngineer.com

 

[dhoward26]

The angles aren't at every joist...they are at 48" o.c. per the detail in the original post. I hadn't ran the calcs on it yet and was just placing them at that spacing for purposes of viewing the detail.

In the past, I have used weights of materials for seismic calcs as there is little to no wind for out-of-plane loading...I was just kind of curious if there is some "Minimum" out-of-plane standard to use as I haven't seen or heard of anything in the code.

I do have some disagreement with the IRC (and its language) though and it's suggested use of the hold-down tension ties. When your decks get long, you have to add more tension ties to meet your diaphragm length to width ratios and it doesn't necessarily say that in the code...at least if you are treating your deck as a true diaphragm.

 

[RFreund]

Aaah, I see. Makes sense.

EIT

http://www.HowToEngineer.com