Deck Seismic Analysis

[dgengineering]

Hi everyone,

My client is adding a deck to an existing 2 story house ( See the sketch attached). The city requires that we check existing house for additional load per IEBC section 1103.2. My plan is to do a seismic analysis of the existing building alone and then with the deck. After that we compare these two cases to see if changes are needed to the existing house. . Also, I consider the exterior existing shear wall (adjacent to the deck, transverse) as my structural element to be checked. But before I do these calculations I have a few questions.

1. What R value to use for the deck? Is it R=1.5 if we treat the deck as timber frame cantilever system? I couldn't find any info about deck R values
2. Should I add the deck force to the existing house story shear at each level? Then compare the existing and proposed conditions, right? because with different R values I have to consider them as separate
structures attached to each other. That's what I think but correct me if I'm wrong
3. Let's say the demand/capacity ratio exceeds the 10 % Should I reinforce the exterior shear wall? I know it's my decision as a designer but If you have other solutions please share

Thank you in advance for your help )

 

[BR0]

I have struggled with this quite a bit, but here are my 2 cents.

I do think you have to check both directions, but perhaps you know the controlling direction is transverse.
1. I have tried using the timber frame cantilever system concept, but have never been able to make it work for any significant loading. I either use a cantilever diaphragm and the R of the building or add a steel cantilever system and use the reduced R for that. Or use narrow proprietary shearwalls like Simpson Strongwalls or Hardy Frames. Decks are the diaphragm so it wouldn't have an R in the table.
2. I agree that the exceptions in ASCE 7 12.2.3.3 do allow you to use a different R in a given direction. I usually do a vertical distribution based on the main R used for the rest of the building but then ratio the forces up only at the line with the lower R. This sounds essentially the same thing you are doing.
3. If I am over 10%, I do reinforce. I'm unsure how you would not as I don't think there is wiggle room on that. The IEBC allows you to use reduced seismic loads.

 

[driftLimiter]

If you have the deck anchored into the building sufficiently and you can resist all of the lateral loading with the existing building, then the deck is another mass for the MSFRS of the building to resist. If your deck structure resists its own lateral loading in same way, then yes you need an R factor for that. Timber frames or cantilever columns perhaps.

As far as retrofit. Yes you may need more sheathing nails, more hold downs.
I would strive to get it anchored back to the building in each direction. One direction would need to be a setup.

 

[lexpatrie]

From the FAQ - The September 2013 issue of Wood Design Focus addressed seismic loads on decks. I'm not sure how useful it is, I've not tried to employ it in a design sense, but the researchers are generally top notch.

Wood Design Focus, September 2013

Seismic Load Determination for Residential Decks, Lyman, Bender, Dolan

Regards,
Brian