How do you brace home on addition project following the IRC?

[Polar24]

The IRC lays out requirements for wall bracing based on spacing of braced wall lines (60')
These braced wall lines take the whole house footprint into account.

If you are doing an addition project, how do you take into account the new
as well as the existing portion of the house?

Do you create a braced wall line at the existing/transition line? - which then treats the addition as fully braced new
construction and ignores the existing portion?

Do you have the 60' braced wall lines continue into the exiting portion and make assumptions
about braced wall construction? Or better yet, wherever the braced wall line occur in the
existing portions, ensure they conform to the code for braced walls at the time - which, prior to say
2009 was basically not well defined?

Thanks

 

[phamENG]

Generally, your options are to look at the entire house and do braced wall calculations for the whole thing, or design the addition to be structurally independent.

The first option is usually pretty tough, unless the house is new(ish), followed braced wall rules more or less comparable to today's, and you have the drawings and calcs from original construction. Otherwise, identifying braced walls and ensuring they are properly constructed is very difficult and potentially costly, especially in comparison with option 2...

The second option is usually my preference. The IRC allows licensed engineers to step out of the prescriptive wall bracing and into the NDS and SDPWS, allowing you to detail diaphragms and shear walls with loading found through rational analysis. This keeps all the work in the new construction, so you don't have to mess with finishes in areas they weren't planning on working in to make the addition on the far side of the house work.

There is a third option that can work in some cases, especially when the addition is small. You can use the IEBC (assuming it's adopted in the jurisdiction where this building is) and see if, along the wall line you're considering, the lateral loading doesn't increase by more than is allowed by the IEBC and not have to check it. You have to be careful with this one, though, because there are plenty of little holes to fall through - make sure you or your boss are familiar with how these codes interact and make sure you're dotting all your i's and crossing all your t's. I always check this option, since it's the least cost if it works, but it rarely does for the additions I work on.

 

[Flotsam7018]

because there are plenty of little holes to fall through

My favorite in the 10% DCR exception for Level 2 alterations is that you "shall account for the cumulative effects of additions and alterations since original construction". Many times, particularly in residential, this is damn near impossible.

 

[XR250]

Honestly, I don't look at this too closely unless the existing house is swiss cheese. Most PE's around here don't either. Sometimes I will sheath a wall inside the house. I try to justify a 3-sided model for the addition when possible.

 

[gte447f]

drawings and calcs from original construction

 

[gte447f]

@XR250 is telling the truth. 90% of the time, I doubt anyone gives much thought to the overall lateral force resisting system when dealing with residential additions, other than to assume that the side that gets tied into the existing structure is automatically braced by the existing construction. I'm not saying it's right, but that's the way it is.

 

[Ron247]

I would like to have a general idea of your loading requirements and the house itself. In general, I agree you will probably need to consider the 3-sided model. The general geographic areas I have worked do not have calcs for residential and most never had any drawings beyond a "Builder's Set" with door swings and light switch locations. I know we built 2 houses off of sketches on a grocery bag in the 70s.

Part of what I would use in making decisions is the age of the house and the "typical" construction in your area at the time it was built. It would be rare for anyone where I have been to even know what lateral stability means let alone considering it. The 3-sided model is also what some houses function as due to all the openings on the "garage side" of the house.