Should I Do a Seismic Retrofit of a 1970 Bungalow? 8

[Logan82]

Hi,

I have bought a 1970s house. There is one basement and 1 story only (it's a bungalow). I am a structural engineer, but I work mainly on heavy steel structures. I don't have much experience with residential homes.

I talked with an engineer specialized in residential structural engineer, and he told me his company sometimes install X bracings for houses to do a seismic retrofit.

Due to its age, it's almost impossible that there are bracings in my wood framed house. The interior is made of gypsum. There are bricks on the exterior. The area is moderately seismic. The ground is mostly made of clay.

The residential engineer told me it's rare he sees clients with a bungalow doing a seismic retrofit.

To perform the retrofit, he would need to remove approx 1/3 of the gypsum in the walls of my first floor.

Should I do a seismic retrofit of my bungalow?

 

[beav-eng]

Is there a specific reason that you think your residence needs a seismic retrofit?

 

[SWComposites]

where is the house located?
is there a framed stub wall between the top of the basement concrete wall and the first floor joists?
is the sill plate anchored to the basement concrete wall?

 

[JoshPlumSE]

IMO, the key is to evaluate the bungalow's lateral load path. Adding X-bracing somewhere to stiffen a wall might work, but if there is no load path to that wall then it wouldn't do all that much good. If there is a brick wall that isn't anchored to some diaphragm, then your roof could separate from the wall and "pancake".

The most efficient type of retrofit (if a retrofit is needed) is hard to determine without really getting a good idea what the load paths are and where they need to be improved.

 

[SWComposites]

see:

https://seattle.gov/documents/Departments/SDCI/Forms/EarthquakeHomeRetrofitPlanset.pdf

also:

https://www.seattleretrofit.com/pos...etrofit-here-s-a-detailed-look-at-the-process

 

[Logan82]

My house is located near Montreal, Quebec, Canada.

 

[Logan82]

I wanted to explore the necessity of doing a seismic retrofit because wooden frames used to not have bracings and I was wondering if it was safe, and also because it appeared feasible budget-wise although I would prefer to put my money elsewhere if I could determine that it's not required.

 

[SWComposites]

My opinion having owned houses in Seattle and CA:
First worry about:
- house attachment to foundation/basement wall
- then if there is a cripple/stub wall in the basement, reinforce that
Fixing those two items will address the typical failure modes in a quake
I would not worry about the first floor walls of a one story house.

But also consider the non structural mods:
- tie down the water heater and gas meter
- install auto gas line shutoff
- fasten shelves and cabinets to walls

 

[Logan82]

Thanks for the all the tips!

I would not worry about the first floor walls of a one story house.

Thanks a lot SWComposites! But I'm still curious, is there a way to calculate the lateral resistance of a first story wall that has no bracings?

Nice I have no cripple/stub wall in the basement, and it's super accessible to attach my house frame to the foundation since I have suspended ceiling tiles that are easily removable.

I have no gas. I will definitely fasten my furniture and my heater tank to my walls. I thought about using these from Ikea

https://www.youtube.com/watch?v=iTM2s12Xu6c.

 

[TLHS]

Wooden houses generally don't have bracing because it's an awkward type of lateral system for the construction method. The lateral system is typically shear walls using OSB, plywood or some other cladding material.

A one storey bungalow is a pretty low risk building unless you know about something specific.

What renovations are you planning? If you're taking this down to the bones, you could make sure it gets re-built up with shear wall segments that align with the heavy seismic portion of Part 9 of your local version of the NBCC. If you aren't doing major construction I wouldn't personally rip it apart just for seismics.

 

[Logan82]

I was not planning to perform renovation in the walls of my first floor, unless it was necessary to perform a seismic retrofit. Initially, I saw no real big problems associated with that. I was curious to see if it was necessary.

 

[lexpatrie]

Most existing structures are grandfathered, so to speak, so it's not required, per se.

If you wanted to prioritize the reinforcement or retrofits, there are the FEMA and ATC (Applied Technology Council) tools, and they have web seminars from time to time, but cripple walls in the foundation tend to get the priority early on, not interior walls.

 

[chris3eb]

But I'm still curious, is there a way to calculate the lateral resistance of a first story wall that has no bracings?

ASCE 41-17 chapter 12 has default values for a variety of light frame wall systems. They list the default lower-bound strength for a gypsum wallboard sheathed wall as 85 plf.

 

[Logan82]

Most existing structures are grandfathered, so to speak, so it's not required, per se.

It's true, but I still wanted to know if it's earthquake proof.

here are the FEMA and ATC (Applied Technology Council) tools

What type of tools are these? Softwares?

They list the default lower-bound strength for a gypsum wallboard sheathed wall as 85 plf.

Wow so the gypsum is taken into account into the lateral resistance calculation? I'm very surprised!

 

[TLHS]

There's no such thing as earthquake proof. The standard of care in Canada is effectively, "has a better chance than not of not failing in a way that kills people during a design level earthquake"

People are mentioning FEA because we like to do fun analysis. Practicing engineers are unlikely to do a complicated FEA for this kind of problem though. You're also unlikely to do an ASCE 41 evaluation in Canada for a small structure, although some people might. It seems more typical to reduce loads based on the structural commentaries and then use the new construction code with additional inputs as necessary from things like ASCE. There's also no code requirement for this though, so the standard of care is likely going to be whatever you want it to be. You're generally not going to have a requirement to do a seismic upgrade unless there's a change in use as defined in the building code, or significant building modifications. Even then, it's in the hands of the local jurisdiction.

Since there's no requirements, it would basically be up to you. You could say you want to upgrade to current building code. You're in a zone that triggers the high seismic stuff in Part 9 of the code, so there's more shear wall calculations to do and some load path validation. Or you could argue that you're a retrofit so you get to reduce your ground motion level in line with the structural commentaries. Now you likely don't trigger the high seismic stuff in part 9 and now your small building has no fancy requirements and is maybe fine. Or you could go to part 4 and have an engineer to decide something based on the general engineering principles in there.

Nobody's going to be able to give you a solid answer, because what you're asking is not a requirement. It's all strictly a judgement call based on your specific building features and your risk level.

 

[Greenalleycat]

Not sure why everyone is getting so worked up over this. A single story house should be pretty easy to brace.
I worked in residential in a seismic country so this kind of stuff is pretty bread and butter

If you were operating in my country then you'd be legally required to complete the upgrade to bring it up to code as it looks like you've gutted the place?
So, does a similar obligation exist for your jurisdiction?

Here we'd use plasterboard for seismic bracing cos we're special, I don't think you guys can do that though
But surely you can just slap some plywood around and get it up to code
I'd fix any obvious/gross deficiencies in load path or other detailing but really wouldn't get too hung up on assessing the entire house