Seismic retrofitting

[Stan in Oly, WA]

I'm taking care of some of the problems in the crawlspace of my house before I have it insulated. One of the things I want to do is to tie the framing to the poured concrete foundation. The house didn't jump off the foundation during the 7.1 quake of 1949, the 6.5 quake of 1965, or the 6.8 quake of 2001, but there's no guarantee that it won't in the future. (It did sustain minor damage in 2001, the only quake to occur while I've owned the house.)

The 2X10 floor joists rest directly on the mudsill, no cripple wall. (That actually makes the job more difficult because the clearance is too low for me to move around on my hands and knees.) The mudsill is not bolted to the foundation. Most of the research I've done about seismic retrofitting recommends attaching the mudsill to the foundation by using steel plates which are lag screwed and anchor bolted. Then L-plates are used to strengthen the attachment of the floor framing to the mudsill. But one site I looked at showed the use of angle iron (steel) to tie floor joists directly to the foundation. This struck me as cutting right to the chase. Does anyone see any drawback to doing it this way? I could weld plate to the ends of the pieces of angle iron to spread the area of attachment. Does this seem like a good idea, or merely excessive (obsessive)? If the floor framing was tied to the foundation, I can't see that there would be any reason to put any effort into strengthening the attachment of the mudsill to anything.

I tried, without much luck, to get information about this from the local building department, and from my homeowner's insurance company. State Farm has some generic guidelines, but neither the building dept's website nor their complicated telephone menu system made any mention of seismic retrofitting (or any related terminology.) I got the distinct impression that I could get them to issue me an over-the-counter permit if I insisted, but no guidelines, or inspection.

Thanks, Stan

 

[Anonymous-0]

The dominant destructive force in an earthquake is horizontal sliding. You can visualize how best to strengthen the house by imagining a giant grabbing hold of the base of the foundation and shaking it back and forth. This also helps you to understand the probability of damage -- tall houses are worse than short houses, heavy houses (as with brick veneer walls) are worse than light houses.

The standard detail for modern construction is to bolt the mudsill to the foundation, and then secure the floor joists to the mudsill with something like a Simpson A35. A rim joist or band joist is secured to the floor joists and mudsill, and the wall sheathing and its nailing ties the rim joist, wall studs, floor joists, and mudsill into a single unit (this latter detail, together with the weight of the house, and the mudsill bolts is usually considered sufficient to resist vertical acceleration).

Getting back to horizontal acceleration, the bolting and A35s tie the floor diaphragm to the foundation walls, creating a shear wall system for the foundation (more important for a basement with its tall foundation walls than a crawl space). They also act to resist having the house slide sideways off the foundation along the axis of the joists. If you alternate the A35s from one side of the joist to the other as you proceed down the row of joists, then you also add strength to prevent the house from sliding sideways perpendicular to the joists. (This is for extra credit, the rim joists and wall sheathing already provide a lot of resistance to sliding in this direction).

Retrofitting a house could be accomplished with an angle drill (for installing foundation bolts) and a palm nailer (for installing the A35s). You can use expansion anchors or epoxied anchors for the sill bolts.

I am dubious of using angle irons bolted to the joists and foundation walls to resist horizontal sliding. The angle irons will tend to twist as the house moves sideways, tending to "work" the bolt holes in the joists and enlarge the holes. One or both of the wall bolts will be loaded in withdrawal as the house moves back and forth. This is the weakest direction for bolts retrofitted into concrete -- they are much stronger when loaded in shear perpendicular to the bolt axis.

The angle iron idea seems OK for holding the house DOWN, but not so good for holding it from sliding sideways.

Building departments in California offer a number of "standard details" for seismic retrofits, and you could pick one of those that is the most workable for your specific house. Washington and Oregon are not so enlightened, perhaps because the Northwest's subduction zone earthquakes are more rare (though no less severe) than California's active fault zones.

You might ask around and find a real structural engineer to look at your house and draw up a detail for you. Considering how much of your time you are going to invest in this project, a few hundred dollars is not that much money to invest, and in similar situations in the past I've found the engineering to be less expensive than I'd feared.

 

[TyTX]

Stan, Here's another answer to your problem. How difficult would it be to open the wall from the inside and drill through the wall plate, subfloor, mudsill and into the concrete? You can use epoxy packs to "glue" in a piece of 1/2" threaded rod to bolt down the house to the foundation.

Texas windstorm requirements mandate the use of plates or "Go bolts" from the foundation to the top plate. You are only trying to attach the house to the foundation so you only need to go from the foundation to the wall plate.

The problem with this plan is that you will be tearing up your wall board around the perimeter of the inside of the house but it may be easier than trying to do all that work in the limited access of a crawl space. Sheetrock is easy to repair.

 

[Stan in Oly, WA]

Thanks for taking the time to respond in such detail, David. Your advice is first rate, as I've noticed for several years that it always is.

Stan

 

[Stan in Oly, WA]

Thanks for the useful suggestion. Good example of thinking outside the box---if that isn't too worn out a phrase by now. I'm probably going to do what seismic upgrading I can in the crawlspace, then later open wall cavities as per your idea. I'm good at sheetrocking and taping, and I'm not timid about doing something that seems drastic when I know it will save time in the long run.

Stan

 

[Slowpoke]

Stan,
Go to Home Depot and look thru the Simpson Connector catalog. Lots of drawings and engineering info on seismic connectors.
I recently used their Titan HD bolts to bolt existing mudsills to a foundation. Drill the specified size hole, put a square washer under the head and screw it in with a 1/2 ratchet.
I first tried drilling with a wood bit thru the sill, then a hammer drill into the concrete. I gave up after about 15 minutes on the first hole. I went to Harbor Freight, got an SDS rotary hammer, Model 47606 for about $70 on sale, then bought a German made 5/8" SDS bit at a pawn shop. Drilling a 5/8' hole thru the mud sill and 5" into the concrete now takes about 20 seconds. It's the slickest tool I have.

 

[T_Bone]

Hi Stan,

To go along with TY's thoughts, I'd consider using 1-1/2" x 20ga galvanized strapping from the foundation wall thru to the roof joist, then screw No8's on 12"oc max into the stud face then use 1/8" or 3/16" red heads to attach the strap to the concrete foundation wall.

I have not checked code but I would think a strap would be required every 36"oc. That seams to be a popular number on vertical wall supports on typical residental construction.

You can also use the same size SM strapping to "X" brace the vertical & ceiling walls. Just use a 1-5/8" wide router bit on a guide to cut 1-5/8" slots in the sheet rock then screw the strapping to the studs/joists.

Any SM shop can shear the strapping cheaply in 10ft lengths and you'll get 400ft per 5ftx10ft sheets at approx $150/sheet sheared at todays steel/labor costs.

Any SM strapping offsets can be made with a wood mallet formed during install. A wood mallet doesn't stress SM when bending the SM like a steel hammer does. A very old "tinner" tip!

Also while I was working at the nuke plants, I was responsible for running a SM screw shear test. A 1/2" No7 SM tek screw (self drilling) thru two pieces of 24ga glav SM had a 2200psi shear point.

I was amazed at the high shear point and lost the test "pool" money...LOL, however I usally cleaned house on football and baby pool's.

T_Bone

 

[T_Bone]

I'll second that comment Stan as it's always a pleasure to read what David has to say about a subject.

T_Bone

 

[Anonymous-0]

Hadta check this thread...thought maybe "seismic retrofitting" was akin to "percussive maintenance"ie:"whackin it with a hammer" only on a huge scale...Guess not

 

[Stan in Oly, WA]

Hi Slowpoke,

H/F has two models of 1" rotary/hammer drills. If you type in the number for either of them on their site, the other one will show up as a small picture in the sidebar. I've had the other (cheaper) one for at least 4 years now, but I've never used it. I got pretty excited when you described how well the job went with that tool. I thought I had the same one because they look very much alike. Your model sells for $100 when it's not on sale, mine for $80. But mine is on sale right now for only $40. I guess I have to hope it's one of those rare instances where you get more than you pay for. H/F does have tools like that, but, unhelpfully, they won't tell you which ones they are.

Did you use the SDS bit to drill throught the mud sill and just keep on going into the concrete? That's what it sounds like you're describing. You must have had cripple walls on top of your foundation, but I don't. There's no way for me to drill straight down through the sill into the foundation without opening the wall in the living space above the floor as TyTX suggested. I was calculating clearances to see whether I could do it with an angle drill, as David suggested, when it occurred to me that there probably couldn't be such a thing as a hammer/angle drill. By the recommended spacing pattern I'll need to tie the sill to the foundation in about 28 places. My guess if that long, long before I was done using a regular non-hammer drill and masonry bits, I'd be praying for an earthquake to bring the house crashing into the crawlspace with me in it.

Thanks for the good news (I hope), Stan

 

[guido]

Hey Stan
If you have a concreted crawl space, a mechanic's
creeper is what you need. You can slide around pretty good and carry what you need a little easier.

Guido.