Sistering studs

[kfc510]

Yeah, uh, as already mentioned, two engineers have already weighed in on the foundation.

Neither was particularly interested or concerned about doing details for the simple foundation replacement. (There are other structural improvements that may or may not get done, and there has been some disagreement about the details there but that is where both engineers said, "yeah I should draw that up for you.")

There is no attempt to strengthen the existing wall above the foundation. There will be no hold-downs for any shear panels in this run of wall. (At the risk of confusing things there may be hold-downs for perpendicular walls, but forget that for now.) This is (at this point) simply a voluntary seismic upgrade to replace a failing run of foundation.

[Bill Robinson]

Keeping it simple is always best.
It appears you understand the structural issues.

When I was doing work on SB I partnered up with an engineer who understood my needs and was willing to do details on seismic and structural retrofit details.
I would go to him with ways I would approach a job, he engineered details to fit the approach and BAM, we had details and since we had a relationship it did not cost an arm and a limb.

Tho I do have to wonder about the term

simply a voluntary seismic upgrade to replace a failing run of foundation

Sounds more like a voluntary foundation repair

Seismic upgrade speaks to actual seismic details.

It is just the terminology I guess<:}

I would be looking for an engineered detail that could be used again in other applications for a nominal wet stamp fee.

[kfc510]

Tho I do have to wonder about the term

Sounds more like a voluntary foundation repair

Seismic upgrade speaks to actual seismic details.

It is just the terminology I guess<:}

I would be looking for an engineered detail that could be used again in other applications for a nominal wet stamp fee.

Yes, for anyone who isn't doing structural retrofit work here on a regular basis it probably is confusing.

"Voluntary Seismic" is the key code term that gets you through the permit process easier and faster. The bay area cities have active programs to encourage "voluntary seismic" upgrades to the existing housing stock. It means the work is not required (as part of a larger project, for instance, like adding a floor or addition.) Nothing else is changing other than making the existing house safer in an earthquake.

Now, where it gets interesting is that there is no code for this. There is no law in CA to generally bring existing buildings up to modern structural code. So in a voluntary seismic job you get to pick and choose where you decide to strengthen the structure.

To date I have met with three other licensed contractors who specialize in structural work and two licensed engineers who have looked at the house. Every one has had some different ideas about what ought to get fixed second, but every one of us has agreed that the one run of foundation needs to be fixed first.

The way it works is you get a call from a client. They have either recently bought a house (and therefore have a local tax rebate coming if they put the $$ directly into structural repair) or they have become concerned about earthquake risk because of recent little shakers, or whatever.

They say: "I have x thousand dollars that I want to put into making my house safer. What should we do?"

You then go over the property with them, and point out where you think x thousand dollars would do the most benefit for that structure when the Hayward Fault goes. Sometimes (not always) you bring in an engineer (or two) to do the same paid diagnostic work.

If you bring two engineers out, they will have their own approaches and concerns, and will often come up with different structural repairs that they think ought to be prioritized. In this case, everyone agrees that the beach-sand foundation on the one side is priority 1. If you can fix one thing, fix that. ASAP.

It is enlightening to hear two different experienced engineers walk through the same building (or two experienced builders, for that matter). They walk through the same structure, but they see different priorities. Again, we are dealing with a limited amount of money, the question is "where do we get the most structural return for our $$$", not "how do I bring this house up to current structural code?".

There is no official "right" answer. One guy will tell you you'd be nuts not to address issue "x", the other guy is saying if you blow your budget on "x" but don't upgrade "y" it's terrible.

You begin to lose the mindless reflexive response ("you need an engineer to tell you how to nail those boards together", "code for new buildings says blah blah blah"), and you start to look at it more holistically. It helps if you have a degree in architecture, and a year's worth of structural engineering under a very good professor, and/or a whole lot of experience.

You got 10k to throw at your house, and a 40' run of foundation is failing terribly? Fix the damn foundation. Don't spend all your money to fix a ten foot section of the foundation, complete with engineered shear panels all the way to the roof, in perfect rigid compliance with "the rules", and then have to leave the other 30' of beach sand concrete as is because you spent all the client's money in a fit of self-indulgence on that "up to code" 10' section, fix the entire damn run of foundation. ASAP.

This is what the two engineers paid to visit the site have said (in addition to me and the builders too). Both engineers said specifically that they were not needed to draw up the foundation detail- the basic detail is the basic tee footing with the basic rebar and the basic AB's. The city has a boilerplate detail I can copy if I want, but it's the same as what's in my textbook.

And, no, the engineers do not generally want to draw up things like how to actually connect the (E) studs to the (N) sill plate. Thank god. The last time an engineer did that kind of detail for a job I was on it was physically impossible to actually do what he drew- he wanted us to slide the new 3x plate into a perfect 2 1/2" space between the top of (N) concrete and the bottom of the (E) studs, so that the studs would rest perfectly on the new 3x plate, LOL. Anyone with any experience at all is going to know those studs will need to be trimmed well short to actually wrangle a real, wet, twisty P.T. 3x into that little strip of space. No shims in his drawing though, uh oh, is that ok?

Never mind that he had spec'd wet-set AB's that were sticking up in that 2 1/2" space, to hold down the very plate he wanted us to slide in, so it was physically impossible to do what he drew up, even if you magically were able to slide a 3x into a perfect 2 1/2" slot.

What did we do in that case? Cut the studs high, dropped the plate in and over the anchor bolts and sistered down to the new plate after it had settled in. I didn't actually do the sistering, but the carpenter who did it did it much like dgbldr talked about- sisters on either side, block (with shims) underneath. No one even mentioned that particular connection as finally carried out, not the engineer, not the inspector, etc.

[Ted S.]

When I was a young college kid, this is what my structural engineering instructor said:

"Nine times out of ten, if you draw it and it looks good, it will stand up. Structural engineers aren't absolutely necessary to make a building that will stand, they just make the construction more economical"

I never forgot that.

[Bill Robinson]

Yes, for anyone who isn't doing structural retrofit work here on a regular basis it probably is confusing.

Thanks for that, it helps me to understand where you are coming from.

Confusing in that it does not make sense.

You are right about that, I no longer do seismic retrofits.
However in the middle 90s we did a lot of it.

From what you are saying it has changed, and it does not sound more stringent but sorta, well....

whatever.

voluntary seismic used to mean doing some structural strengthening, this does not look or sound like that.

Wonder is Dick has had a hand in this?

[kfc510]

whatever.

voluntary seismic used to mean doing some structural strengthening, this does not look or sound like that.

Geez, Bill, on what planet does replacing a failing foundation not mean structural strengthening? You seem to think "structure" starts at the studs and goes to the roof. In a seismic event the foundation is literally the first structural element to receive force. Whatever, indeed.

[Bill Robinson]

OK,I appreciate the lesson.

Thanks

[kfc510]

When I was a young college kid, this is what my structural engineering instructor said:

"Nine times out of ten, if you draw it and it looks good, it will stand up. Structural engineers aren't absolutely necessary to make a building that will stand, they just make the construction more economical"

I never forgot that.

And the economic benefits of good engineering are not to be sneezed at. Or good architectural design, for that matter.

Still, when all the engineers have stood there saying a) "That right there is your single weakest element in event of an earthquake" and b) "there's no reason for me to draw that for you" it would seem presumptuous to ignore them...

kevin

[Ted S.]

If I remember correctly, that junction you are working on is pretty hot when it comes to seismic, it experiences "base shear" ... lateral design was much less intuitive topic than gravity, I don't much remember it.

[kfc510]

If I remember correctly, that junction you are working on is pretty hot when it comes to seismic, it experiences "base shear" ... lateral design was much less intuitive topic than gravity, I don't much remember it.

I actually really enjoyed structures. I was in school to learn real technical things about building, so even though I was not required to take the class for my BA I did so gladly. (edit to add: I also plainly sucked at the drawing/rendering skills that the future architects I was in class with excelled at.)

I actually was invited to join a graduate structural seminar the next semester because of my aptitude. And, the professor for both was R. Gary Black, a very good teacher. He also works professionally as an engineer, formerly with Christopher Alexander, now for his own company, Integrated Structures.


As to lateral forces, the ground moves (generally modeled as a simple horizontal movement creating shear) which yanks sideways on the foundation, which yanks sideways on the anchor bolts, which yank sideways on the sill plate, which yanks sideways on the attached sheathing and the end-nailed studs, and on up. The shear force is basically the same at each connection from the foundation up to the center of gravity.

So (in the most basic analysis) you kind of start at the bottom and ask, "when the ground yanks on the foundation, will the foundation fail?" If yes, then you fix the foundation. Then you ask "when the foundation yanks on the sill plate (via the anchor bolts) will the plate and/or bolts fail?" if the answer is yes, then you add strength there, and so on.

In terms of the performance of the house, the horizontal yanking is usually theoretically modeled as a force yanking laterally in both the x and y axis acting *at the center of gravity of the house*, but this is actually the counterforce of the house's inertia. The ground is doing the yanking, not invisible strings attached to the house's belly button.

The distance between the force at ground level and the inertial counterforce acting at the c.g. induces a rotational force to the body as well. The rotational force is harder to counteract in the narrow direction, and is less of an issue in the long direction. The weight of the house itself is often enough to counteract much of the overturning force in the long direction. Not all of it, but much of it.

In the case of the particular project I'm looking at there are a number of openings in the exterior wall above the failing run of concrete, so that whole run of wall is a poor area to throw money/work at to carry the loads from foundation/sill plate to the top plate/rim joist. I could put a number of narrow site-built shearwalls, Strong- Walls or Hardie-Frames in the exterior wall at great cost, but that would be stupid from a cost/benefit standpoint.

There are better (easier, cheaper) ways to tie the 2nd floor diaphragm to the footing and to the ground in that axis. The center bearing wall (in the same axis) has a long uninterrupted span that will be utilized to tie the ceiling/floor diaphragm to the central footing if the client decides to go that far.

But even if she doesn't go that far, fixing the damn foundation is still a great idea.

kevin

[Ted S.]

Sounds like you have it covered, kevin.

I never got much joy out of lateral design, gravity was cool. The Gothic cathedrals were my favorite to learn about.

You definatley retained more about the lateral then I did.

[kfc510]

Thanks Ted.

I actually really enjoy geeking out on how structures are working in their real situations.

But honestly, sometimes I feel like I need a letter from my old professors when I post around here. A little thread about sistering studs and I get multiple warnings about engineers and wife-beaters. Sheesh. I realize it should be said once, but once is enough.

I wonder what will happen when I start the next thread about shims in a load-bearing wall. Maybe I should get a wet-stamped letter from my engineer first.

kevin

p.s. at some point a real engineer will weigh in and point out the gross generalizations in what I wrote too; I actually look forward to that.

kevin