DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Bricks, Masonry and Concrete > How close can poured footing get to design height? #seismic




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Old 08-21-2011, 01:45 AM  
redwood1922
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Default How close can poured footing get to design height? #seismic

What is reasonable to expect of a concrete contractor who helps repair or retrofit an existing house by adding partial new concrete footings with anchor bolts? I expect that carpenter will use no shims at all to support the house, because Simpson and USP connectors seem to require continuous wood surfaces for seismic rating. If a moderately skilled craft person produces a footing, how close is the top of the concrete footing to design height - or equivalently, how close will the result be to the design clearance for top-of-concrete to bottom-of-girder? Like, within 1/4 inch of whatever specification is on the drawing? Or more like 1/2 inch or 1/8 inch? Will the tops of footings be close enough to adjust for any difference by using a planer to set final thickness of new supporting members? Is the planer idea practical?

I've been looking at readymade plan sets for voluntary seismic upgrades. For example, 2x blocking can lay flat over a new section of mud sill to support an existing girder. Actually the blocking is "thickness as required" according to a FEMA plan set.

The house will be sitting on this blocking. So I think Simpson/USB connectors had better have a nice solid grab on the blocking to connect to girder above and sill or other member below. If the blocking can't be shaved down too much then the wall can't be excessively higher than design. Also, the Simpson and USB catalogs seem to rule out shims for connectors to remain load-rated. So the wall can't be excessively lower than design. I've seen plenty of concrete work that would not be accurate enough for this job I'm describing. That's why I want to know.



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Old 08-21-2011, 10:54 AM  
BridgeMan
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Not really sure exactly what you want to know. Most industry and building codes have construction tolerances, although they are not always adhered to. Depends on the level of inspection provided by the permitting agency, some of which can be pretty loose, for lack of a better term. When I was in the bridge building business, we were happy to get within an eighth of an inch, both vertically and horizontally, in the structures we were building.

I think if you are concerned about major earthquakes and the damage they can produce, there are better things to worry about than fraction of an inch tolerances. Such things as concrete strength, rebar placement, etc. Brings to mind a new spec home being built near ours in SW Colorado a few years ago. One Sunday afternoon, I had occasion to check out the footings and stemwalls had been poured the previous Friday. I happened to kick one of the many piles of waste concrete scattered around, and it broke completely open and literally crumbled. This is definitely not normal behavior for 48-hour-old concrete, so now my curiosity was aroused. Further examination showed that all of the concrete on site was similarly weak and degraded, and none of the exposed piles had any aggregate in the mix larger than a quarter inch in diameter. The concrete had the appearance of very weak mortar instead of concrete.

I called the County Building Department the next day, and was bluntly told by one of the inspectors that they weren't concerned about the quality of the products being used, as that was strictly the homeowner's and contractor's responsibility(!).



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Old 08-21-2011, 09:37 PM  
nealtw
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I agree with Bridgeman here You should spend your time understanding the concrete to be ordered and be onsite to make sure someone isn't changing the slump when your not looking, Most times the footings a pored low and a box is installed to raise concrete to height at the bearing point with level pore strip at proper height. Which simpson tie are you planning on using.

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Old 08-21-2011, 10:50 PM  
redwood1922
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Default Thanks BridgeMan, and clarification

Thanks, that's great, 1/8 or even 1/4 inch accurate is great. Understand I'm not worried about "tolerances" for some nerdy reason. I have very low clearance in crawlspace. Where some people have a 2 foot pony wall, I'm gonna have 2-inch, maybe 3-inch blocking, possibly planed down slightly if concrete comes out a little high. What worries me a little is concrete coming out a little too low,and needing shims. I need those connectors to hold the loads they're rated for, else the job is pointless. As far as I know seismic spec connectors are not load rated to handle shims.

Good point about concrete details. Engineer will see standard plan sets but I'm sketching how the standards apply to my house specifically. I'm not gonna pay an engineer to start from scratch.

As for inspections, permits, and such...off topic but there are way fewer bureaucracies involved than you might think. I had a long talk with building and planning recently. Most of the job is new spot footings where posts are resting on grade. There is no inspection of new concrete spot footings! For a whole host of reasons. There is no code for voluntary seismic improvement of a 1922 post and pier house. House doesn't meet current code and never will, and they only inspect to code. My state and some others in earthquake country do not want to do anything to discourage a homeowner from making a voluntary seismic improvement. OK, I exaggerate a little: There are some restrictions, plumbing and electrical changes subject to normal rules, value of work cannot exceed 50% of assessed value of structure, contractor must be licensed to do this kind of work. Heck, engineering isn't absolutely required, if contractor doesn't see need. Like I said I do expect to run my plans by an engineer though.


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Not really sure exactly what you want to know. Most industry and building codes have construction tolerances, although they are not always adhered to. Depends on the level of inspection provided by the permitting agency, some of which can be pretty loose, for lack of a better term. When I was in the bridge building business, we were happy to get within an eighth of an inch, both vertically and horizontally, in the structures we were building.

I think if you are concerned about major earthquakes and the damage they can produce, there are better things to worry about than fraction of an inch tolerances. Such things as concrete strength, rebar placement, etc. Brings to mind a new spec home being built near ours in SW Colorado a few years ago. One Sunday afternoon, I had occasion to check out the footings and stemwalls had been poured the previous Friday. I happened to kick one of the many piles of waste concrete scattered around, and it broke completely open and literally crumbled. This is definitely not normal behavior for 48-hour-old concrete, so now my curiosity was aroused. Further examination showed that all of the concrete on site was similarly weak and degraded, and none of the exposed piles had any aggregate in the mix larger than a quarter inch in diameter. The concrete had the appearance of very weak mortar instead of concrete.

I called the County Building Department the next day, and was bluntly told by one of the inspectors that they weren't concerned about the quality of the products being used, as that was strictly the homeowner's and contractor's responsibility(!).
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Old 08-22-2011, 08:53 PM  
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Hi, nealtw, thanks for your attention, and also on the other thread.

What Simpson ties am I planning on using you ask. Short answer is L90. Or maybe twice as many A23 or GA2, which I think would be useful in several other places so there'd be fewer different parts to buy.

Longer answer: Standard plan set shows 2'x2' spot footings every 8-10 feet, with four #3 horizontal rebar but I'm asking for some vertical also. I'm hoping engineer will OK 3/4 inch galvanized anchor bolts, 10+ inch embedment, 16 inches apart, for a 2x10 x 24 inch "mini mud sill" centered under existing 4x4 girders and 2x6 end joists. Before releasing the jacks lay down and connect a 2x8 x 24 "mini plate" to make up gap between sill and girder, yes it's that low clearance down there, the plate is all there's room for, right where some other house has a 2 foot pony wall. Similarly use 6x8 x 24 where there's a gap between sill and end joist. Sill to plate could be L90 each bay, or maybe twice as many A23 or GA2. Plate to girder, again L90 or pair of A23 each bay. Standard plan set also shows MTSM20 (kind of like a long H hurricane tie) from concrete to girder, to resist uplift. Joists to girders, there's H10A, though those don't work on end joists. Your idea from other thread of blocking and strapping every other bay sounds like some kind of blocking could also make it easier to secure end joists, thanks again for that.

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I agree with Bridgeman here You should spend your time understanding the concrete to be ordered and be onsite to make sure someone isn't changing the slump when your not looking, Most times the footings a pored low and a box is installed to raise concrete to height at the bearing point with level pore strip at proper height. Which simpson tie are you planning on using.
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Old 08-22-2011, 09:56 PM  
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I wasn't paying enough attention to see both posts were for the same house.
When poring your footings I would be looking for a tie down that is included in the concrete and ties everything together all the way to the wall above in two places on the wall. Block between the joist just there and your floor will be stable.



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