Ledger on concrete wall Calculation Check


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Old 04-12-13, 06:50 PM
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Ledger on concrete wall Calculation Check

Hello,

Through unforeseen circumstances I had to change the design of my deck. I was going to build a freestanding deck but will now need to use a ledger attached to a solid concrete basement wall. I am in the process of designing it and have proofed calculations for everything except the wall itself. I took a stab at the calculations; does this seem right?

I will be using hilti wedge anchors with a min embed depth of 2.75. To figure out whether or not the concrete will hold up I calculated the following:

Concrete Strength: Assumed 1000 psi. I dont have original plans so I erred on the side of caution here. Home was built in 1977. Any idea if a standard strength requirement was used then?
Bearing Surface: 2.15 Square Inches. Calculated by multiplying {[circumference (1.57) x Embed Depth (2.75)] divided/ by 2 (only half of total surface area will bear on concrete)} = 2.15 square inches.
Bearing Strength of Concrete holding Wedge Anchors: 2,150 Pounds 2.15 square inches x 1000psi = 2,150 Pounds

My calculated live and dead load for the ledger is 736LBS/Foot and I plan on using two anchors top and bottom spaced every 12. So, every foot I will have 4,300lbs worth of concrete bearing strength which is much greater than the planned load of 736 LBS/Foot. Do those calculations add up for you experts out there?

If it matters, FYI; top of deck will be 2-5 high, 11x9(ledger side), 2x12s 12 oc

I am still trying to figure out how to calculate lateral load, but that may be better suited for a difference topic/post so this one doesnt get off track.

Thanks for the help.
 
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Old 04-12-13, 07:07 PM
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I can't help you with the concrete strength (if you need it) but here is the prescriptive deck guide with W.A. spacing, spans, loads and lateral resistance hardware; http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf
Did you ask the local AHJ if they needed that info?

Gary
 
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Old 04-12-13, 07:51 PM
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You could even assume 2000-psi 2500 psi for concrete basement wall. That is because that strength is dues to the excessive water used in the mix for poured basement instead of the normal 3000 psir used.

I would check into you method of calculating the strength of the hilti anchors, but in any case, the strength of the concrete is not a factor. They usually have tables. Around here you cannot even get a good supplier to deliver less than 4000 psi concrete for a driveway.

dick
 
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Old 04-13-13, 06:25 AM
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Old 04-13-13, 10:39 AM
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Thanks for those attached references/guides as they should really help out.

Dick, none of the bolt specs, for any manufacturer, show a concrete strength of less than 2000 psi, which is why I was trying to compute for 1000psi. But, if I assume in my design that the concrete strength is 2000psi, I can avoid the hassle of calculating it myself and just use the bolt tables. Ill just do that; Im sure the reviewers might have a good idea of the strength of mix used in this town back in the late 70s. Like you said, I seriously doubt its less than 2000psi, but I was just erring on the side of caution. Any of the hiltis or simpson wedge anchors I use will hold up to my planned deck load.

Gary, that guide is great. It says in one of the first notes that other guides should be used when planning large concentrated loads for your deck (i.e, hot tub), but I should be able to use the basics out of it.

Chandler, those Tolcos seemed to be some attachments when used in combinations with wedge anchors. Im not really sure what benefit it would add to my design, but thanks for sharing anyways.
 
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Old 04-13-13, 02:31 PM
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Jgavin -

I was being very conservative with my numbers without knowing if it was site mixed, ready-mixed or just a really, really cheap builder.

Around here, you cannot even get concrete delivered for a driveway/garage unless it is over 4000 psf, but areas differ on acceptability.

Dick
 
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Old 04-13-13, 03:24 PM
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jgavin;
re. your figures.
You say the combined live and dead load is 736 lbs/foot; Is that right?
If the 9ft side is against the wall, that would mean the wall ledger is carrying 9 x 736 lbs, =6624 lbs, and presumably that will be one-half the deck, in which case your combined live and dead load would be about 130lbs/sq.ft., or perhaps I've not read your dimensions right?

If you allowed a total of say 50lbs/sq.ft, that would mean the total load on the ledger would be around 275 lbs /ft, which is a lot more manageable.

As regards your calcs on the bearing of the bolts in the concrete. It doesn't quite work like that because the bearing stress is not equally distributed along the length of the fixing due to the eccentricity of the load. It tends to be higher at the outer edge of the bolt and reduces towards the inner end of the bolt.

As you have seen, manufacturers are wary of giving precise figures for bearing as there are too many unknowns. Having said all this, the strength of your concrete is likely to be much higher than 1000lbs (as you have been advised).

I can't see any problem at all with your scheme, I think you have gone in for overkill, which far better than going the otherway!

cheers.
 
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Old 04-13-13, 04:24 PM
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Hey Tony,

You figured correctly, ~130lbs/sqft. I am designing to allow a hot tub to be put on top of the deck.

Good point on the bearing area; that makes complete sense. Thanks for the reply as I would have never thought about it like that.
 
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Old 04-13-13, 06:08 PM
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Vertical loads applied at anchor bolts in a c.i.p. concrete wall do not transmit straight down at and through the area of each bolt. Rather, compressive stresses are distributed outwards in the plane of the wall, from each bolt, much like a cone. You are more likely to have the anchor bolts fail in shear than having the concrete fail in compression.

Don't forget to stagger the bolt pattern, instead of placing them in pairs, directly in a vertical line. Doing so won't weaken the wall quite as much, and it also enables the anchors to develop more strength. Just follow the bolt manufacturer's spacing instructions, and you should be all right.
 
 

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