Bolting bracket to concrete wall

[Ralph2]

Hello
I have a bracket, 50" horizontal with a 24" vetical leg that I want to bolt to a concrete wall. The approximate load on this (spread evenly over the horizontal member)will be ~1000 pounds.

I plan to use (2) 3/4" bolts passing through the concrete and vertical leg.

My question... common sense would indicate the higher (closer to the horizontal)the better for the first hole. But.... what would be the minimum distance from this first hole to the second.

Both bolts will share the shear forces but the top one will see most of the tensile force. The farther away the second bolt is the less tensile it will see so it would seem to me that it should be as close as possible to the first. But the strength of the concrete with 2 holes drilled in close proximity concerns me..

Anyone with any thoughts... advise??
Thanks
Ralph

 

[UcfSE]

The farther away the second bolt is, the less tensile force it will see is right. That is a GOOD thing. Less force on bolts is good in this case. The minimum distance between the bolts is that which will induce tension that, when combined with the shear, will overload neither the bolt itself nor the concrete to which it is attached.

Will you have some sort of bracing or something to make the bracket rigid enough that you will not also have forces due to the bracket moving out-of-plane?

It is up to you to either calculate the forces and bolt capacities or hire someone who can. We cannot do that part, of course.

 

[Bagman2524]

Anchor bolt manufacturers such as Hilti have information on the minimum spacing between anchor bolts. The spacing is typically governed by a pullout shear cone, so that the bolts are not within the cone of another bolt.

Are you bolting the 50" horizontal part of the bracket anywhere? That seems like a bit of a long span to go without any bolts.

 

[Ralph2]

Thanks for the comments.
The horizontal portion will have a diagonal brace.. from 2/3 the distance (at a 30 degree angle.. mating with the vertical at 60 degrees. The bracket will be 4x4 x .375 structural steel angle, welded .. There will be 4 of these to support a 10 foot x ~50 inch wide addition to my house.. Making room for a main floor laundry, part of which will be within the current walls.

The comment about "Hilti" I will look into, but this is not quite like a pullout cone as I am drilling through.. and with a back up plate / washer..

Ralph

 

[Bagman2524]

How thick is the concrete you're anchoring to? The top bolt will be in tension which will try to pull your anchor plate through the concrete. The perimeter of the anchor plate and the thickness of the concrete will determine how much pullout force the concrete can handle.
Sounds to me your structure will be plenty strong enough, I would guess the top bolt should go somewhere near the top of the 24" L and the bottom bolt approx 12" below it.

 

[Ralph2]

Thanks Bagman..
The determining factor will be how close to the top of the concrete wall I can have the top hole.. Tenativly I had thought a minimum of 8 inches.. then the next one 10 inches below that.
My basement wall thickness is unknown at the moment.. I am assuming a standard 8 inches.. which seems to be the norm around here.

 

[UcfSE]

You will still have a truncated pullout cone. It just won't have a bottom. This situation yields less trength than a bolt with a similar embed in thicker concrete. The washer will be pushing on the back of the wall the same as a headed bolt would be pushing on the pullout cone in a cast-in-place application. The washer size and wall thickness will determine how much cone you have. So you still have to check the strength of the concrete as well as the bolt even though you have a through-bolt and not a typical anchor set in concrete.

Don't arbitarily set distances between bolts. The distance between bolts affects the tension each bolt gets. Too little distance will give you large forces. Too much and you may adversely affect your frame.

Again, you should make sure you check all of your numbers thoroughly and sufficiently or hire someone who can.

 

[Ralph2]

Thanks UcfSE..
The "steel" numbers have been worked out and verified as being sufficient with ample safety margin...
But the.. "So you still have to check the strength of the concrete".. part concerns me. The only tests I have ever seen on concrete are essentially compression factors. How much sideways force a concrete wall can sustain is kind of an unknown as there are many variables which are pretty hard to quantify.

Using simple geometry.. I assume a 1000 pound load at 32 inches from the wall then one bolt would have to hold 32000 pounds, if this bolt was lowered 8 inches the force would increase to 48,000 pounds (assuming a pivot point at the bottom of my 24 inch leg)which is not true as there will be another bolt to assist... but that calculation exceeds my simple geometry skill...

There will be 4 brackets spread over 10 feet. My basement wall is ~30 feet long and a standard 8 feet deep. Grade is some 12 inches from the top of the wall.The addition will be centered on this wall

My gut feeling is... the wall can handle it.
(should I be worried??)

 

[UcfSE]

If you have not done the proper engineering then yes you should be worried. 32000 lbs is a lot of force is it not? We cannot say "no, don't worry it'll be fine" over the internet. Anyone who does is out of line. That is a clear liability as well as ethics problem. The advice I can give you is to hire a professional. From reading your posts it seems you know just enough to get in trouble, but not enough to perform all of the required engineering calculations and judgment to do a thorough and complete job. The amount of sideways force a concrete wall can sustain is only unknown until you check it with a rational engineering analysis using the proper building code. Then it becomes known.

Again, you should make sure you check all of your numbers thoroughly and sufficiently or hire a registered professional engineer in your area who can. If you are not a strucutral engineer yourself, and you do not have the proper knowledge to actually check everything and not just the few things you know, then you need to hire someone. We're not going to check it for you or give you a "yay or nay". That is neither our purpose nor the function of this site.

 

[zcp]

I agree with UcfSE. You are not talking about a lot of investment for peace of mind. You may also get a cheaper / better / easier way to do what you are trying to do.

ZCP

http://www.phoenix-engineer.com

 

[Dinosaur]

I don't want to appear self-serving, but you should talk to an engineer about this. He will need access to your home to inspect the condition of the walls where you want to make these attackments. Checking the steel is the easiest part because all that will be new. Assessing the capacity of what is existing will be the real trick. One thing that has to be checked is the capacity of your basement wall in bending were you are making the attachment.

I think the guidance produced by the Hilti corporation is worth reading for your first question, but I think there are questions that need answers of which you haven't even considered.

Good Luck.

 

[Bagman2524]

I agree. Sounds a bit more involved then what it appeared at first. My intention was not to give an ok to something that requires a structural evaluation by a civil/structural engineer, merely an opinion that your structure should pass a proper analysis.

 

[Ralph2]

Hello all..
After I shut my computer off I realized I had made some major errors in my calculations.. What I have is moments of force around a pivot point. So 1000 pounds at 32 inches to the pivot= a reaction of 1333 pounds at 24 inches. Considering then only the vertical leg a force of 1333 pounds at 24 inches = 1999 at 16"

Much more reasonable numbers and I am sure the wall can handle that.

Thanks again for all the advice and concerns. For what it is worth... the city did require a PEng to approve this design.. but the matter of spacing the attaching holes was not specified.. nor was the wall inspected. It is still currently covered over (finished basement) and there may be some surprises when I remove the wall covering for access.. The whole thing may go back to the drawing board if I find large existing cracks in my wall.

Ralph

 

[BSE05]

I hate to say I'm old fashioned but I have never seen this done before in home construction. I don't know your site situation but a couple of posted foundations would be a more positive and safe solution.

I agree with Dinosaur, follow this advice.

I highly suggest you get a local structural engineer to check this out. You should be able to find one by calling a local land surveyor or civil engineer. They usually have a structural engineer that they use.

I prescribe to an analysis of ---"is the risk worth the reward?" In this case the risk of failure is high and after the fact the fix could be expensive!

Also you might have trouble getting a building permit without engineering sign-off.

 

[Ralph2]

Well the traditional way was considered however there were a couple of major drawbacks..

To keep my costs reasonable this is a one person project.. me..

As I can not get mechanical equipment into the back of the house I would have to drill the footing holes by hand... not something I fancy when the depth has to be in the 12 foot deep range. Then, placing the concrete into those holes would also be a problem..

And... in my limited experience an addition resting on footings seperate from the house will forever cause cracking. The house moves the footings move and the walls crack.

So... As I work in the steel industry.. my materials are at cost and my labor is free it "seemed" like a reasonable solution.. If.. in the end it did not work.. it will be an expensive repair.. but that is the risk I am taking