Attaching tension cable to concrete block wall

[DeSimulacra]

I'm doing some charity work that involves putting a wire rope under tension between two block walls.
I can't find anything down and dirty on this subject.
Can someone point me in the correct direction?

Specs.
>12" block walls
>distance between walls is 60'
>load on cable is 350# max. with possible doubling of weight due to shock.
> plan to sandwich blocks between steel plates with eyebolt thru center.
> 3/8 DIA. wire rope is available.

ATTACHED IS A QUICK SKETCH

 

[btrueblood]

What is the droop/sag allowed for the cable? This will set the pre-tension necessary in the cable, and allow you to calculate the tension force (total) in the cable.

Then you need to determine pull-thru strength of the concrete block. And overturning resistance of the block wall. And probably a few more things I'm forgetting.

I notice you posted here instead of the structural engineering forum...

 

[GregLocock]

What you got there, me old mate, is a great way of pulling fixtures and bricks out of walls, or walls out of the ground. What you have drawn will work fine for washing lines, 3/8" steel cable, not so much.

Anyway, the tension in the cable has to support half of the maximum shock load+any temperature effects+wind+settling of the walls+all the other excellent suggestions you will receive in this thread.

The simple answer is T*sin(theta)=V/2 where theta = atan(centre deflection under max load*2/span), so paradoxically the greater the unladen sag the easier the job (except that your shock loads will probably increase).

If human beings are ever anywhere in the vicinity think about using a factor of safety of more than 10.

Also see the thread on snapping winch cables, being tickled by a 60 ft bight won't make anybody happy.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tmoose]

Not knowing the construction of the wall, I'd be concerned about pulling out some blocks. If it's a freestanding wall I'd worry about tipping it over.

So I'd contemplate-
-A vertical strong back attached to floor and upper deck.
-Surface bonding both sides of the wall, floor to ceiling, for maybe 4 feet either side of the attachment.

 

[BobM3]

Is there a vertical compressive preload on the concrete block (like from a roof)? You have to make sure the concrete block is in compression at all times.

 

[chicopee]

I came across a situation similar to yours,however, it was to mount an exhaust ventilation hood and duct system loaded with automatic fire extinguishing system. Not having details like those mentioned above by the participants about the wall, I decided to design a free standing frame instead to support all equipemnt including the roof exhaust fans.

 

[DeSimulacra]

Thanks for all the replies. I feel I can design this safely now with the info you've given me and that is by deciding I don't know enough to use the block mount. So I changed my design.
The cable size had gone down to 1/4" (got into sag) and is still oversize, may go down to 3/16".
I also decided to go with a steel cross member and attach to trusses. While the load I'm sure would be ok on the blocks, with bonding plates. I know how to figure the steel beam factors and loads.

Thanks for the great advice,
Mark

 

[chicopee]

Be careful about the trusses. If the trusses are open web steel bar joist make sure that they are designed for concentrated loads. On OWSBJ's, check out one of the ends as they have a metal tag atteached by wire indicating wh is the manufacturer.

 

[Ron]

Guys...this is commonly done. Make sure there are reinforced, grouted cells in the masonry and you'll be fine. Your load is compared to what a reinforced masonry wall can take.

If the wall is not reinforced and grouted, then you have to be concerned about pull-through on a block. If that's the case, make sure your backing plate engages more than a single block.

Since you have a 12-inch masonry wall, it is likely to be reinforced and grouted. Further it will have better shear resistance in the mortar-block interface.

If you consider that you have about 96 square inches of mortar bond contact for each block (8x16 block, inside face mortar and outside face mortar, joint at least 1" wide on block contact shell....before someone yells at me, I'm not talking about the thickness of the mortar joint which is 1/2" to 5/8"), and even if you consider a minimal shear bond strength of 25 psi, you have a resistance of 2400 lbs.

Your backing plate needs to be larger than the core of the block, so your plate needs to be at least 8"x8".

 

[hokie66]

DeSimulacra,

In summary, it's all about the wall, not the wire rope. If you know how that the wall is reinforced and core filled, fine. If not, don't do it. There are far too many kids killed each year because well-meaning fathers have hung a basketball goal on an unreinforced section of masonry.

 

[DeSimulacra]

Hey Hokie I know exactly what you are talking about. I'm overly cautious I guess but in my 30 years I have yet to design something that hurt someone. Building one off's I intent to keep it that way by being scared, very scared to under design something. My motto is when in doubt build it stout.
Ron I'm sure you are right and if absolutely necessary I'll look at this again but feel that I would rather spend a few extra bucks and stay in my safe zone.
chico I work with steel everyday and have designed buildings so I'll be ok there but thanks.