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Author Topic: Mount 12" box truss to confer block wall  (Read 1734 times)

Nathan Riddle

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Re: Mount 12" box truss to confer block wall
« Reply #10 on: October 14, 2017, 11:29:05 pm »

Then they do it without you and you have no part in it. 

How do you know that will never change?  "Cool - look at this cool truss thing!  We can have our youth group pull up competition on this thing!"

Agreed.

But that same argument can be used for anything that was done properly, but then is used outside of the initial safe means. (Wording??? Hopefully this conveys what I'm trying to say...)

I would put flags on it saying no more than 100lbs total loading. Just like other installed structures.

If I wasn't to do it but they 'really' want to do it, and I could somehow (unlikely) convince them to do it correctly.... who would I send them to? AllProSound is close by...? I ASSume they have people who can do that safely...?

P.S. I find your tongue in cheek statement about the kids doing pull-ups to be hilarious :) Thanks for the laugh!
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TJ (Tom) Cornish

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Re: Mount 12" box truss to confer block wall
« Reply #11 on: October 16, 2017, 09:03:39 am »

Agreed.

But that same argument can be used for anything that was done properly, but then is used outside of the initial safe means. (Wording??? Hopefully this conveys what I'm trying to say...)

I would put flags on it saying no more than 100lbs total loading. Just like other installed structures.

If I wasn't to do it but they 'really' want to do it, and I could somehow (unlikely) convince them to do it correctly.... who would I send them to? AllProSound is close by...? I ASSume they have people who can do that safely...?

P.S. I find your tongue in cheek statement about the kids doing pull-ups to be hilarious :) Thanks for the laugh!
Nathan,

There's a difference between a well-engineered system that is abused by the people that follow and an unengineered/unsafe from day one system that "hasn't fallen down yet so it must be OK".  An engineer would look at how something could be (reasonably!?) abused and factor that into the design.  For example, playground equipment doesn't have a sign saying "no one over 100 pounds may use this structure"; rather they build it so that if Cletus the 300lb drunk frat boy goes down the slide it won't collapse.  Do they make it out of solid concrete and able to withstand someone using a crane to drop a car on the slide?  Probably not, but that's a lot less likely.

Closer to home, a stick of truss is impressive looking, and it looks like it can hold a significant amount of weight, which it can if correctly supported.  I am not a structural engineer and I don't play one on internet forums, but common sense to me would be to design this system so that it could hold however many light fixtures (assuming that's the purpose) could fit on the truss - something like 500lbs rather than 100lbs.  Would I worry about someone trying to use it to winch the scissor lift up stairs?  Probably not.

Churches and non-profits are funny animals, and sometimes the almighty (lack of) dollar(s) is the only thing they fixate on.  Neither you or I can fix this mentality, other than doing our best to educate.  If that's not enough of a deterrent, all you can do is get the heck out of there.  I might go as far as mailing them a certified letter saying you were not involved this "solution", and you have informed them that you believe it is unsafe.

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Nathan Riddle

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Re: Mount 12" box truss to confer block wall
« Reply #12 on: October 16, 2017, 09:30:40 am »

Nathan,

There's a difference between a well-engineered system that is abused by the people that follow and an unengineered/unsafe from day one system that "hasn't fallen down yet so it must be OK".  An engineer would look at how something could be (reasonably!?) abused and factor that into the design.  For example, playground equipment doesn't have a sign saying "no one over 100 pounds may use this structure"; rather they build it so that if Cletus the 300lb drunk frat boy goes down the slide it won't collapse.  Do they make it out of solid concrete and able to withstand someone using a crane to drop a car on the slide?  Probably not, but that's a lot less likely.

Closer to home, a stick of truss is impressive looking, and it looks like it can hold a significant amount of weight, which it can if correctly supported.  I am not a structural engineer and I don't play one on internet forums, but common sense to me would be to design this system so that it could hold however many light fixtures (assuming that's the purpose) could fit on the truss - something like 500lbs rather than 100lbs.  Would I worry about someone trying to use it to winch the scissor lift up stairs?  Probably not.

Churches and non-profits are funny animals, and sometimes the almighty (lack of) dollar(s) is the only thing they fixate on.  Neither you or I can fix this mentality, other than doing our best to educate.  If that's not enough of a deterrent, all you can do is get the heck out of there.  I might go as far as mailing them a certified letter saying you were not involved this "solution", and you have informed them that you believe it is unsafe.

Hi TJ,

Thanks for the in depth clarification and your stance.

And I do understand the engineering process. I was simply trying to provide an example where people exceed ratings (which is why engineers should over-engineer to a given safe extend [can't protect everyone, from everything because costs would be astronomical; but we can protect against 'normal' abuse].

As well as the HOW 'no money' yet 'gonna do it anyway' attitude. [which is why I'd rather help than not, because I'm going to do it more 'right']

---

I think we differ on how (myself or another) would engineer this truss stick on the concrete wall.

I would use no less than 8 concrete wedge anchors with wrap around 2" clamps.

When using a 3/8" anchor with 3" depth I have a tension force of 3229 lbs or with a 5:1 safety factor 645.8 lbs per wedge anchor.

That means a total force of 5166.4 lbs could be pulled out of the wall straight (sheer forces would be more, but the force will be a combination of the two [simple to calculate, no real need as the forces always be much less]. The truss max loading is: 5746.5 lbs. Thus both the truss and the wall would fail at the same time roughly with a safety factor of 5:1 for concrete anchors (unknown on trussing).

In the end, yes I over engineer because I know the end user will probably exceed any initial expectations as they are uneducated fools.

I never said I would only make the trussing safe to 100lbs I would make it safe to 5000lbs with a max load sticker of 100lbs (make it 300lbs so they can have more) if necessary.

That's enough to make it both safe for the '300lb kid who wants to do pull-ups' and any other idiotic things they wish to do.

At the end of the day I've accounted for ALL pieces of hardware in the install and made sure it was safe. Rather than running away because it is a liability for my company. (I would rather accept the minimal risk from my over engineering than them doing it wrong and hurting/killing someone).

https://www.confast.com/products/technical-info/thunderstud-anchor.aspx

Quote
Values shown are average ultimate values and are offered only as a guide and are not guaranteed. A safety factor of 4:1 or 25% is generally accepted as a safe working load. Reference should be made to applicable codes for the specific working ratio. *Tested by ATEC in accordance with ASTM E488-90 and ICBO ES AC01. Minimum embedment for satisfactory anchor performance is 4-1/2 bolt diameters. Deeper embedments may yield higher tension and shear capacities.
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Re: Mount 12" box truss to confer block wall
« Reply #13 on: October 16, 2017, 09:50:07 am »

You can talk all you want about the hardware involved, but it's a "cinder block wall".  You have to take into account the properties of the wall and the possible consequences resulting from a truss anchor point failure.  To maintain an acceptable load safety margin may well require reinforcing or re-engineering the wall itself.

I would look long and hard at providing support under the truss from the floor up and anchored to the wall to eliminate the whole issue of shear and cinder block anchoring capacity.
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TJ (Tom) Cornish

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Re: Mount 12" box truss to confer block wall
« Reply #14 on: October 16, 2017, 10:02:45 am »

Nathan, I agree that it's possible to be overly concerned with liability and therefore be afraid to be involved when there is some risk of injury or damage (which is in virtually every situation).  That said, attaching to permanent structures and modifying them is a different level.

You mention the set of anchors rated at 5000lbs pulling force after a safety factor.  What is the wall you're putting the anchors in rated for?  What is the condition of the wall?  If there is some spalling of the concrete, all bets are off.  BTW, a 3" anchor into a block wall doesn't have the holding power of a 3" anchor into a slab as the cinder block is only 1" thick or so. 

You haven't said how much truss - is this one 10' stick?  50'?  What are they intending to put on it - lekos/pars?  Video projectors?  Movers?  Where did you come up with your 100lb max weight number?  Will this be over people's heads?

One last question - why truss instead of schedule 40 pipe which is a more accepted way to hang lights (assuming lights)?

People like you and me can make reasonable guesses and to some degree reduce likelihood of failure by adding more things - wall anchors in this case, but an engineering/theater consulting company will have generally accepted methods for doing things like this that are backed by testing and engineering stamps.

One last point - I think you will find it difficult to adequately line up 4 pairs of holes for cheeseboroughs tied directly to the wall.  A better solution would be a set of metal brackets that you attach to the truss first, then hold the truss in place, align/level, then drill  through the bracket holes into the wall.

Not all of the above may be material issues n your circumstance; however I think it's worth pointing out the legitimate issues for consideration, especially the idea that someone who has done this before may know things that we don't, and the result may be different or better in ways we haven't thought of.
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Robert Healey

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Re: Mount 12" box truss to confer block wall
« Reply #15 on: October 16, 2017, 12:51:36 pm »

Nathan,

It would probably only cost a few hundred dollars to have a structural engineer give you a stamped drawing that calls out all the hardware you need to use. Then the liability of the design is on the engineer.
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Nathan Riddle

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Re: Mount 12" box truss to confer block wall
« Reply #16 on: October 16, 2017, 03:10:22 pm »

I think I wasn't very clear about my intentions. Sorry.

I'm not doing this 'yet' and I haven't spoken to them yet to say they're insane to think the aforementioned plan would work.

I've switched into thinking about how to do this properly IFF they ask me to do it. Currently, I'll stay away unless I can it done 'right' = structural engineer, proper hardware, etc.

Nathan,

It would probably only cost a few hundred dollars to have a structural engineer give you a stamped drawing that calls out all the hardware you need to use. Then the liability of the design is on the engineer.

$750-1k I would imagine for my area. Dunno why so expensive. That's what I've been quoted on for other jobs.

You can talk all you want about the hardware involved, but it's a "cinder block wall".  You have to take into account the properties of the wall and the possible consequences resulting from a truss anchor point failure.  To maintain an acceptable load safety margin may well require reinforcing or re-engineering the wall itself.

I would look long and hard at providing support under the truss from the floor up and anchored to the wall to eliminate the whole issue of shear and cinder block anchoring capacity.

Hmm good point, though I guarantee the stage floor would break long before the block wall breaks.

I'm ASSuming they are going for the 'look' of flown trussing without the risers from the floor (as they've done that before).

Nathan, I agree that it's possible to be overly concerned with liability and therefore be afraid to be involved when there is some risk of injury or damage (which is in virtually every situation).  That said, attaching to permanent structures and modifying them is a different level.

Good point. Wisdom and caution should be used even if the task seems 'easy/simple'


You mention the set of anchors rated at 5000lbs pulling force after a safety factor.  What is the wall you're putting the anchors in rated for?  What is the condition of the wall?  If there is some spalling of the concrete, all bets are off.  BTW, a 3" anchor into a block wall doesn't have the holding power of a 3" anchor into a slab as the cinder block is only 1" thick or so. 

True, though going into the mortar joint would make sense. Also, i'm not sure if the blocks are filled are not (probably not).

You haven't said how much truss - is this one 10' stick?  50'?  What are they intending to put on it - lekos/pars?  Video projectors?  Movers?  Where did you come up with your 100lb max weight number?  Will this be over people's heads?

One last question - why truss instead of schedule 40 pipe which is a more accepted way to hang lights (assuming lights)?

9ft (3m) 3x sticks one on each wall of the /---\ stage walls.

One mover on each truss stick.

ASSuming, over head height, yes. (though people aren't going to stand directly under this doesn't change the safety though).

Probably because it isn't 'pretty' they're going for a new stage design.

People like you and me can make reasonable guesses and to some degree reduce likelihood of failure by adding more things - wall anchors in this case, but an engineering/theater consulting company will have generally accepted methods for doing things like this that are backed by testing and engineering stamps.

Hmm, very good point. Though, I doubt anyone in our area that will do it within their price will provide those 'testing & engineering' stamps. I could be wrong though.

One last point - I think you will find it difficult to adequately line up 4 pairs of holes for cheeseboroughs tied directly to the wall.  A better solution would be a set of metal brackets that you attach to the truss first, then hold the truss in place, align/level, then drill  through the bracket holes into the wall.

Haha good point, Yeah I've butchered lining things up in the past for sure. I think the grout lines would help.

Perhaps I should think about using 6x anchor points instead of 8x, though if an actual company came in here and did that I betcha they would use 4 ;)

I thought about flying to the structure above with approved methods.

I didn't think about brackets (because who makes brackets that are acceptable for these purposes). [I'm not inclined to make my own or have some welded up (because who knows if local fab shop can provide engineering load stamps on the brackets).]

Not all of the above may be material issues n your circumstance; however I think it's worth pointing out the legitimate issues for consideration, especially the idea that someone who has done this before may know things that we don't, and the result may be different or better in ways we haven't thought of.

True, I'm going to call them today and try to convince them to both: NOT use the TV wall mounts. AND go with a engineering/theater company who can provide stamped documents proving the suitability of the mounts as being safe.
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Re: Mount 12" box truss to confer block wall
« Reply #17 on: October 16, 2017, 05:10:50 pm »

Nathan...

Don't use the floor as a base of support.  You'd need to go down to foundation level, pour or install a proper pad, then use proper sized beams up to or through the floor continuing as high as needed to properly bear the load.  This would be proper even if the walls were solid masonry or concrete rather than block.  Lagging mounts into side walls like that is just not right.
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Nathan Riddle

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Re: Mount 12" box truss to confer block wall
« Reply #18 on: October 16, 2017, 05:18:16 pm »

For a 100lb load?

Nathan...

Don't use the floor as a base of support.  You'd need to go down to foundation level, pour or install a proper pad, then use proper sized beams up to or through the floor continuing as high as needed to properly bear the load.  This would be proper even if the walls were solid masonry or concrete rather than block.  Lagging mounts into side walls like that is just not right.



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Re: Mount 12" box truss to confer block wall
« Reply #19 on: October 16, 2017, 06:11:46 pm »

For a 100lb load?

Serious.  The joke is assuming that you can lag a load into concrete block.
And once the truss is there, who's to say what someone might try to hang from it or hoist with it in the future.

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« Last Edit: October 16, 2017, 10:03:45 pm by dick rees »
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