Bending a bolt 1

[asixth]

Hi guys

I have been asked to rate/certify the structural components of an existing ceiling. I have managed to get into the ceiling space to measure the ceiling framing which is supported from the roof structures at 4000 centres (13') by bending a 12mm dia bolt (1/2") and hanging from the web of the purlin. My initial reaction is that this connection cannot be accepted and that the connection detail should be replaced with a more acceptable alternative.

Would anyone else be prepared to accept the bolt in its current state?

I estimated the weight of the wood ceiling frame to be 2,000kg (4.5kips) and 3,400kg (7.6kips) when the ceiling is fully clad with plasterboard with the maximum tension in the rod to be approximately 1/10th of this load.

 

[ACtrafficengr]

Is it redundant or fracture critical (US bridge language for a structure likely to fail catastrophically if a key member fails)?

Have you done any calculations to see how much of a factor of safety the bolt has, or are you bothered because it's a non-standard detail?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[rowingengineer]

I have no problems with the concept, looks very similar to suspended ceiling situation. However I would want to know the layout and % capacity. My preference would be that you could remove some rods and still have a functioning ceiling.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[hokie66]

asixth,

I think you have answered your own question. If the strength of the rod is 1/10 of that required, it is no good. There are ceiling hangers with flat plates on the end, designed for bolting through the web of purlins. I would think the spacing of hangers needs to be closer than 4000 for such a heavy ceiling.

Certifying existing suspended ceilings is a high risk activity. Invariably, there will be non-typical things which you don't see. Make sure to certify only that which you are certain about.

 

[rb1957]

i think we're confused by this ... "the maximum tension in the rod to be approximately 1/10th of this load."

do you want a safety factor of 10 on a 7600lb load ? a 1/2" bolt won't carry that.

 

[apsix]

There is some confusion.

I'm reading a total ceiling weight of 3.4 tonne and a maximum rod tension of 340kg.
That's possibly OK for the detail shown.

 

[dhengr]

Asixth:
As I understand it; you say 7.6k total weight of ceiling structure, evenly distributed ?? to ten (10) - .5" round rods, that’s 760 #’s per rod, or about 3.9ksi, tension in the rod. That certainly doesn’t sound like a problem if we know a little more about the steel the rod is made of. Are you sure that the ceiling weights aren’t distributed more like .375wl to two exterior walls and 1.25wl to the rods down the center of the room?

This looks like an existing ceiling structure, not a new one, under construction. What can you say then about the fact that this ceiling structure has already withstood the test of time. I would want to know a lot more about the connections of the rods at the purlins and at the 2x wood joists. Can these connections take the 760 #’s at each end? What is the rod bearing doing to the purlin web, any yielding in bearing btwn. the rod and the purlin web or web buckling? What prevents the rod from pulling out of the web if the rod straightens a bit? Isn’t the rod at this point a 2" long cantilever with a 760# load from the purlin web, plus the tension in the rod? With the rod you show, how do you adjust its length to level the ceiling? What happens if one or a few of the rods do relax a bit, is the ceiling just a bit out of level, or what. Is there any question about the purlin’s cap’y in bending? Any significant mechanical loads on a few of these rods?

I might try to improve the rod connection to the web of the purlin, and put in some sort of length adjustment for the rods, turnbuckles. That’s kind of a crude detail for .5" round rod hangers, but a fairly common detail here in the states, using about 1/16" wire tied around the bottom chord of a bar joist, or some such, for a drop ceiling. Sorry, I offer more questions than answers.

 

[Ron]

Based on your info, the rod is not overstressed;however, I would be concerned about the spacing, particularly from a creep deflection consideration.

Apparently the ceiling is not yet fully in place. There are mechanical connections made for this application that clamp to the bottom flange of a beam. Add some to cut the wood spans down a bit.

 

[asixth]

Just to clarify:

The roof structure weighs 7.6k (3,400kg) and the maximum tension in one rod is 0.76k (340kg).

Because the ceiling frame is supported from the roof structure. My calculations show that the roof structure itself is overcapacity when the additional ceiling load is suspended from it.

I still cannot come to grips with the bent rod. I know that there are propriety 'hooked' rods on the market but as soon as this rod is bent and is used in bending instead of tension than I cannot quantify through calculations that the flexural stresses in the rod are not overcapacity.

hokie,

I believe that we do have a duty of care to the community. If we know something is suspended in the manner which it is than we cannot allow it to be. I'm sure there are many situations of collapsed ceilings similar to this.

dhenger,

Yes, there will be some elastic distribution of reactions. There is more than 4 hangers total so I have increased the reaction in the rod by 15% to account for this.

I don't think the argument that it has stood the test of time is relevant. The wood code that I use penalizes the capacity of wood and its connections based on the duration of loading. This suggest to me that the wood is more susceptible to failure with load duration. I cannot quantify that the fixing of the connection works so I do not know on what basis that the ceiling can be certified as structurally adequate. PE's duty of care.

There is a nut on the other side of the rod which prevents it from pulling out of the purlin.

Are their any fire considerations for suspended ceilings?

Again, thanks to everyone for there continual help and support.

 

[rb1957]

760 lbs on a 1/2" steel rod doesn't sound too much. obviously it is the bending loadpath into the support that is the issue, probably a loose fit so the rod would cock over slightly, loading the two faces of the support, sort of like an overhanging beam. i'd expect it'd work, but maybe not; you know the steel, the dim'ns and can work it out.

as you said in your OP, "I have been asked to rate/certify the structural components of an existing ceiling." and you're asking for other thoughts/opinions. we're having our discussion, some of us are having "shouldn't be a problem" but at the end of the day it's your call.

good luck, let us know which way you go.

 

[wannabeSE]

If the distance between the center of the vertical rod and the center of the web is assumed to be 1", then M = 0.76k x 1". Using the elasitic section modulus with a quick calc, the fb = 62 ksi. A 36 ksi rod will yield.

 

[rb1957]

this detail is something easy enough to test, if people don't like the calcs you run.

 

[BAretired]

I would be inclined to go with your initial reaction. The existing connection is unacceptable.

BA