Framing connection question

[aayvat]

I'm designing a connection between two light guage steel framing members. The connection will need to withstand a load of 3100lb. I'm looking to use teks fasteners if possible. Otherwise 1/4-20 hex bolts. Need to determine number required and configuration. My question is: when designing for such a connection, must allowable screw loads be met or is it ok to move towards the ultimate screw loads. Big difference between the two. Two 12-14 teks would be enough using ultimate loads whereas I would need atleast 5 considering allowable loads.

 

[JAE]

You have to have a safety factor on your design. Using ultimate screw capacities leaves you with nothing.

 

[aayvat]

Thanks for the reply. So I take it that if I were to design closer to but slightly above the allowable load, I should be ok?

 

[JohnWylie]

What type of load are you referring too? Are the fasteners going to be subjected to withdrawal, just lateral loads, or a combination of both? Depending on the type of load is going to determine number of fasteners and configuration.

Typical safety of factor is four (4) if the manufacturer is showing the "ultimate" values. It would be up your engineering judgement on what safety factor you are willing to use.

 

[Ron]

In pullout, as JW noted, a factor of safety of 4 or 5 is used by manufacturers. This is consistent with code-required safety factors for anchorage to concrete and similar anchorage requirements.

For shear, some references require a safety factor of at least 3 (ASCE 8).

I would use a safety factor of at least 4 and then check interaction (shear/tension) as well.

 

[msquared48]

One or two extra connectors are small price to pay to avoid failure.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[abusementpark]

In pullout, as JW noted, a factor of safety of 4 or 5 is used by manufacturers. This is consistent with code-required safety factors for anchorage to concrete and similar anchorage requirements.

I have never quite understood why they always summarily put a factor of safety of 4 or even 5 on these fasteners for anchorage. I can understand if there is a large coefficient of variation, but sometimes the manufacturer gives the standard deviation and the coefficient of variation is usually less than 10%. For a normal distribution, they say 4 standard deviations away from the mean has practically a zero percent chance of occurrence. With a factor of safety of 4 and a coefficient of variation of 10%, even if you assume your nominal strength is 4 standard deviations away from the mean, you will still have a factor of safety greater than 2.

I don't get it. Perhaps, they are worried about improper installation, which is a valid concern, I suppose.

 

[jberg]

I had a similar discussion with Lindapter last week about their Hollobolt in HSS connections. Granted, it is a different company, but last week I was told by a Lindapter engineering rep. that they list the safe working load (F.S. = 5.0 I believe). I was told that capacity is for the "lay-man" installer since they are probably not checking all of the limit states that a structural engineer would check. He mentioned that I could go to a factor of safety of 2.0 (similar to some limit states in ASD) as long as I was checking the limit states per AISC.

I agree that as Mike stated if it is a question of a couple of extra connectors, then that is a small price to pay. Just thought I would share that discussion with everyone.

JWB

 

[abusementpark]

I agree that as Mike stated if it is a question of a couple of extra connectors, then that is a small price to pay.

Yes, I agree. I haven't found screw capacities to generally be a problem. My comment was more geared towards something like the capacities given for mechanical anchors into masonry. I'm always having a hard time making that stuff work and its frustrating because it is due to these very high safety factors.

 

[hokie66]

I think you know the answer. Per your above post, improper installation is the problem. In my experience, the majority of masonry anchors are installed improperly.

 

[rowingengineer]

Having just had load testing carried out on a site for anchor in connections to masonry, I can assure you that you need the safety factors. Had a 20% failure rate on this site, I think this is because these were installed by unskilled labour, even though my spec says they should be installed by a trade and signed off.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

RE, I think the problem with that is that there is no trade where we are whose job description includes installing anchors.

 

[abusementpark]

rowingengineer,

what exactly do you mean by a 20% failure rate? 20% failed for the design loads? or 20% failed to reach ultimate capacity?

 

[rowingengineer]

Hokie,
unfortunately hindsight is a b*tch, I had hoped they would be installed by the steel erector or a carpenter. Maybe we need to create a trade.

Abusementpark,
We were testing to working load. I wasn’t there so I can't say what the reasons are/were. We had a few different anchors on site, am yet to see the breakdown of failures.




Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it