Steel erector bolt preference. TC or Hex?

[buck1017]

Hello all,
Has anybody seen a trend towards the preference of tension controlled bolts for steel erection recently? I'm involved in a pretty large steel project relative to the town that we are located in and recently got an RFI from the steel guys wanting to switch from snug-tight hex bolts to tension controlled bolts. Most of the jobs we design are pretty small (1-2 stories), so this is the first time I have come across this request. For those of you who work on larger steel projects, is this the new normal? Should we be considering specifying TC bolts going forward on larger buildings or is it really a contractor preference and we should continue to spec snug-tight and let the contractor ask the question?

Thanks,
Brandon

 

[canwesteng]

Tension controlled is for pre tensioned bolts not snug tight connections. Snug tight connections the erector should be bringing the plies into contact.

 

[UcfSE]

They do that because it's easier to inspect, particularly when you have a visual indicator like twist-off TC or direct tension indicating washers. Then the workers and inspectors only have to look at the bolts to verify if it has been tightened.

 

[buck1017]

No. Tension controlled are the snap-off type of bolts. They are wholly different from pre-tensioned bolts. I understand their benefit with respect to inspection. I'm just curious if anyone has noticed a trend toward using them recently BECAUSE of their ease in inspection. Does the premium for the material outweigh the cost of the inspections on larger jobs so they are more cost effective for the contractors? Or does it come down to personal preference?

 

[canwesteng]

I'd suggest reading up on some of the AISC and Bolt Council resources available online, as you seem confused with respect to the meaning of pre tension and the purpose of TC bolts. TC, DTI, or turn of the nut are acceptable methods to get the required pretension in bolts where it is required. In snug tight connections your inspection should just be to make sure the plies of the connection are in contact, so TC bolts don't serve much purpose.

 

[WinelandV]

canwesteng,

I've had a contractor ask for these TC bolts (with the snap off portion) for all the bolts on the job - yes, even for beam shear tabs. They didn't want any confusion as to which bolt, where, what procedure at which location, etc. Is it overkill? Yes. But, it saves them installation headaches.

buck1017, as far as cost, I don't think that is necessarily your concern. As long as the contractor doesn't charge the owner a extra cost for it, you should be able to approve it as acceptable.

 

[canwesteng]

wineland,

Agreed that contractors will want to use them for all bolts on a job to save on confusion - but OP should have a better idea of what TC bolts are meant for. Worth noting AISC recommends against using pretension bolts all over the place to avoid "banging bolts" - up to the engineer to decide if this is a concern.

 

[buck1017]

I did look some things up regarding the differences between TC and pretensioned. My understanding is that elongation won't occur with TC bolts because the head snaps off before you get adequate torque to pretension. Is that not correct?

 

[nutte]

Properly installed TC bolts do reach the full bolt pretension, making them appropriate for connections where pretensioned joints are needed. But TC bolts can also be used in snug-tight joints, even if the splines are twisted off. Some erectors prefer TC bolts for the ease of visual inspection mentioned above.

There is a serviceability issue of bolt banging. For this reason some Engineers of Record do not allow pretensioned bolts in snug-tight joints. I do not think AISC recommends against this, though. Rather they leave it to the EOR to determine if it is an issue on a particular project.

 

[thaidavid40]

Most smaller erectors around here can't afford the expensive tension-control guns needed to tighten these bolts. We only see these types of requests from the larger outfits. The requests seem related to construction firm size - not project size. However, the two do conflate at times.
Dave

Thaidavid

 

[vmirat]

DoD requires either TC bolts or DTI washers for all connections (UFC 3-301-01). Their philosophy is that it adds assurance that the bolt is tightened. The only problem I've had with them is access with the gun. Even the "right-angle" guns can be too big to fit in some tight joints where standard hex bolts would work. I had to let a contractor use standard hex bolts in one location for this reason. The other problem is clean-up. I still find the knobs in buildings.

In contrast, I can't tell you how many times I've gone into a building, such as mech rooms or above ceilings, and found hex bolts loose. I've even found a few bolts missing nuts.

Although I give the contractor the option to use TC's or DTI's, they always use TC's. The "squirty" washers are a pain to use.

 

[wannabeSE]

Contractors also like pre-tensioned twist-off bolts because they can be tightened from one side. On the west-coast, it is common to see pre-tensioned F1852 bolts specified for all connections.

Where does AISC recommend against pretensioned bolts to avoid bolt banging? One Modern Steel article from 1999 states:

“ . . . the snug-tight condition typically results in bolt tensions close enough to the prescribed pre-tension for slip-critical connections. As a result, slip under thermal conditions and service loads will be a random occurrence independent of the installation technique.”

https://www.aisc.org/globalassets/modern-steel/archives/1999/11/1999v11_banging_bolts.pdf

 

[Ron]

As wannabeSE notes....one side tightening. Sometimes conventional HS bolting needs two people to accomplish....not so with TC bolts....labor savings.