Tension Indicators 1

[steve123]

On a recent project visit, I was made aware that a client is performing structural steel erection using A325 and A490 bolted connections. These connections are being made using direct tension indicators. The client has waived any type of testing of impact devices based on a preliminary test of a representative samples of these devices.

The client also specified that these connections are not "shear". Being new to AISC specifications, I have a few questions.

DTI's are designed to "squirt" upon a certain tension being attained from an impact device. If a device has not been recently tested, how is OVER tensioning of the bolt avoided ? The client actually said that as long as the bolt does not break it's not over-tensioned .

What constitutes a shear and moment connection ?

Any and all help would be greatly appreciated from a person who has had structural steel erection thrust upon him !!

 

[Qshake]

I assume that by saying the connections were not "shear" they were referring to bearing connections which transfer load through shearing of the plates and fasteners. The other method to transfer load in steel connections is through slip critical connections which transfer load through friction imposed by the normal force introduced to the joined surfaces via the bolt tension.

The latter seems to be likely your case as I see no reason to have specific and otherwise expensive inspection procedures for establishing bolt tension. The fact that the client chooses to use load indicating washers is evidence of the nature of the connection and desire to reduce inspection costs as well assure quality.

From my perspective, the bolts/washers should be tested using the Skidmore-Wihlem apparatus (commonly used on site or in tesing labs). The SW reads bolt tension and can be used to determine when the washer activates. This will ensure that the washer is working properly. THis may have been done already and the lot of washers certified. Then the inspector need only watch to see that the ironworker doesn't needlessly sit on the bolt longer than necessary. It will otherwise be obvious that the proper load has been achieved.

Obviously, the client is wrong in stating that "as long as the bolt doesn't break its not overtensioned". We know that the bolt may yield prior to fracturing. However small.

 

[Ron]

Steve123...I agree with QShake that a Skidmore-Wilhelm device should be used as a check on the "Squirters". Squirters are a class of DTI's known as self-indicating DTI's. When the silicon bubble under the washer "bumps" breaks open and bleeds out, you have the visual indication of having the desired tension in the bolt (presumably!). This still needs to be checked with on-site bolts, to check what is actually being used and to verify the actual bolt tension. As a secondary check, you can still use the "feeler gage" on these DTI's to check flattening of the "bumps" on the washer.

Also as QShake noted, only slip critical connections really need to be checked for proper tensioning. On bearing connections, it's nice to have all the bolts tightened consistently, but actual bolt tension is somewhat irrelevant, assuming they are reasonably tight (snug tight is the preferred industry term).

 

[Rowland33]

Steve, since you have indicated that you are inexperienced with steel erection, I have the following comments. I agree that the usage of squerters indicates that the connections are probably 'slip critical'. Make sure that the faying surface of the connection complies with the requirements of slip critical connections, particularly that the faying surface should not be painted.

 

[ishvaaag]

In any case the fact of transfering the full friction loads just on say 6 bumps is not a feature I like. For the risk of local corrossion heightens and the destruction of the bumps on any relative displacement is more likely than in full pressing bearing.