Another bolt connection question?

[jiamin]

Hi, Guys, I have One more question on bolted connections, please see drawings in attached document. My question is:

1. In practice, there is always certain length of bolt that extrude out beyond the nut, is there any code requirement that specifies the minimum bolt extrude length?

2. If in a bolt connection, the bolt does not extrude out of the nut, but just flush with the nut (see lower drawing in attached document), will it reduce the bolt shear/tension capacity? If it does reduce the bolt shear/tension capacity, then how much it will reduce?

Thank you very much for your help.

 

[kelowna]

I seem to recall that that is specified in the code. Is it two full threads for non tensioned bolts and one full thread for tensioned bolt?

I think it is a question of good practice but not aware of any, if there are, technical explanations.

Maybe other forum members can help?

 

[kelowna]

Further to my previous message. CAN/CSA-S6-00, 10.24.6.7 states that when properly installed, the tip of the bolt shall be flush with or outside the face of the nut. This is the bridge design code in Canada.

I still recall the 1 or 2 threads sticking out. I worked in Europe for a while, so it is probably a requirement in some European code.

 

[TehMightyEngineer]

Per RCSC section 2.3.2 "The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed."

Some contact documents and/or local codes may require you to have "stick-through" but this is not recommended.

I remember seeing in a Modern Steel Construction Q&A that there is a reduction for having the bolt not fully engaged in the nut but I couldn't find it. However, there is no reduction in the capacity of the bolt as long as the end of the bolt is flush with the nut.

EIT with BS in Civil/Structural engineering.

 

[neffers]

See the following link....

http://www.modernsteel.com/steelinterchange_details.php?id=821

neffers

 

[racookpe1978]

The usual requirement is 1-1/2 to 2 thread sticking out from the nut. (I'll accept that apparently some rules say the bolt must be flush, but I can also disagree with them - or that the real intent is that they "extend beyond" the face of the nut.

Reason is the development of the strength of the male threads in the bolt (or stud), particularly when many bolts are threaded to the end, but the last part of the bolt is ground smooth so the thread is slightly tapered at the end. You need a little bit extending past the face of the nut to get the first engagement of the bolt threads into the (female) nut threads - which are often also recessed (or ground in the nut head) slightly.

I just checked several 1/2" and 5/8" coarse threaded bolts I've got available: It takes about 5/8 to 3/4 of one revolution just to get a male thread deep enough to begin pretending that it can develope full strength. A 1/2 square nut only needed 4-1/2 revolutions to get the bolt "flush" with the end of the nut, therefore the nut threads and bolt threads were actually only engaging for three revolutions. Same thing with the larger diameter 5/8" dia bolts: 4-1/2 to 5 revolutions are all are engaged, and since it takes 1 revolution to get a thread to full strength, by allowing "flush" bolts, you have cut your strength by 23%.

I know why the contractor/builder wants to allow "flush" nut engagements rather than "1-1/2 threads showing": - he doesn't want to check and replace lots of "too short" and "almost long enough" bolts.

I've been taught 1-1/2 threads minimum for that reason. But have not seen it in the Codes.

 

[BAretired]

It would seem to make sense to size the nut so that the threaded length is 1.5 threads longer than required to develop the ultimate tension of the bolt. Then it would be perfectly acceptable to grind each bolt flush with the nut.

BA

 

[jedward]

BAretired hit it on the head- do not use thin nuts.