bolts failure

[sybie99]

Hi

I am looking at an existing fixed end plate(moment connection)of steelbeam to uc column. Calculations show the bolts capacity to be exceeded in tension. The beam has the capacity at midspan to take extra moment, so if some rotation could occur at the connection, the tension would decrease. My question is this: how do bolts fail when in tension and also when in shear, is it a sudden failure or is there first deformation, so plastic failure. The plate is too stiff to bend before the bolt fails. If the bolt does deform slightly before failure, this should redistribute some of the stresses.

Whilst on this topic, another question: If you have a connection where bolt holes are slightly out of position, say the bolts are only in shear, this will result in some bolts carrying more shear than others. When calculating shear on simple bolted connection you can usually just devide the shear by the amount of bolts. In this case, will the overloaded bolt deform slightly and then put more load on the other bolts so that all the bolts carry more or less the same load. Or will it shear off?

Hope someone has some better understanding of this.

Thanks

 

[rob768]

I don't know what the connection is, but usually the pre-tension in the bolts creates friction between the surfaces connected to each other. That way, if there is enough surface area, the actual shear on the bolts is negligible.

 

[Ron]

High strength bolts will elongate some under load and will yield plastically, but ultimate failure is likely to be brittle and fast. I would not count on the bolt elongation to re-distribute moments. Further, if you do get elongation, you will demobilize friction restraint and shear will come into play again.

Have you checked bending in the column or are the column loads balanced?

 

[nutte]

Ron, any decrease in normal force at the tension side (due to the bolts elongating) is matched by an equal increase in normal force at the compression side. The net normal force is the same, so the friction resistance is not changed.

 

[paddingtongreen]

If it is a moment connection, with bolts below the beam, the normal force below is not increased, the stress in the bolts in the compression zone is reduced.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

Nutte...for your premise to be correct, it would require two things...plate bending (not likely) and a balanced stress block...also not likely if you have an odd number of bolt pairs.

As Michael noted, the pretensioning in the bolts is reduced, thus a likelihood of no net gain in compressive contact stress.

 

[nutte]

Michael, AISC disagrees with you. See the top of page 12-9 in the 3rd edition LRFD steel manual.

If bolts are to be slip critical, all bolts may be designed for shear only and the shear-tension interaction equation may be ignored. Although some of the bolts lose a part of their clamping force due to applied tension, the overall connection suffers no loss of frictional resistance because the bolt tension produced by the moment is coupled with a compensating force on the other side of the axis in bending. Thus, the net clamping force is maintained in the connection.

 

[Ron]

Nutte...I agree with AISC while the bolts are in the elastic range, but not so after yielding.

 

[weab]

sybie,

I don't see any responses on your second question. If you are talking about slip critical bolts, then your question is irrelevant since the clamping creates the shear resistance.

HOWEVER, even in slip critical connections, one should check the design in case it does slip. I know that shear tabs and other connections are designed to a maximum thickness so that the bolt can "plow" into the plate. Eventually all bolts will go into shear.

But my explanation goes up in flames if you are bolting to a thick column flange. I'm sure it won't shear off though. Someone else may be able to elaborate.

 

[sybie99]

Thanks guys, maybe someone can help with the second part of the question, as jsdpe started to elaborate on

Sybie

 

[paddingtongreen]

I'm not clear on "where bolt holes are slightly out of position", How did they insert the bolts? I presume they reamed them till the bolt would go in

You don't say whether these are bearing or friction bolts. AISC and RCSC have rules on tolerances for bearing bolts and for oversize and slotted holes in friction connections, but they must be internal plies or covered by plate washers.

For field fit up, see 7.14 of the AISC Code of Standard Practice, at:

http://www.aisc.org/WorkArea/showcontent.aspx?id=6878

Michael.
Timing has a lot to do with the outcome of a rain dance.