Cross Beam Bolted Connection

[MiketheEngineer]

I have two I-beams that cross over each other at 90 degrees - like a cross.

The lower beam is shackled at the ends to the roof and supports the upper beam where they cross - in the middle.

There are no real shear or axial forces involved between the two beams. The upper beam simply sets on the lower beam. The only reason I see to even connect them is to keep everything square and to prevent any side slippage.

I suggested a couple of 3/4'' bolts in opposite corners to secure the beams together.

One of my young engineers is trying to convince me that the moment in the upper beam - which is sizeable will now somehow transfer moment into the lower beam thru the bolts. The upper beam has already been sized to carry the moments and shears generated by the loads on the upper beam.

I am trying to convince him that 3/4'' bolts in 1'' holes will not transfer any moment (or minimal) to the lower beam...

Who is crazy here??

 

[RARWOOD]

I think if you bolt the two beams together they will share the load based on their stiffness, assuming the lower beam also carrys load. The only load I can see the bolts taking is axial load as the lower beam tries to hang off the upper beam.

 

[civilperson]

The stiffer beam ,(the top big beam), will determine the deflection due to loading. The smaller, (bottom beam), will defect an identical amount since it is connected to the top beam. This is a simultaneous equation with two beams having identical deflection from one loading.

 

[Lion06]

Well, they will both take load since they are both deflecting. They should share the load by the ratio of I/L^4.
If the bottom beam supported the top beam at a location other then the center of the upper beam, I might agree that it will put some torsion into the lower beam because those bolts will make the lower beam go along for the ride when it rotates. At the center, however, rotation is zero so this is not an issue. Even if this were an issue, just provide stiffeners in the lower beam at that location and it will behave similar to a beam over column connection with stiffeners.

 

[MiketheEngineer]

Yes - thanks - I agree

 

[civilperson]

The simultaneous equations for the two beams have PL^3/48EI as the deflection of the bottom beam, (not uniform loading but point load at crossing and a little uniform load due to self weight). The ratio of the beams L^4 is really L^3 and uniform weight of qL for the top.

 

[MiketheEngineer]

Let me re-state the question.

The upper beam has moments/shears generated by the loads on it.

If I did nothing and just set this beam on the lower beam - it would be OK. The bolts are just to keep things aligned and prevent any sliding.

If I introduce this bolted connection - two simple 5/8'' or 3/4'' bolts - will moment from the upper beam transfer into the lower beam due to compression of the upper beam lower flange and then the bolts introducing a shear into the top flange of the lower beam....

Not worried about simultaneous deflection - that is under control.

My opinion is if there is any transfer - it has to be minimal..

 

[Lion06]

you mean a torsional moment in the lower beam, right?
If it is at the center of the span of the upper beam, I would say no.

 

[MiketheEngineer]

Yes - it would be a torsional moment on the lower beam... but given the size of the beams and loads - it will just be minimal.

Thanks

 

[nutte]

I think you (or your young engineer) are thinking way too hard about this.

 

[MiketheEngineer]

Nutte -

That is exactly what I was trying to explain to him.. but he kept coming back and back and finally got me thinking in crazy directions.

Glad my head might still be in the right place

 

[engco]

you are both crazy!))

 

[PanamaStrEng]

In my humble opinion, I would disregard any effect (with the exception of the already noted shared deflection between the upper and lower beam). It will be very hard to apply a torque through the kind of connection you have described , even if off the center of the beam. For a torque to exist, you would need to be able to resolve a tension-compression couple to the top flange of the lower beam. On the other hand, if the bolts really apply a shear to the top of the lower beam, I would be more concerned with weak axis bending.