Deflection of steel beams with bolted moment splices

[rtpnd]

I have a 40' steel beam which requires two splices in it, one about 10' in from each end, where the bending stresses are relatively low vs capacity at the middle of the beam. I'm designing a moment splice connection with steel top and bottom splice plates which will be field-bolted to the flanges with slip-critical bolts (as well as a field-bolted web plate to handle the shear.) I have a couple of questions:

Will the beam deflect significantly more (than in a full-length non-spliced beam) due to the presence of the splices? Is the deflection greater if the splice connection is designed for the full bending capacity of the beam vs. if it is designed for the required moment of the beam at the splice location?

The connection will be substantially more expensive if it has to be for the full moment capacity of the beam, but I suppose I can justify it if it reduces deflection.

How do you go about checking this?

Thanks,
-Ryan

 

[KootK]

With slip critical connections, I typically don't worry about this. I would expect a stronger moment connection to be stiffer but I wouldn't know how to quantify that and I wouldn't pursue that as an explicit, deflection reducing measure.

As long as the connection doesn't slip, I'd expect deflection to remain pretty much unchanged. If it does slip, than the extra deflection will come from localized rotation at the moment connections. I suppose that you could try to calculate the localized rotations based on flange plate strain and a presumed 1/8" slip in all of your bolts. There are analogous procedures for calculating wood diaphragm deflection where nail slip in the chords is a contributing factor.

If your concerned about localized rotation, one strategy may be to add some more bolts. A mentor of mine once told me that any bolted connection with standard holes and at least six bolts is likely to have at least a couple of bolts indirect contact with the connected steel (and thus no slip). Granted, that was just his opinion and I can produce no science to support it.

Another measure that could be taken would be to provide flange plates of a similar cross sectional area to your beam flanges regardless of the actual moment demand. That shouldn't add much cost and would reduce the extent to which you'll have a rotational soft spot at the splice.

Lastly, if the moment connections will be field installed, you could use them as an opportunity to introduce camber, remembering that camber will only affect final position, not deformation.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rtpnd]

Thank you for your input. My intuition tells me that the splice has to to introduce some additional deflection from the rotation at those two joints--stiff as it may be--it just doesn't seem like it will be as stiff as the original. I like your idea to investigate the rotation based off the 1/8" bolt slip, and also to increase the plate thickness slightly to add a bit more stiffness.
Thanks!
-Ryan

 

[KootK]

You're most welcome rtpnd.

One thing to keep in mind is that deflection is the integral of curvature taken over a considerable length. As such, even if your very soft at the splices, as long as the splices are short, the contribution to deflection will be minimal. That's why I focused most of my response on the bolt slip issue. I see that as being the primary contributor to any deflection increase that you may see.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[gwynn]

I agree with KootK for the most part. The main times I would be concerned about bolt slippage and/or splices increasing deflection are with cantilevered beams.