Field splice of girder with different web thicknesses 5

[OSUCivlEng]

I am reviewing some shop drawings of a plate girder with a field splice with different web thicknesses. One web is 3/4" and the other web is 13/16". I think this is rather odd, but I'm not the EOR.

There is no filler plate called out in the construction drawings or shop drawings. There will be a 1/32" gap between the 3/4" web and the web splice plate assuming the webs are centered on each side of the splice. The filler plate would be 22 gauge thickness.

Personally, I would have used the same web thickness for both sides of the splice. I am struggling to find any reference in regards to this small of a difference in thicknesses of plates joined in a field splice and a requirement for a filler plate. It would appear to me that this splice doesn't meet the requirements for a slip critical connection in the AASHTO LRFD Bridge design specs.

 

[Agent666]

I'd say its potentially pretty close to the normal tolerance variation on web thicknesses for rolled sections. Obviously if you were to get each section at the opposite end of the tolerance spectrum it might cause more of a differential thickness variation.

FYI, the code I work to requires shim plates to be a minimum of 2mm thickness. It also says you might want to think about filling narrower gaps where corrosion might be an issue. Imply from that what you want, but it seems to me some very minor misalignment is all part and parcel of normal erection tolerances considered by codes.

 

[BridgeSmith]

Unless the splice plates are much thicker than what you'd except for a 3/4" web, the splice plates will bend a little, and the effect on the clamping force should be minimal. If you want to quantify the effect, you'd have to calculate the force needed to bend the splice plates that 1/32" over the distance from the CL splice to the first row of bolts, and subtract it from the clamping force of the bolts in the first row. However, the friction values for the faying surfaces carry quite a bit variation, anyway.

If the applied slip force is much smaller than the slip resistance, I'd try ignoring the slip resistance of the first row of bolts, as a quick & dirty check.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bridgebuster]

On NYSDOT projects, different web thicknesses wouldn't be permitted on a new bridge without a compelling reason but fill plates would be required. The plates would have to be of equal or greater strength.

 

[Lomarandil]

Aside from the slip-critical issues to be worked out as mentioned above, it's odd but not concerning from a design perspective.

I would have it filled with some sort of shim plate from a maintenance perspective.

----
just call me Lo.

 

[STrctPono]

Take a look at AASHTO 6.13.6.1.5 which states that for bolted web splices with thickness differences of 1/16" or less, no filler plates are required.

Not saying I would ever do this, but this is what is clarified per AASHTO.

 

[OSUCivlEng]

STrctPono a coworker informed me of that this morning. I still think it's not the best idea either.

 

[r13]

Insert shim under the splice plate?

https://www.mcmaster.com/metal-shims/material~steel/shim-stock-6/

 

[hokie66]

Not a bridge engineer, but using such a thin filler plate doesn't sound like a good idea. Corrosion initiation in that filler would be my main concern.

 

[r13]

Corrosion is a valid concern. But if the 1/32" gap is undesirable, the thin sheet is easy to cut to the size of washer. And, stainless steel sheet is available too.

 

[BridgeSmith]

If AASHTO doesn't require filler plates for 1/16" or less differences, I wouldn't recommend putting one in. I definitely wouldn't use a stainless steel plate for a slip-critical connection, without adjusting the slip coefficient appropriately.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

Understand. But can it be used? What are other ill-effects you can think of? Also, is the 1/16" referred to each side of the thinner web (1/8" total), or both sides combined (1/32" each side)?

 

[BridgeSmith]

If AASHTO doesn't require filler plates for 1/16" or less differences, I wouldn't recommend putting one in.

But can it be used?

For bolted web splices with thickness differences of 0.0625 in. or less, filler plates should not be provided.

Per AASHTO, apparently not.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

Got it. THanks.

 

[STrctPono]

r13,

Not to mention that putting a more noble metal such as stainless steel sandwiched between A709 would be a terrible idea from a corrosion mitigating standpoint.

 

[BridgeSmith]

It's probably too late on this project, but my question would be why the plate girder was designed with a 1/16" difference in the web thickness to begin with? We would never do that, because it causes too many fabrication and erection problems (cross-frames that are 1/16" different in length, etc.) We'd have just made the whole thing with a 13/16" web, or more likely found a way to get 3/4" to work everywhere. 13/16" plate is not a common size, so it might be the most efficient, but it's not likely to be the most economical.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

STrctPono,

Not to argue with the corrosion concern. Since we are on this subject, text below is excerpted from a trade blog for information.

"The existence of two dissimilar metals in direct contact can be no problem whatsoever if there is no electrolytic material present.

In most atmospheric applications the only potential electrolytic material that can be present is rainwater or dew. Both of these forms of water are poor electrolytic materials since they do not contain many salts and ions which would make them conductive. On the other hand, marine environments and areas where the melting snow includes road salts can be very good electrolyte materials. Bimetallic couples are more easily formed in immersion situations where the assembly will be underwater when it is in service. Salt water is especially tough on two dissimilar metals in contact. The best guide as to how various metals will react in contact with zinc under different environments is the following table.

 

[STrctPono]

retired, how about salted roads? Seems like a plentiful supply of chloride electrolytes to me.

Honestly, I think you would be hard pressed to find many Engineers who would think this is a good idea.

As BridgeSmith and bridgebuster stated, this design is not ideal and should have been detailed differently from the get-go. Since OSUCivlEng is not the EOR they are merely asking what is allowed to satisfy their review. I think they got what they needed in AASHTO 6.13.6.1.5

 

[r13]

Great point. No, I don't think it was a good idea to start with, same reasons as pointed out by Hokie66 and BridgeSmith. Just wonder if someone is crazy enough to provide webs with different thickness in exceed of 1/16". I am still confused on the allocation of the 1/16" toleration by AASHTO, is it refer to the offset on one side, or both sides combined thought.

 

[STrctPono]

The 1/16" difference is in the web thickness so that would be 1/32" each side of centerline.