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.

 

[r13]

Thanks. It make sense.

 

[OSUCivlEng]

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.

I agree 100%. It makes no sense to me.

I forgot to mention this bridge is skewed 0.27 degrees or thereabouts.

¯\_(ツ)_/¯

 

[BridgeSmith]

I forgot to mention this bridge is skewed 0.27 degrees or thereabouts.

Whaaaaat? It's skewed a fraction of a degree? Tell me it's at least on a horizontal curve...

Was there an architect involved?

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

 

[STrctPono]

I forgot to mention this bridge is skewed 0.27 degrees or thereabouts.

That's ridiculous! That mistake just made the design and detailing that much more unnecessarily harder. Sounds like the Civil Engineer didn't know what they were doing when they laid it out and the Structural just never called them out on it.... Now this makes me more curious on the whole different web thickness splice issue and whether there was even a good reason for it. What was the maximum span length on the bridge? If less than 200ft, it sounds hard to justify such a decision.

 

[r13]

Forgot to ask, why designate slip critical for bolts on girder splice, is there a gap between the girders?

 

[BridgeSmith]

r13, bolted splices of bridge girders are always slip critical connections, as required by the AASHTO bridge design spec.

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

 

[r13]

Do you know what is the reason?

 

[Agent666]

So it doesn't slip.

 

[r13]

What is consequence of the slip? If there is no gap between girders, where the slip goes, through slotted hole? Is slotted hole on one girder only, or both? I think AASHTO must have good reasons for this requirement on splices. That is what I am looking for.

 

[BridgeSmith]

It is not allowed for the girder pieces to have a change in angle between the pieces. The tolerance for the gap between the ends of the girder pieces is up 1/4", which is more than the clearance of the bolts in the holes (1/16"). However, the tolerance on the position of the girder pieces is very small. The elevations along the tops of the girders have to be within 1/8" everywhere, over spans of sometimes hundreds of feet; even a change of a fraction of a degree can throw the alignment off by an unacceptable amount.

If it slips during the pouring of the concrete deck, which is when it would happen, the consequences would be very detrimental to the vertical alignment of the bridge deck.

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

 

[OSUCivlEng]

Whaaaaat? It's skewed a fraction of a degree? Tell me it's at least on a horizontal curve...

Was there an architect involved?

That's ridiculous! That mistake just made the design and detailing that much more unnecessarily harder. Sounds like the Civil Engineer didn't know what they were doing when they laid it out and the Structural just never called them out on it.... Now this makes me more curious on the whole different web thickness splice issue and whether there was even a good reason for it. What was the maximum span length on the bridge? If less than 200ft, it sounds hard to justify such a decision.

It was a two span girder on a straight alignment, no curve. I think the spans were around 120 feet. No architect that I am aware of. I would have done a few things differently.

 

[r13]

The paragraph below answers my question.

"Slip-Critical Joints are required in the following applications involving shear or combined shear and tension:
(1) Joints that are subject to fatigue load with reversal of the loading direction;
(2) Joints that utilize oversized holes;
(3) Joints that utilize slotted holes, except those with applied load approximately normal (within 80 to 100 degrees) to the direction of the long dimension of the slot; and,
(4) Joints in which slip at the faying surfaces would be detrimental to the performance of the structure"

But attention shall be paid to the sentences below for connections with offset on contact surfaces.

"Faying surfaces in slip-critical joints must meet the requirements in RCSC Specification Sections 3.2 and 3.2.2. RCSC defines a faying surface as “the plane of contact between two plies of a joint.” NOTE that the surfaces under the bolt head, washer and/or nut are not faying surfaces."

 

[BridgeSmith]

Criteria number 4 is also applicable to splices of highway bridge girders. It's actually critical than than number 1, in most situations. The fatigue stress levels at a bolted field splice are typically fairly low and the fatigue resistance of the bolted connection is fairly high.

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

 

[r13]

Rod,

You have answered my next question satisfactorily, as I was doubting the value of point 1 to this case. But I couldn't comprehend the importance of point 4 for bridge girders, especially when it is fully braced. If the bolt tension was not checked/maintained routinely, it seems a waste to me. Fuse/buffer type connection is an absolute necessity for piping fixed on a rigid bent though.

 

[BridgeSmith]

Bolts in bridge girder field splice are fully tensioned high strength bolts. Properly installed, they will maintain compression on the plates for the service life of the structure and beyond.

Bridge girders are not "braced" longitudinally. The splices are made at the contraflexure points (generally around the 1/4 points of the span) and not at the supports. The girder pieces for continuous spans must be made so that they function as a single beam over multiple supports, maintaining the very exact geometric shape prescribed by the web cutting diagram, or camber diagram in the case of cambered rolled beams (Wide flange shapes). The web cutting accounts for the deflection of the girder when the weight of the deck is applied, so if the connection slips even a little at that point the final profile of the bridge deck will be off and driving across the bridge would be like riding a roller coaster. As I said, when it must have the proper contoured shape within an 1/8" vertically and the pieces are sometimes over 100' long, any slip at the field splice would be detrimental.

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

 

[r13]

Does the slip matter after the girders, deck slab and everything have put together? Thinking 1/16" maximum slip over standard hole.

 

[BridgeSmith]

Yes, slip would be detrimental anytime during the service life of the bridge. If the splice moves, the moments at the splice location get redistributed to other parts of the girder, increasing the loads at those locations.

For composite superstructures, it's often a moot point, since the moments at the splice are many times greatest during the deck pouring sequence. The section modulus increases dramatically once the deck becomes the top flange of the girder.

...and yes, it's about a 16th of an inch, but 1/16" one direction at the top of the splice and 1/16" the other way at the bottom of the splice is enough to effectively create a hinge in the superstructure, producing a vertical movement of an inch or more 60 feet away at the center of the girder piece.

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

 

[r13]

Rod,

Your geometric effects got me lost in this conversation If slip is unacceptable, then wouldn't be standard bearing type connection work too, if not better?

 

[BridgeSmith]

If slip is unacceptable, then wouldn't be standard bearing type connection work too, if not better?

Nope. The entire girder gets laid out on its side in the yard at the fabrication shop, aligned in the shape it will be erected, then the splice plates are attached and the holes are drilled through all 3 plates. Then when the girders are erected, they align the girders and tension all the bolts. If they let it slip into bearing after it was erected, it would fall out of alignment. The only way it would work with the bolts in bearing, is if the holes were exactly the size of the bolts, but then it would be impossible to assemble.

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

 

[r13]

Wow, the requirement and tolerance are so tight. Nice to learn. Thanks.