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Field splice of girder with different web thicknesses
5

Field splice of girder with different web thicknesses

Field splice of girder with different web thicknesses

(OP)
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.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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)?

RE: Field splice of girder with different web thicknesses

Quote (BridgeSmith)

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

Quote (r13)

But can it be used?

Quote (AASHTO LRFD 8th Ed.)

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

RE: Field splice of girder with different web thicknesses

Rod,

Got it. THanks.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

Thanks. It make sense.

RE: Field splice of girder with different web thicknesses

(OP)

Quote (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.

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

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

¯\_(ツ)_/¯

RE: Field splice of girder with different web thicknesses

Quote:

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

RE: Field splice of girder with different web thicknesses

Quote (OSUCivlEng)

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

Do you know what is the reason?

RE: Field splice of girder with different web thicknesses

So it doesn't slip.

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

(OP)

Quote (BridgeSmith)

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

Was there an architect involved?

Quote (STrctPono)

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.

RE: Field splice of girder with different web thicknesses

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."

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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.

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

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?

RE: Field splice of girder with different web thicknesses

Quote:

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

RE: Field splice of girder with different web thicknesses

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

RE: Field splice of girder with different web thicknesses

Rod, what's the tolerance on pretensioned concrete girders in your area?

RE: Field splice of girder with different web thicknesses

The tolerance on prestressed girders varies, depending on the situation. We try to come up with a range of camber that is reasonable for the type of girder and the span, and accommodate that range in the finishing of the driving surface. We don't do many prestressed girders, and the ones we do are typically small, low-speed county road bridges. We've found the long-term camber is difficult to predict accurately, so we choose where we use prestressed accordingly.

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

RE: Field splice of girder with different web thicknesses

In my part of the world we cater for variation in prestressed girder hogs by estimating and documenting at various time intervals (transfer, 30, 60, 90 days typically). The surveyors and site team then can compare what's actually achieved in the precast yard and prior to erection and adjust bearing pedestal levels as required. Deck thickness will typically vary from the nominal/minimum at mid-span and increase over piers/abutments.

RE: Field splice of girder with different web thicknesses

The 'final' camber (increasing camber slows, but never stops) of prestressed girders can be controlled much better by extending the time from casting to transfer, but it's not very popular with the precasters around here.

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

RE: Field splice of girder with different web thicknesses

(OP)
Where I work we have standard haunch thicknesses for each size of prestress beam. The longer the span the thicker the haunch. We don't worry about camber and I have never heard of a problem with negative haunch or deck thicknesses less than 8". Prestressed concrete is commonly used for spans from 40' to 130'.

RE: Field splice of girder with different web thicknesses

Nearly all of our prestressed girders are either decked bulb tees or trideck sections. It we're going to use a CIP deck, we pretty much always opt for steel girders, as the more economical option in this area.

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

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