Screed rail support on top of girder

[MIKE_311]

I have a contractor that want to set the screen rail on top of a girder. They want to use a pipe receiver set on the beam and tack welded to the shear studs since they aren't allowed to weld to the flange. The receiver would have a smaller pipe and adjustable threaded rod that would be pulled after the screed passes, leaving the receiver in place

Concrete finishing aside, any issues with type of support or tack welding to studs? I can't seem to find anything in the agency specs or others that forbids this.

I've never seen this before so i'm not sure what other alternatives exist. I know its done, many agency specs discuss using over girders and provide clearance over top of chairs, etc, but they are all surprisingly light on attachment requirements or restrictions (besides welding to the flange) to steel girders.

If this is a normal method of attached, what does the contractor need to demonstrate, if anything, via calculations, for this method of attachment?

 

[STrctPono]

Do your specifications disallow them to support the screed from the girder as opposed to forcing them to support it off the deck overhang brackets and form? If this is not exclusively addressed in your specs then you may not have a leg to stand on to reject it. Sure, you can nitpick about the details (how it's supported and clearance) but that's probably all. Our specs have a clause that states: "The supports for the screed rails shall not be placed within the full width of the bridge." Because we don't want them to hand finish the remaining deck outside of the exterior girder, especially if you have a large overhang. Blanket grinding your entire bridge deck may alleviate this concern but that is expensive (we still do it). I cannot say that I have ever seen the screed chairs welded directly to the shear studs before but maybe someone else on this forum has. Do you have a detail you can share that shows the connection detail? That to me seems like the most suspect proposition about the plan especially since they are "tack" welding. Other than that detail, you would only be able to weld to the top flange and then you may want to consider limiting their welding to only positive moment regions between points of contraflexure and not in the negative moment regions.

 

[MIKE_311]

The agency spec doesn't limit this. Honestly, I don't have a problem with it either. VDOT discussed this in their manual and allows it, including how to finish properly however they don't say how you are allow to attached to a steel beam.

This is a stage line very close to an existing structure, so overhang brackets aren't really needed here and in all honesty, probably would be more problematic. My main concern is the bridge as the EOR. The other side of the bridge has the rail on the overhang bracket.

There are tack walking to to the stud head, so as long as they don't damage the stud?

Also, does leaving the rail chair so close to the stud, impact the ability of the stud to perform since it wouldn't be fully surrounded in concrete. In the tension region I'm not so concerned but in areas were we are relying on composite action...

I think I would prefer to see the placed longitudinally between the studs and tied to the rebar.

 

[jorton]

I've worked up details similar to this for PSC girder bridges. The owner accepted them. If this were mine, I'd might slide a length of pipe over the shear stud to receive the rail yoke instead of welding the pipe to the stud. Then I'd pull the rail, yoke, and pipe as finishing progressed.

 

[STrctPono]

The sketch clears up some confusion. After seeing this, I wouldn't be concerned.

In regards to damaging the studs and the studs impacting the ability of the stud to perform properly.... I remember when I went out to the field to see my first steel plate girder lifted onto the piers, I noticed that 5%-10% of the studs were all bent over at a 30 degree angle. I was upset and found the steel erector supervisor and asked him what the heck happened. The dude was nice and explained to me about the bend test on welded shear studs and how they whack it in the shop with a sledge hammer to test the ductility and soundness of the weld. I was shocked because I felt that this test compromised the integrity of the structure. If you read AASHTO, however, they state somewhere under Chapter 6 that testing has shown that the designer shouldn't be so worried about calculating the horizontal shear flow requirement down to the gnat's eye at every section along the girder but rather ensure that there are enough studs along the full length between points of contra flexure or end-to-end (for simply supported beams) to satisfy the requirements. Maybe a quick review of your calcs and if you have at least 5% contingencies in your shear stud quantities then I wouldn't worry about it.

I do agree with you that I would have them place the rail chair longitudinally between the studs and (unless constrained by your railing bars above) I would see if they could center the chairs directly over the web of the girder. Other than that, I don't see any issues with their plan.

 

[MIKE_311]

Thanks. This is all great information!

 

[BridgeSmith]

I also wouldn't be concerned. The weld to the top of the shear stud does not compromise its strength or fatigue capacity. If something was being welded near the bottom of the stud, well that could be a fatigue issue, but not what is shown.

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