How to avoid negative fillet(haunch) for plate girder design on bridge

[t230917]

Hello

I am designing a 2 span bridge (concrete deck on plate girders). The bridge has 7 girders with varying spacing. On 2 of the 7 girders, I am getting a fillet(haunch) height which is less than the minimum allowed for plate girders (3/4"). The remaining 5 girders are OK. How do I get around this? Do I increase the flange thicknesses?

I am thinking that the fillets are due to negative deflection that I am getting just before the center pier. This is the only region where the fillets are less than the minimum. How do I avoid this uplift in this section.

Span 1 is 130 feet and Span 2 is 150 feet.

Thanks.

 

[bridgebuster]

By fillets, do you mean deck haunch?

 

[t230917]

Yes. Sorry, thats what I meant. The slab haunches between the deck and top flange.

 

[bridgebuster]

@t230917 - no problem about the terminology. 3/4" haunch/fillet is pretty shallow for a new bridge. I think most agencies would require minimum 2". Aren't your splices going to be protruding into the deck, even with the girders that are OK in terms of the fillet depth? How shallow are your fillets at the two girders? Anyway, You could increase the flange plate thickness to reduce the deflection. It's an iterative process but it may not do much other than add weight and cost. Can you drop the elevation of two girders down to get what you need; that could be the easies thing, assume the underbridge clearance isn't affected? You could also try reducing the web depth and increasing the flanges to maintain the same overall girder depth.

 

[BridgeSmith]

For plate girders, adjustments are typically made in cutting the profile of the web to compensate for profile grade and dead load deflections. We don't use haunches on our plate girder bridges; we just have them fabricated so that the top of the top flange elevation matches the bottom of the deck elevation everywhere. Sometimes, individual girders have different web cutting profiles, but we've never used a concrete haunch except on a widening where we needed to raise the grade over some existing girders.

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

 

[bridgebuster]

Rod - it sounds to me like OP has a combination girder elevation & cross-slope problem, which is eating into the haunch. Here in the northeast, plate girders are fabricated to the deflection and vertical curve profiles but we always provide haunch.

A deck replacement on existing girders could be a different story. A lot of older bridges here were built without haunches; the framing was stepped. Sometimes to meet, or at least try to meet, current geometric standards we have to build up the hunches. On one project I did recently we had haunches up to 10" (bridge had crazy geometry - 2 "S" curves, among other things.) On a subsequent project, just finished, that was similar, the client relaxed to cross slope requirements, which did help. We couldn't raise the stringers because of the staging. On average we're about 3" with the haunches but in some locations we have very shallow haunches 1/2" or less. To me it's not a big deal because these are all simple spans, no splice plates, we get the full deck thickness.

 

[BridgeSmith]

Here in Wyoming, we just vary the theoretical thickness of the deck, so that the bottom of the deck meets the top corner of the flange on the high side, and the flange is embedded as required to maintain the design deck thickness on the low side.

That is for composite girder sections. For the occasional non-composite superstructure, the flanges are embedded a minimum of 3/4" into the deck.

All of this is theoretical, and it's a moot point when stay-in-place formwork is used (which is almost always), since we effectively get 2 - 1/2" or 3" of embedment at the flanges due to the configuration of the support angles and form pans.


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

 

[t230917]

@bridgebuster In Illinois, the minimum required per the bridge manual is 3/4". The splice locations would have deeper haunches as necessary so that the plates do not reduce the overall deck thickness.

Thanks all for your input. I thought about this more, and yes the profile was also an issue. However, by reducing the camber, I was able to increase the fillet/haunch heights and work around it.

 

[BridgeSmith]

The splice locations would have deeper haunches as necessary so that the plates do not reduce the overall deck thickness.

Interesting. We've never considered that to be a concern...

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