Deck Haunch Reinforcement

[bridgebuster]

I'm working on the rehab of a viaduct and we have to raise the deck upwards of 9"-10"; we'll have some deep haunches. The DOT standard is to reinforce the haunch when it exceeds 4", using 2 - #4 bars and #4 stirrups at 18".

This has been the standard for as long as I remember. Now it's got me thinking, what's the basis for designing haunch reinforcement? I can't come up with one; other than "we've always done it this way".

 

[Qshake]

Hi Bridgebuster - I've pondered this question too at no real length several times in my career with no real answer. Like you I've heard the old "we've always done it that way". The longitudinal rebar I've always written off as temperature reinforcing and the tophats as additional horizontal shear reinforcing.

I suspect that between us we've been in this business quite a while and if we haven't seen an explanation yet then we're not going to now, not very easily anyway.

Some ideas that come to mind:

Tophat bars - Effectively extends the shear stud into the deck proper and provides good development length via the horizontal legs. It's easy for us to know the haunch is going to have to carry signicant horizontal shear stress (over a relatively small width). There again, I've never seen any failures or hauches that have cracked to the point of being useless.

Longitudinal bars - due to horizontal shear stress and strain compatibility with the flange, the haunch will undergo flexure, though not as much as the "single girder analysis" may have us believe. Hence the longitudinal bars could aid in this respect as well as temperature.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[cooperDBM]

The studs have to extend above the deck steel. If the haunches are larger than planned then the studs may not be tall enough, so reinforcing the haunch should take care of that.

The ultimate interface shear design also assumes some slippage along the top if the flange so that all studs are assumed to be mobilized equally. This requires that the studs bend to accommodate the slippage. Reinforcing the haunch will maintain it's integrity while the studs are bending otherwise there would presumably be significant cracking. As Qshake says the stirrups also help to transmit the horizontal shear along with the studs in a tall haunch.

 

[bridgebuster]

Thanks for the responses guys. I didn't think of the longitudinal bars in terms of temperature reinforcement. Your opinions on horizontal shear were somewhat similar to what I was thinking; I was looking at the haunch reinforcement as being akin to a bond beam in masonry construction.

Roughly 75% of this mile long viaduct will require haunch reinforcement. We're going to allow the contractor to piggy back the studs where necessary. I did it on a project about 25 years ago and that deck is still holding up.

Qshake - you're absolutely right about being in this business for a long time and we're probably not going to find some explanations easily. Although sometimes, it could be, the way we've been doing it turns out to be the right way. For example, there's a drawing with rivet replacement criteria that regularly appears in NYSDOT steel bridge rehab projects. It's been floating around for at least 35 years and no one knows how the criteria came about. Funny thing, there was a study by the USACOE in 1999 on the subject; they went into FEA and came up with similar results. It doesn't appear that they were aware of this drawing.