## Extent of negative moment reinforcement

## Extent of negative moment reinforcement

(OP)

Good day.

Suppose we have a multi-span continuous bridge deck. We need to provide negative steel to the deck on both sides of the pier. The question is how far from the pier should we terminate the negative steel?

1. Draw dead + superdead load moment diagram. Terminate negative steel at the point of contraflexure.

2. Same as 1 but extend a little more to include development length.

3. Use dead + superdead + live load moment diagram, and terminate at the point of contraflexure (or extend to include development length)

Can someone please provide me a reference (Code) on what is the correct method? I have heard from other people, but I need solid reference.

Personally, I would go with 2., but I cannot explain it properly.

Also, if you follow a thumb rule, please share.

Thank you

Suppose we have a multi-span continuous bridge deck. We need to provide negative steel to the deck on both sides of the pier. The question is how far from the pier should we terminate the negative steel?

1. Draw dead + superdead load moment diagram. Terminate negative steel at the point of contraflexure.

2. Same as 1 but extend a little more to include development length.

3. Use dead + superdead + live load moment diagram, and terminate at the point of contraflexure (or extend to include development length)

Can someone please provide me a reference (Code) on what is the correct method? I have heard from other people, but I need solid reference.

Personally, I would go with 2., but I cannot explain it properly.

Also, if you follow a thumb rule, please share.

Thank you

## RE: Extent of negative moment reinforcement

## RE: Extent of negative moment reinforcement

-----*****-----

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

## RE: Extent of negative moment reinforcement

Our DOT stagger every other rebar 3' longitudinally.

## RE: Extent of negative moment reinforcement

## RE: Extent of negative moment reinforcement

We have never staggered the ends of the bars, but we don't terminate them at the end of the section, either; they're lapped with #4 bars that extend through the positive moment regions.

For an integral abutment, the #4 bars get lapped with #6 bars at the 1/4 point of the end spans, and those run to the abutment and get bent into the end diaphragm.

## RE: Extent of negative moment reinforcement

-----*****-----

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

## RE: Extent of negative moment reinforcement

## RE: Extent of negative moment reinforcement

Steel Bridge Design Handbook Chapter 17 seems to indicate that it was a past practice to define negative moment region by the points of dead load contraflexure. It is relatively easy to find this distance from the piers.

Then the handbook goes on to talk about the tensile cracking requirements.

It is however, difficult to determine exactly up to what length from the pier the tensile stress in deck exceeds the factored tensile strength. So approximations like 25% mentioned by Bridgesmith is helpful, though it is not per the code.

Bridge decks have typically 10 mm of waterproofing layer. So, in my opinion even if there are some cracks developed beyond the point of dead load contraflexure, they are not that harmful. So I will go with Option 2 I mentioned in the original post.

## RE: Extent of negative moment reinforcement

We haven't found it to be that difficult. It's just M/S. Calculate S for the composite section with the deck transformed to an equivalent area of steel (divide by the modular ratio, n), divide the max moment at 10 or 20 points along the span by S to get the stress, and transform the concrete back (multiply by n). Easy peasy with a girder line analysis program and Excel.

## RE: Extent of negative moment reinforcement

They do? Ours typically don't unless it's needed, which usually isn't until 20 years or more into its service life.

## RE: Extent of negative moment reinforcement

Typically we don't do calculations for deck steel. Empirical method, which takes into account the arching action of the slab, indicates that we could provide 15M @ 300 mm the entire length of the deck, and then only in the "negative moment regions" we need to add 20M @ 300 mm alternating with those 15Ms. This gives the 1% steel required. This is similar to what 3Fan above was referring to. But then the question comes, how far does this "negative moment region" extend? If we use live loads (in addition to dead loads), then it becomes computationally difficult because - depending on the position of the vehicle - we could see the presence of the negative moment far away from the piers. Then we would end up providing "negative" steel nearly the whole length of the bridge save near the abutments! That's why defining "negative moment region" in terms of dead load contraflexure only is perhaps more reasonable.

Frequent use of de-icing salts from late November to early April, and the freeze-thaw cycles of penetrated water makes it mandatory for bridge decks to have 10 mm waterproofing.

## RE: Extent of negative moment reinforcement

concretedue to the Service II load combination is less than 0.9*fr (See AASHTO LRFD 6.10.1.7). Depending on the DL to LL ratio, this may be more or less than the DL contraflexure point, but we've always found it to be less than 25% of the longer span adjacent to the pier under consideration.## RE: Extent of negative moment reinforcement

Yes, I understand that it is the tensile stress in

concretethat we are talking about. The mention of waterproofing came because even if there is some tensile crack in concrete deck, it is covered by the waterproofing layer - so salt and water cannot penetrate.CHBDC design philosophy is similar to AASHTO. But of course,the load factors and resistance factors of SLS1 are different from AASHTO Service II.

## RE: Extent of negative moment reinforcement

FYI, the Service II load combination factors component DL by 1.25, wearing surface DL by 1.5, and LL by 1.3.

## RE: Extent of negative moment reinforcement

DL = 1.0

LL = 0.9

But the resistance factor phi = 0.75

Waterproofing material is rubberized asphalt, plus fibre board and special fabric.

## RE: Extent of negative moment reinforcement

Interesting. And this is the 10mm thick overlay that goes on the bridge deck?

## RE: Extent of negative moment reinforcement

Waterproofing consists of two layers of rubberized asphalt, a fabric reinforcement (white sheet) and a protection board. All together make 10 mm.

The fabric reinforcement adds an extra impermeable layer. The protection board protects the waterproofing from getting melted and dislodged when hot asphalt (wearing surface) is placed on them.

https://www.youtube.com/watch?v=DLEpNQnOD7g

## RE: Extent of negative moment reinforcement

## RE: Extent of negative moment reinforcement

My thought would be 3 + past the inflection point would apply? Is there any reason why we wouldn't at least consider LL in your negative moment envelope?

This is from the CHBDC. The Concrete Design Handbook has Figure N12.12 describing the clause.

You would also need to check a particular section for minimum (and maximum) reinforcement per Clause 10.11.5.3.2 as you indicated and reinforcement ratio and cracking moment. The 2014 CHBDC used to also require you to check the crack width associated with your reinforcement, with 0.25 mm being the max crack width for a bridge deck subject to de-icing chemical. In 2019 they have changed it to a stress based check.

@BridgeSmith

Bridge deck in Canada are almost always exposed to de-icing chemical. We rarely have bare concrete decks.