Potential Typo in AASHTO Section 6.10.10.3? 1

[BridgeEngineer21]

The section in the 2020 version of AASHTO reads: "For members that are noncomposite for negative flexure in the final condition, additional shear connectors shall be provided in the region of points of permanent load contraflexure". However the commentary reads "the purpose of the additional connectors is to develop the reinforcing bars used as part of the negative flexural composite section." My question: is the main section text supposed to say composite instead of noncomposite? Logically, you need extra shear connectors if you're trying to achieve composite action, and that's what the commentary seems to confirm. Does anyone have thoughts on this?

 

[BridgeSmith]

If you don't have shear connectors through the negative moment regions, those regions cannot be designed as composite sections, so they are considered noncomposite for design. However, if you have composite section on both sides of a negative moment section, the reinforcing bars will still develop tension stresses as the section bends in negative flexure. To maintain the continuity of the deck (and not have huge cracks develop), the tension force in the reinforcing bars must be transferred to the girder top flange.

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

 

[BridgeEngineer21]

So then, if you are already designing the girders to be composite for negative flexure, the check in Section 6.10.10.3 doesn't apply? It seems kind of circular - if you're not designing composite, then you have to follow 6.10.10.3, which if you meet that check means it is now acting compositely, which means 6.10.10.3 doesn't apply? Maybe I'm still misunderstanding this..

 

[OSUCivlEng]

Back in the day shear connectors were not provided in the negative moment regions of continuous steel girders. I have done quite a few deck replacements where we added shear connectors in these areas.

My guess is this language was added to the code to encourage the use of shear connectors in the negative moment regions. However, things can get confusing and messed up when you are writing by committee.

 

[BridgeSmith]

So then, if you are already designing the girders to be composite for negative flexure, the check in Section 6.10.10.3 doesn't apply?

Correct. If it has shear connectors throughout, it can be designed as fully composite (composite the full length). The forces that develop in the deck are transferred to the top flange all though the negative moment region, rather than accumulating at the end of the region.

It seems kind of circular - if you're not designing composite, then you have to follow 6.10.10.3, which if you meet that check means it is now acting compositely, which means 6.10.10.3 doesn't apply?

There a difference between full composite action you can count on for design, and composite action that may be present under some circumstances, in some regions without shear studs. Composite action may occur in certain portions of the negative moment regions without shear studs, by virtue of the deck being locked in to the steel girders at the ends of the negative moment regions.

Also, consider that because moving live loads typically compromise a majority of the load on a bridge, the contraflexure points for total loading shift substantially. There may be significant negative moment in the 'positive moment region' and significant positive moments well into the supposed negative moment region.

The bottom line is that you need to maintain continuity of the forces in the reinforcing steel so you don't have large tension forces transferred to the concrete deck.


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