balanced reinforcement requirements

[sccci]

I am working on a redesign for a bridge deck replacement. The original structure was a poured in place deck and girder bridge. The new structure will use the existing substructure, but replace the deck and girders. The new girders will be be precast stems which will be set in place and decked with metal deck forms. The stems were initally designed to accomodate the required AASHTO HS-20 loads. When I got around to checking the stem for construction loads I discovered the reinforcement ratio (rho balanced)was higher that the allowable (0.0212 vs 0.0187). The stem is approximately 4X stronger than required for the construction moments. Is there any way to get around the rho requirement or can it be ignored since this is a "temporary" situation?

 

[wiktor]

The reinforcement ratio for the stem shuld be ignored, as it represents only temporary condition for the beam designed to be composite with the deck.

 

[rapt]

Wiktor,

I will disagree with you there. Even though the construction condition is temporary, all ultimate strength and short term service checks must be carried out.

Ductility is not a long term requirement. It is a strength requirement and must be satisfied for the construction condition as well as the final condition.

 

[wiktor]

Rapt,
I agree that all strength checks have to be carried.
However, the reiforcement ratio for the precast beam is different case.
The reiforcement is designed for the service condition, i.e. with the composite deck, and checking of the beam alone yields no meaninful results, or leads to the wrong conclusions.
There is no way to satisfy the reiforcement ratio for the precast beam when checked without the flange.

 

[rapt]

Wiktor,

While the amount of prestress added must satisfy transfer and service conditions for the precast element by itself, the precast element must also be checked for ultimate strength and cracking and deflections for the construction condition and possibly for lifting and transportation conditions also. This ultimate strength check requires ductility checks and ductility must be satisfied.

If the precast element is only sized on service conditions then the design is not being done properly.

 

[sccci]

Thanks for the lively discussion on the topic, but the stems in question aren't prestressed elements, they are precast with 60ksi #10 rebar.

 

[wiktor]

So that's exactly was the point - there is no way to keep the reinforcement ratio for the beam alone within the limits specified, as the bottom reinforcement is designed for the full loads with composite deck.
The ductility eqiurements are applicable to the prestressed beams (by code), and, in the case discussed will be satisfied by fullfilling the requirements of 8.16.3 for the final section. (to provide balanced condition)

 

[rapt]

Sccci,

The same logic applies to RC precast if it is being required to support the construction loads. If it is fully propped during construction, there is not problem but this is not the case. For construction loads it must be designed for ultimate capacity and ductility is one of the design requirements of this. It must also be checked for deflection abd cracking.

Wiktor,

In that case the design does not work. The construction condition must be satisfied if the precast member is supporting the construction loads. Just because the deck is not in place does not preclude the need for the check. The precast member is being expected to carry out a load carrying function during construction and must be checked for this.

 

[wiktor]

Rapt,
I have to strongly disagree. The design does work, and there is no need to satisfy the balanced condition reinforcement requirement - because it's a temporary condition, and beam is designed with the composite deck.

The ductility requirement was introduced to assure that the beam will fail due to yielding of the reinforcement, and in no case due to exceeding crushing strength of the concrete, to prevent sudden collapse.

It's quite obvious, that the precast beam without top flange will have very high ratio of reinforcement and ultimately will fail due to crushing of the concrete. But I have to stress it again - that's not the service condition, and the balanced ratio does not apply.

Without understanding the logic behind the code, it's rather difficult to interpret the meaning of code, or decide how to apply it. Otherwise anybody who can afford few bucks for AASHTO could claim to be a bridge engineer.