development of positiv moment / anchorage of rfm in joints

[bender36]

Dear colleagues,

We are facing a problem in understanding and applying the ACI 318M-02 for our project. It is the first project we do acc. ACI. It would be very nice if somebody can help us:

Problem: anchorage of reinforcement at joints / development of positive moment

12.11.1. (…) … such reinforcement shall extend into the support at least 150mm.

12.11.2 When a flexural member is part of a primary lateral load resisting system, positive moment reinforcement required to be extended into the support by 12.11.1 shall be anchored to develop…

In our design, we have lots of continuous beams on columns and continuous grade beams on piles. Even if we apply all load combinations including earthquake & wind, the moment envelope shows no positive moments at connection points beam-column or beam-pile cap.

Acc. 12.11.1 we have to extend ¼ of the positive moment behind the face of the beam by 150mm. Now, there is this clause 12.11.2. I understand that our frames and grade beams carry the lateral loads into the ground – so they are part of a primary load resisting system.

Question: Is it like this?
What is then a secondary lateral load resisting systems ?
When are 150mm for restraint beams / continuous beams sufficient?

Thank you very much and best regards from Germany !

MARTIN

 

[JAE]

Neither of the two clauses above require you to develop the positive moment "IF you have positive moment at the support". They simply require you to develop the bottom reinforcing at the support, regardless of the moment.

So even without positive moment you still need to develop the bottom bars as they require.

 

[shin25]

Depending on what seismic design category that you are in, chapter 21 (seismic detailing requirement) will require some positive moment capacity at the joints regardless. These requirements will also need considerations of rebar development at the support.

 

[bender36]

Thank you for your comments !

@ JAE: I have understood that I have to extend the bottom reinforcement anyhow in my support but question is fully developed or only 150mm? This is not clear!

@ shin25: and what to do if there is only wind to be applied ?

Thank you,

MARTIN

 

[JAE]

I'm not familiar with the 318M version but generally you follow these steps:

1. Bottom bars must extend into the support some minimum distance (6 inches in my world - perhaps 150mm in yours).

2. Per chapter 7, under the "Structural Integrity" section, [bold]some[/bold] of the bottom bars and some of the top bars in some cases must extend and be developed further than what is normally required in item 1 above.

3. Per shin25, you must also consider chapter 21 seismic.

4. If the building is in a low seismic area, then wind obviously controls and you would then have to do items 1 and 2 above, but also develop into the support any reinforcing that would resist tension at the joint.