Seemingly Academic Question 1

[Lion06]

But it's for a real problem.

I'm going to pose this for a reinforced concrete beam design. Say you have a simply supported beam with applied moments at each end such that the beam has a constant moment along its length.

Is there anything that says the reinforcement needs to at the same location along the length of the beam? Specifically, let's say I have three top bars and three bottom bars at the end of the beam, then at some point I need to change to skin reinforcing (Say I just don't want three top and three bottom bars anymore). Can that transition happen without having to use a lap splice?

I'd like to design the reinforcing so that phiMn for both sets of reinforcing exceeds Mu. I'd like to pick some convenient location and fully develop both sets of reinforcing on both sides of this arbitrary location. Let's say the beam is deep enough such that the three top and bottom bars cannot be considered to lap with the skin reinforcement where I change it up.

I'll admit this doesn't feel as clean as running the reinforcement continuous, but I don't see anything technically incorrect with this approach. I actually have a need to do this, but would appreciate some input.

 

[Lion06]

I just want to point out that the reason I chose to fully develop both sets of bars on both sides of my arbitrary section is that no matter where I were to look at a section I have a complete set of bars that is capable of resisting the entire moment that is fully developed and I'm not relyiing on any transfer from one set to the next like you would with a typical lap splice (where one of the bars is taking some load depending how far it is from the end and the lapped bar is taking the remainder).

 

[a2mfk]

Why not provide a sketch of your beam elevation and show the rebar too...

 

[BAretired]

You could place the bars in an 'X' configuration in elevation. Three bars are in the top at one end and go diagonally down to the bottom at the opposite end. The other three bars do the opposite, so at midspan you have six bars at midheight. Can't think of a good reason for doing it, however.

BA

 

[Lion06]

Here is a sketch of what I'm talking about. I feel pretty comfortable with it, but I would appreciate some feedback.

 

[BAretired]

Why not extend the bars in Section 2 throughout the full length?

BA

 

[Lion06]

There is a portion of the length where it's just not physically possible.

 

[BAretired]

I can't see anything technically wrong with the proposal. Will you have ties throughout?

BA

 

[Lion06]

There will be stirrups along the entire length.

 

[BAretired]

Over the length of the lap, you might want to add confinement steel.

BA

 

[Lion06]

By confinement steel, do you mean ensuring the stirrups meet the requirements for column ties?

 

[OHIOMatt]

The below comes from ACI 318-08. My interpretion is, that since you require tension reinforcing throughout, that you would only be able to count the bars that meet this criteria.

12.14.2.3 — Bars spliced by noncontact lap splices
in flexural members shall not be spaced transversely
farther apart than the smaller of one-fifth the required
lap splice length, and 6 in.

 

[Lion06]

Matt-

I did read that and that's exactly why I'm "lapping" (I say lapping only for lack of a better word here) the two sets of bars by ld1 + ld2. If you read the commentary to 12.14.2.3 it says that the spacing limit is provided to ensure you don't get an unreinforced section failing with a zigzag crack.

The "lap" of ld1 + ld2 ensures that at any section there is adequate moment capacity from either set of reinforcing. What this means is that at any location along the beam, one, and only one, set of reinforcing is capable of providing phiMn>Mu.

That's not true of a splice condition. At a true splice condition, you need both sets of bars in the splice to provide phiMn>Mu.

 

[BAretired]

Yes, I believe the confinement steel should meet the requirements for column ties.

BA

 

[Lion06]

Let me pose a similar question. Say you have a precast concrete shearwall that is going to sit on a pile cap at each end. At the base of this wall there is a net overturning moment and a lateral shear.

Let's say that the wall stacks two panels high. There is distributed reinforcing in both panels that provides adequate moment capacity all on its own for both panels.

I believe, and someone tell me if I'm off the mark here, that if you provide localized chord reinforcement for shear friction (to get the shear into the pile caps) and uplift (just as a hold down mechanism), that the localized chord reinforcing need only extend up the height of one panel, not both panels.