Yes, to control diag. cracking at reentrant corners.

Quote:

Yes, to control diag. cracking at reentrant corners.

I get that that's the idea, but why can't the other 4 bars control cracks? Especially when forces are low. Is there a study or code reference you can cite, or is it just one of those "that's how we've always does it" things?

www.hellodwell.design

I started in a place that used the diagonals all the time. Then I moved to a place that never uses them. No change. I know this has been debated here before if you can find it.

see: https://www.concreteconstruction.net/how-to/site-p...

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Remember stress concentration at member geometric changes? Corner of opening represent one. And, remember concrete failure plan is 45° from the principle plans? The crack on the corner is usually extended approximately 45° (from horiz. or vertical) out, the diagonal bars minimize the crack width and length by taking control of the tensile stress developed on each side of the crack. Make sense?

Member self weight, temperature effect, deflections, quality of the concrete, and different rate of shrinkage, all are contributors to the complex stresses around corners. The influence of member forces is minimal if the wall/slab is properly reinforced. The diagonal bars are sometimes joked as "feel good bars", but a good insurance for sure, especially for large openings.

If you still have doubt, walk around a residential area and look around, see where you can spot most cracks, and the pattern of the cracks. Pay attention to houses with brick, stucco, and concrete facade. Have fun.

I know the diagonal bars are shown in Chapter 7 of CRSI's design guide for enconomical RC Structures, but they do not provide much additional detail other than "The main purpose of the diagonal bars at the corners of openings is to arrest cracks that can form at these reentrant corners."

We never do, as it introduces more layers, but you can if you want. Diagonal bar is better at arresting the cracks.

Always have and always will - do need to verify bar clearances though as previously mentioned.

Diagonal bars are just more effective than the vertical and horizontal bars in resisting the diagonal tension loads.

Mike McCann, PE, SE (WA, HI)

Mike:
If you have double bars... you already have 4 of em (only working part time)... How many do you need? I haven't used diagonal bars for likely more than 30 years. It congests an area already prone to honeycombing.

Dik

Dik:

I see your point, but I just prefer a direct tie. Plus I have never had a problem with congestion related Deterioration.

Cheers.

Mike McCann, PE, SE (WA, HI)

I usually design and specify a minimum clear cover of 1.5", on the cross section views, to the outer/top & bottom most reinforcement. The diagonals are mostly 2 - #3 or #4 bars placed locally, thus congestion wouldn't be a problem. But if the member thickness is too thin to maintain required covers to the main reinforcement, then it is better not to have the diagonal bars, which tend to introduce more problems than gain benefits.

"I see your point, but I just prefer a direct tie. Plus I have never had a problem with congestion related Deterioration."

Two engineers will design something three different ways... Cheaper and easier to build.

Dik

At the end, the code does not require it (maybe in the commentary), so the diagonal bar is there by practice preference. Also, every design office tends to have its own standard and details.