Use steel on compression face to Increase phi Mn?

[Blackstar123]

I have a two story RCC structure with multi bays in X direction and single bay in Y direction. The building LRFS is IMRF. Flexure demand on the beams in Y direction due to seismic loads is coming out to be very high such that some compression reinforcement will be required for -Mu. The span of beam is 10.5m

This is what the design forces at the support looks like.

Size of beam = 400x1200
Required +Mu = 1180 KN-m, As reqd = 2961 mm2, Provided: 4-25Dia + 4-20Dia
Required -Mu = 1680 KN-m, As reqd = 4861 mm2, As’ = 30 mm2, Required No, of bars = 12-25Dia

I cannot increase the size of beam due to clear height requirement (i.e., 5m) but also don’t want to provide maximum steel on the tension face of beam.

If I only provide 8-25Dia on tension face and take advantage of skin reinforcement and the steel present in the compression zone, numbers are showing me some decent increase in flexure capacity of the beam, due to compression force in the steel in compression zone (i.e., 4-25Dia + 4-20Dia).

I would like to ask for advice from experts here if they think this approach is workable.

 

[Agent666]

Blackstar, have a look at clause 19.2 which covers the requirements for concrete minimum and maximum strengths, it states a minimum concrete strength of 2500psi (17.23 MPa) is required.

In the metric version they have rounded it down to 17 MPa for some reason. They butcher quite a few formulas as well like this...

https://engineervsheep.com

 

[Lomarandil]

I don't know... in my neck of the woods (Asia, but not Pakistan) 25MPa cylinder strength is the floor (30MPa in the bigger cities). And we have pretty good aggregate and cement quality here. My colleagues in Africa assume 15-20MPa unless they're taking special care to get more.

Skilled labor still makes around $10/day. That's right around the cost of a single 14-16mm bar.

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just call me Lo.

 

[jayrod12]

People I've chatted with that designed for firms in the Philippines was amazed we could specify 25 MPa concrete and not have people scream about it. He was used to 10-15 MPa design strength since everything was site mixed with questionable materials.

 

[BridgeSmith]

He was used to 10-15 MPa design strength since everything was site mixed with questionable materials.

Sounds a little like what I saw in Haiti. The mixing was done in a pile on the ground, with shovel-fulls of aggregate, sand, and cement thrown together, and then water is sprayed on while the 6 or so guys turn it over with their shovels.

I suspect even in Pakistan, the labor cost vs. material cost balance has shifted significantly towards the labor being comparatively much more expensive than it used to be.

Skilled labor still makes around $10/day. That's right around the cost of a single 14-16mm bar.

Ok, maybe my suspicions were incorrect.

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

 

[Lomarandil]

I'm sure they are, in some locales... a point I always make is that the "developing" world is in fact developing. And quickly -- it seems standard practices are turned on their head every 3-5 years.

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just call me Lo.