Considering moment In Design 2

[Xeus]

Hi there. This might sound really weired but today I got involved in a discussion with our senior engineer who has 30 years of experience under his belt. (I've joined this company just recently) He was saying that in east coast we do not consider earthquake loads in design and also in areas where basic wind speed is 90 mph (Minimum) we do not even consider connections (beam, column, base, etc.) moment resisting. We assume everything PINNED and do our design. Well, I'm not used to this and have never heard of such a thing. Particularly when it comes to a concrete building, it is automatically a full moment resisting frame and you can't just ignore it! Anyway, that's what he says and none of us could convince the other. Since I haven't been doing this for a long time (and what he says is 100% in contrary with what we learned in university) I thought I might ask you guys. Is he right? Sounds impossible to me but is it really true?

I appreciate your help. (although he wouldn't listen anyway, but I'll know the truth.)

 

[csd72]

If you designed the concrete frame as pinned, it would probably stand up, but I imagine there would be huge cracks at the pinned joints.

I would also think that this would lead to larger beams than necessary with very little benefit.

I have always designed concrete frames as continuous with moment connections to the columns.

 

[woodmeister]

If there are no cold joints in the concrete system, it is inherently fixed at each end. Think of a simple foundation in plan, really the whole rectangle in plan is a rigid box

 

[woodmeister]

I should add, it is inherently fixed, but if proper reinforcing is not added, the system may act as pinned with unsightly cracks.

 

[Xeus]

That's exactly what I told him. He was arguing that you define the behavior of the system by the design concept you choose. I didn't agree and said technically, you can not ignore the fact that moment will be transferred between beams & columns so you can not dictate your thoughts all the time. Well , meybe toy can do that in a steel building where you choose what type of connections you use (e.g. simple or moment resisting or ...) or at connection to foundation, if you assume everything pinned, the whole framing system will become unstable. I don't find his argument valid at all. My main problem is that he says "We design like this in the U.S."

 

[woodmeister]

Xeus, Here on the east coast seismic usually doesnt control the design, wind does. Perhaps it is just a conservative approach by assuming pinned behavior, and adding in shear walls to handle lateral forces with the moment resistive nature of the frame as sort of suspenders along with a belt.

 

[271828]

One of my pals went to work in D.C. and he said kinda the same thing. He said they design 8-10 story flat plate and flat slab bldgs all the time and don't run the first lateral analysis. I've designed a few east coast concrete bldgs and for ones not that tall, the gravity-only LCs resulted in tremendously higher moments at the support and drift was next to nothing, so that assumption would've been ok.

Are you sure your boss wasn't saying to design for wL^2/8 and conservatively throw that into the top and bottom steel? I could see an older fellow doing that. Be careful arguing with him. Remember that it'll probably take you a couple of decades to design what he has, LOL.

If he doesn't use top steel, he still might be right about the lack of cracks. He has some minimal steel at the top right, if for no other reason, to hang stirrups off of. I wouldn't do it, but I can see how that might still work fine.

 

[271828]

Xeus, after reading your last post, all I have to say is "Hang in there." Don't argue a lot with him. It's hopeless and won't get you anywhere in the company. He's been doing this for (maybe) longer than you've been alive, and I assume nothing fallen down yet, so there doesn't appear to be a need for you to save bldg occupants from him, LOLOL.

He also might be completely right and just not communicating correctly the idea to you.

As a side item (sorta), remember that one doesn't always try to define the exact load path. Take steel connection design for example. For bracing connections using the Uniform Force Method, nobody can say exactly how each force component gets from one end of the conn to the other. It's adequate to make sure that _A_ statically admissible load path is strong enough. The conn will be as strong or stronger than that. That's all the Uniform Force Method does--comes up with _A_ load path.

Not saying that this is exactly what he's doing or arguing, but it's something to think about.

 

[csd72]

He is right in one thing, concrete does tend to act the way you reinforce it (to an extent). It just cracks to alleviate the stresses where there is no reinforcement.

I think this may be a violation of section 10.6 in ACI 318 where they talk about minimum reinforcement.

Do not question his judgement directly, ask 'to understand' what the justification is according to the code.

 

[patswfc]

Your boss is right in saying 'you define the behavior of the system by the design concept' but you got to make sure that you detail it such that your design assumption are valid.

As for designing an RC frame as having pinned connections it can be done but i wouldnt analyse it this way.

 

[jike]

In my opinion, ACI section 8.3 clearly spells out the method of analysis with sections 8.4 thru 8.9 providing additional info.

Do the drawings that are being produced have both top and bottom steel and do they seem reasonable in size and quantity as to what you would normally expect from the continuous method? If so, then the building may be built adequately and it is just the particular method he uses that is the issue.

 

[JAE]

I worked with "seasoned" engineers for many years in a firm that did a lot of concrette framed buildings.

We did do a lot of analysis where we designed continuous concrete pan joists as pinned at supports (continuous across the support while ignoring any beam torsional stiffness) and for the concrete beams where we ignored column stiffness and assumed no moment at the far ends of continuous spans.

This of course created higher positive moments which yielded higher bottom steel, higher negative moments at the first interior support, and created zero negative moments in the far ends.

At the far ends, despite the zero moment, we added some top bars, usually about 60% of the top bars at the first interior support. We felt this would keep cracking under control and avoid loosing much of our [φ]V_c at the support.

 

[csd72]

JAE,

How did they calculate the moments in the external columns? That would be my biggest concern, especially with 2nd order effects.

csd

 

[jike]

To obtain the gravity moments for the exterior columns you can do a one cycle moment distribution (oops, I am showing my age) with the column above and below and exterior beam span all fixed at their far ends.

 

[JAE]

csd72,

We obviously didn't have any direct moments to use so what usually happened is the engineer would use wL²/10 to estimate an end moment and use that.

This was pre-PC days so doing a full blown lateral frame was a much bigger effort than it is today with RISA, ETABS, etc.

 

[Taro]

jike's code references are right on track. In particular, look at ACI 318 section 8.4.

Negative moments can only be reduced up to a maximum of 20%. So treating a fixed end as a pin is an obvious code violation.

However (as jike mentioned) he may just be using a conservative method to calculate the bottom steel, then adding additional top steel to satisfy the code.

 

[lkjh345]

Jike:

Did you say ' moment distribution ' ???

There's a term I haven't heard in many a year!!

 

[csd72]

A moment what? You mean you have to do it by hand! - just kidding.

 

[JAE]

shudder

 

[vmirat]

Xeus,
I second what 271828 said. As your mentor, he has an obligation to make sure you understand whatever it is he's trying to teach you. So far, it doesn't sound like he's doing a very good job. However, I suggest approaching him again (in the nicest way possible) and see if there is more to this than meets the eye. Senior engineers have done these calcs so many times that they just know things like seismic are not worth calculating because wind ALWAYS governs. They may be taking the approach that, even though the design may be conservative, it saves the client money by minimizing the time it takes to design the project and maybe adds a little more factor of safety into the project, again benefiting not only the client but your company as well. I would bet that he learned this approach from the guy that mentored him and so forth. It's important that you understand and agree with what the firm is doing. Otherwise, you may want to start looking around for another place to work....after you've learned all you can from this guy.