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






RE: Considering moment In Design
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.
RE: Considering moment In Design
RE: Considering moment In Design
RE: Considering moment In Design
RE: Considering moment In Design
RE: Considering moment In Design
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.
RE: Considering moment In Design
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.
RE: Considering moment In Design
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.
RE: Considering moment In Design
As for designing an RC frame as having pinned connections it can be done but i wouldnt analyse it this way.
RE: Considering moment In Design
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.
RE: Considering moment In Design
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 φVc at the support.
RE: Considering moment In Design
How did they calculate the moments in the external columns? That would be my biggest concern, especially with 2nd order effects.
csd
RE: Considering moment In Design
RE: Considering moment In Design
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.
RE: Considering moment In Design
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.
RE: Considering moment In Design
Did you say ' moment distribution ' ???
There's a term I haven't heard in many a year!! :)
RE: Considering moment In Design
RE: Considering moment In Design
RE: Considering moment In Design
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.
RE: Considering moment In Design
An example I was talking to him about was that he says although we use top and bottom rebars in continuous concrete beams that go across columns and also we have rebars in columns, there's nothing to transfer the moment from the beams to columns because we do not bend the beam rebars into columns! I argued that the core acts like a rigid body and when the end of beam rotates, the core rotates and therefore, the column end rotates. So in reality, assuming connections pinned is nowhere close to truth. What I'm afraid of right now, is that following his methods (which sound completely non-professional to me) might affect my engineering knowledge and if I join another company, saying such things make others laugh at me and ask if I'm coming from 19th century.
RE: Considering moment In Design
I have also recently begun a career of heavy concrete design and it is kind of weird coming from doing mostly steel, wood, and masonry. Concrete design to me is very arbitrary, you can assume all kinds of things in designing a slab and each way may work. Also, I have noticed other engineers that have been doing only concrete for years ignore compatibility torsion all the time in floor systems. And also treat floor beams and one way slabs framing into spandrel beams as pinned connections and provide very little rebar at the connection (the 20% max distribution is not considered). The reasoning is that spandrel beams aren't going to give you any significant amount of end moment resistance.
It would seem to me that always assuming a pin at such cases could be bad for shear resistance. I don't know...
RE: Considering moment In Design
Yes
I think this is a fact that many engineers out there don't appreciate. Seismic is much more "controlling" than most think.
RE: Considering moment In Design
RE: Considering moment In Design
Also, for long narrow buildings, the wind on the short elevation doesn't control the overall lateral design due to the limited exposed area vs. the seismic demand.
Some years ago I was a young engineer working on one-story buildings in Texas. The buildings were for a grocery store chain and we had a UBC zone 0 seismic condition. As I was learning how to design them, I asked how to do the lateral wind analysis and the engineer I worked under scratched his head and said something to the effect that, "...well, we probably should check that but we just haven't in the past."
RE: Considering moment In Design
Are you serious when you say he considers everything pinned?? Even steel buildings? wtf?
RE: Considering moment In Design
Why would you quit doing things your way and adopt his? Is the office so dictatorial that they'd tell a 10 year guy how to design concrete beams? If so, I'd be out of there in a hurry.
This needs to be a "you can do it this way, but I'll do it that way" situation, and nothing more. He's obviously done fine and will retire soon. You're probably technically more correct, so there's no reason to start doing things in a less precise manner.
As for the DC area and ignoring lateral loads, I'd guess that he's right, regardless of whether he uses WSD or USD. My pal who went to work there is one of these ultra-precise fellows and had this "WHAT!?" reaction at first, but proved it to himself pretty quickly. You will probably do the same. It's also good engineering judgment to recognize when one is NOT in one of those severe areas and adjust one's work habits accordingly.
I've never designed anything in DC, but have designed mostly in moderate to low EQ load areas. I've found it pretty hard to make anything other than gravity control in most cases. Usually end up with girder end moments about 5-10x larger from 1.2D+1.6L than with any lateral load combo. It also shouldn't matter if it's WSD or USD load combos, BTW. You usually get the 4/3 increase with one, but 0.75 on the other.
RE: Considering moment In Design
"Well what he says is about DC metro area where earthquake factor is 0.05 and they ignore it all the way. They even say a 90 mph wind is not controlling and just design for gravity loads."
Xeus;
I am getting the impression that he ignores the lateral system altogether and just designs for gravity?
RE: Considering moment In Design
271828 : Well, yes, it's a dictatorial environment in our office. He's so controlling and also in bad mood every day that we have to do things the way he wants. As for leaving the company, I can't do that right now. Long story ...
everybody : Irrelevant to this topic but not worth opening a new thread for. I have a question about checking deflection. I have always checked deflection for LL and limited it below L/360. Then limited span/Depth ratio with respect to DL/LL and finalized the design. Now my question is : Is there any direct limitation for "DL only" or "DL+LL" for deflection, similar to L/360 for LL? I looked everywhere in AISC and couldn't find anything. Are the two limiting criteria I use enough or you guys check other limitations too?
RE: Considering moment In Design
Re: DL or TL deflection.
Answer: Understand the effects of that deflection on the usage of the structure, connecting elements, long term creep effects, visibility of the deflection....
in other words....use good engineering judgement.
RE: Considering moment In Design
As for the deflections under live and dead combinations as well as isolation, the typical method for canadian engineers (as least those from whom I have learnt and design procedures I have read) include dead load deflection only if the system will not naturally assume all dead loads prior to being engaged by live loads. IE, if you have a system who's dead loads are mitigated (pre curve in a steel beam) or automatically assumed during construction (mass masonry construction), the live load is the only component of deflection you need consider.
This is separate from a fundamental frequency estimate of 18 over the root of the total deflection. You must always include dead loads when making this estimate. There are a couple of threads about vibration, and it's outside of this discussion.
The Canadian methods aside, and assuming they are fairly similar to the US methodology (commonly are, as our industries work quite closely together), the New Zealand code DOES contain guidelines for separate dead load and live load only, as well as combination, deflection limitations. They are contained in an appendix to Part 0: General Design Principles. See AS/NZS 1170.0 Appendix C "Guidelines for the Serviceability Limit States". So such guidance does exist, it just may not be a common approach in the US. The format is "Element type | Phemomenon Controlled | Serviceability Parameter | Applied Action | Element responce".
A good example is for wall elements: Columns...
"Column | Side Sway | Deflection at top | Ws | Height/500"
where Ws = Wind load (excluding any dead load).
Please let me know if you would like me to post a copy of the table for your consideration. However it's based on the following BRANZ report, which would be the better reference for what you want. I will post the link once's I'm back at the office (can't seem to find it right now, and the BRANZ website is down). If it's up, check for yourself... www.branz.co.nz Author: Andrew B. King.
Good luck,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Considering moment In Design
Total load deflection limits are listed under Table 1604.3 of IBC 2006 along with the L/360 live limit.
Can you tell me where the buildings he has designed are located so I won't got there? Seriously, there is always some lateral capacity even with shear tab connected steel framing. But to make this assumption as the standard under 90 mph wind? For a basic 2-story office building in 90 mph area I would expect around 15psf*200ft*(13ft+7ft)=60 kips of lateral into the framing. And seismic would likely be more.
The IBC requires the lateral system to be defined for seismic under the general notes. I would be interested to see what he lists..."pin connected moment frames"? What is the R value for those?
RE: Considering moment In Design
RE: Considering moment In Design
There's a big difference between IGNORING wind and seismic and having designed a zillion bldgs of a specific type in a specific location and recognizing that 1.2D+1.6L (or more likely 1.4D+1.7L for most of his career) ALWAYS CONTROLS EVERYTHING by about 5x.
If one has done something enough times to know that it won't control, and still checks it every time ad nauseam, then I'm not sure that says anything positive about one's engineering judgment either.
I have to assume that if he designs a 10 story flat plate bldg in Memphis or anywhere else he's not familiar with, he'd check lateral loads.
And we're talking about concrete bldgs only, right? If he has a steel framed bldg, surely he includes moment frames, braced frames, shearwalls, or something to count as the LFRS.
LOL, if anybody freakin' out about going to DC, better stay out of all those 80 year old multi-wythe brick and wood framed bldgs, just as one example. Some of those suckers were built using methods that we'd never dream of today.
RE: Considering moment In Design
1- Once with factored loads and design the beam for moment;
2- Once with service loads and check actual deflection for use with L/240 & L/360.
Do you do the same thing? Analyze twice?
This comes from our latest project which is an equipment shelter platform which will be installed on the roof of an existing building and because of some sensitive equipment inside that shelter, we have been required to accurately calculate deflections of all beams and not just go with that table. Any ideas?
RE: Considering moment In Design
Yes, use factored loads to come up with moments, shears, etc. for strength design.
Yes again, use service loads to check deflection.
As for the last paragraph, why is it harder than just calculating the deflection due to the new load?
RE: Considering moment In Design
I think he is saying all buildings and not just concrete. That was my question a few back. Also, I would expect the buildings that are 80 years old have a lot of redundancy built in that economized steel framed, metal stud office buildings of today don't have.
RE: Considering moment In Design
RE: Considering moment In Design
Well, what he recommends won't always work, especially for girders.
You can check deflections without another load combination. Take your deflection limit and scale it up by the ratio of the factored load to the service load. For gravity bms (assuming that's all we're talking about), 1.2D+1.6L should control the strength calcs 99.9% of the time. Your D & L should be the same over your entire floor or roof, so figure out a scaling factor. Maybe L/360 goes to L/300 for example. That would be the fastest way to approach the problem without buying software.
Which brings up another question: Why not use a program like Ramsbeam? Last time I checked, it cost next to nothing, like $300 and is very handy. Then you could set it to match your standards and be done with it.
The real question to me is: "How long before you're outta there?!" There have to be other places you can work for.
RE: Considering moment In Design