Is 3D analysis/design modeling really necessary 5

[mtu1972]

As background information I am an "experienced" structural engineer but a relative newcomer to my present firm. We design industrial facilities in various locations accross the US.

Younger engineers/supervisors feel that it is essential to create the complete 3D building model to design any new structure. This means 8 directional wind load cases, additional wind load cases for various roof pressures and suctions, ditto for seismic (whether it will govern or not), secondary members such as girts, equipment loadings, floor openings, etc.

We do not work on essential and/or highly irregularly shaped buildings.

We do not do BIM or 3D CAD drafting unless specifically requested by the customer.

Am I among a dying breed in thinking that the lateral load resisting elements can be done in 2D frames in the two perpendicular directions and the remaining infill framing can be designed using only gravity loads and conventional design methods?

GJC

 

[271828]

Probably a dying breed, LOL. I agree with you that a lot of bldgs can be designed using 2D frames. I'll offer a perspective from the other side, though.

Some firms commonly design bldgs with several unsymm moment frames in each direction and it becomes hard to know how much load to put on each of them. THe firm develops design tools like spreadsheet preprocessors, etc. that make generation of the 3D LFRS fairly easy. Wind loads, seismic loads, etc. generated using spreadsheets also. If a bldg really is easy, then it takes very little time to design it using either approach--2D frames or the typical (for them) 3D approach. Then it's more a matter of being systematic and going through the familiar steps. LOL, it would probably be slightly *harder* for them to figure out how to take their "exact" way and adapt it to an approximate 2D model. Not to mention that some firms use automated systems so much that they're very used to that process.

 

[PMR06]

I see the same type of thing in our firm. I think the important questions are in regards to the accuracy of the final product and the economy with which it can be produced.

Personally, I can whip out a 3D model with wind, seismic, live and dead loads in about the same time some of the other guys can decompose the structure into a myriad of 2D frames, and spreadsheets. The final designs will end up relatively the same, although I would argue that putting together the final calculations is easier with 3D model.

The key is that I have learned to make the 3D program work efficiently for me. I don't dilly-dally around for days trying to put together a model, and I don't let the program take over and do it's black box thing.

 

[asixth]

I would do both. I build up the model in three dimensions, normally I don't model purlins or girts, I make the assumption that they load the rafters/mullions based on their tributary area. For a lateral force resisting system it shouldn't make that much of a difference as the forces find their way to the resisting elements. But I never take the answer from the computer without verifying it with a simple model. It also allows me to better calculate the stiffness of the structure and see how far the building is moving under service conditions.

There has been plenty of threads written about computer methods against hand calculations and if you cannot verify something by hand calcs, then you will need to hit the books and do some more reading up on the topic (not saying this about yourself but there is nothing that I hate more than seeing a young guy use a program without having a clue what the answers mean).

If you are concerned that the younger engineers don't have a good grasp of the load paths, then go up and ask them why the are building up the model and what they are trying to design. They should be able to answer this question straight up. Before building up a model, the engineer should know how the structure is working.

 

[mtu1972]

Thanks to the three of you for your insights.

I've done the calcs by hand for all too many years. My feeling was that it is a waste of time to build the entire model to the level of complexity that is being done, when 90%+ of the members are simple span beams, girts, purlins and/or columns not directly contributing to the lateral load resistance.

Personally, I also would not put in the girts but would add the wind loads to the columns directly. But it seems with wing load generation, et. al. that many of the other engineers want the computer to do ALL of the work for them.

And asixth, I especially agree with not just taking the computer output as gospel. In fact, my experience lends itself to doing quick and dirty preliminary designs as the basis for the computer input, so often times the design output requires just some final tweaking of member sizes.

I can count on one hand the times I have modeled whole buildings so my perspective is undoubtedly skewed.

Perhaps instead of validation of my approach this will inspire an old dog to learn new tricks.



GJC

 

[hokie66]

mtu,

I think it depends on the building. There is nothing wrong with designing regularly shaped industrial buildings with 2D analysis methods. Where 3D helps is with high rise buildings, buildings which depend on a combination of bracing and frame action, and irregular structures.

 

[rowingengineer]

"If you can't describe your structure on one page before your model, then you have no idea what is going on" a quote by a former college. Thus I don’t mind how people model the buildings just understand the limitations of your model, ie drive the model, don’t let the model drive you. Steel in 3-D models is normally hard to model wrong; it behaves in a linear elastic manner generally for deflections. Concrete in 3-D model is “dangerous” if you’re a young player and you don’t understanding the basics (creep, shrinkage, restraint and torsion)and how to include them in your 3-D model correctly, then deflections will be a problem.

I refuse to let my grad engineers near the computer for the first 6 months of work, They must use simple hand calcs to get there approximate sizes and then do all comps by hand. Maybe I am a hard task master, but they soon learn how to get a feel for the buildings.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rb1957]

we have mallets, let's start looking for walnuts ...

besides 1 tonne of electrons is more impressive than 1 oz of insight.

 

[haynewp]

You are required by ASCE to make a 3d model for seismic analysis of buildings with certain torsional irregularities. It would seem like you could break it apart into enough 2d frames and redistribute torsional moments to essentially arrive at an approximately 3d analysis. Even though I would never do that, I am not sure why the code prohibits such a workaround for someone that didn't have a 3d modelling program.

 

[Mixtli]

Wow, I was going to write some on this interesting post.... but you guys said it all, lol.
I may add the following though:
I used to work for a detaling firm and my boss used to do things faster by hand that me with my 3D, but when it came to hip and valey sistems on non othogonal frames, then I was way faster than any of the detailers, and more than fastness, it was the accuracy by boss used to like. I detected one or two interferences steel-concrete that would have been hard to find "by hand", and a few times I was required to build a 3D model on irregular structures to get the 2D detail the fabricator wanted. I actually had a checker for whom I had to do a real 3D model, out of cardboard, to support my detail, lol.
In the end I guess the best tool is the one you master...
Anyways... greetings to you all

Tony

 

[abusementpark]

I think the 3D analysis is important when you are dealing with rigid diaphragms, especially when you have irregular layouts of vertical lateral load resisting elements or vertical lateral load resisting elements of different stiffnesses (i.e. a building with a mix of concrete shear walls, braced frames, and moment frames).

Also, sometimes putting to together a 3d model doesn't require that much of effort. The tools that now come with analysis packages can really expedite the process.

Another thing, sometimes that real value of the 3d model is not the better analysis results, but the enhanced organization of the analysis results. All the information is in the model and any changes that get made are automatically reflected throughout the other members of the structure. This is huge for me.

 

[271828]

Sort of off the subject, but this reminds me of a recent question one of my pals asked me. It was in regard to a beam-column in a moment frame.

The question was posed something like the following:

"I put the frame into (un-named program) and ran it using ASD89 and got a stress ratio of 1.3 for one beam-column. Then I put it into (another un-named program) and ran it using ASD5 and got 0.85. Why is the 05 Spec. so unconservative compared to the 89 Spec.?"

Anybody see something wrong here? No mention at all of manual verification of the program. The "program" was equated to the "spec." It didn't take long to see that the dude had the second program set to ignore second-order effects and this particular beam column was long and flexible.

 

[seny]

well i'm old dog too. and spent many years w/ hand calculations, but now i can not imagine how it was possible to work without 3d modeling. in modern soft 3d model can be done very fast. on side note - critical members i always double-check by hand calculation.
rowingengineer, can i ask you:
"Concrete in 3-D model is "dangerous" ... and you don't understanding the basics (creep, shrinkage, restraint and torsion)and how to include them in your 3-D model correctly, then deflections will be a problem"
when you said how to include them in model- did you mean property modifiers or load combination imitates long term conditions or both of them? or some other technique?
i'm using etabs and safe for concrete
thanks

 

[asixth]

I'll weigh into this debate for a second time. I was at a function recently where the guest speaker was a renown structural engineer who has designed many of the iconic structures throughout the world. It was the second time that I have attended a presentation from this speaker and both times the emphasized the use of computer analysis packages in the preliminary stages of design and particular attention to modelling in three-dimensions.

I don't think that it's ever too late to start trying new methods in hope of finding a more efficient way to improve your effectiveness at work. When I was an undergraduate, I took on a FE class and took a particular interest in it, so much so I started programming my own FE code in Matlab.

I work closely with a very experienced engineer who never took a liking to computer-aided design just because they thought they were more thorough when they performed hand-calcs and where able to keep there designs organised that way, which is fine. Each to their own and whatever they have to do to get the job done.

I however do like to use computer packages and would encourage you to get in their and give them a go, they are becoming more usable every year.

 

[rowingengineer]

seny,
Hopefully rapt will drop by and give a complete run down on the problems with FEA and concrete. Or he will correct what I say.


But since this is now a debate, I shall take the corner of the opposing just for fun, but let me say that my opinion is that everything has it time and place. But getting on with the debate:


Danger number 1. Load combinations to imitates long term conditions. I am/was hoping that they will/would update the long-term multiplier (kcs) in all codes to include fcs (or some concrete strength identifier, as we all know higher strength concrete with higher amounts cement thus higher shrinkage). The approach using Kcs for the calculation of final deflection fails to adequately predict the long-term or time-dependent deflections (by far the largest portion of the total deflections).


asixth you by any chance been at a high-rise workshop.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[asixth]

Nope,

still single storey suspended slabs and highway bridges for myself. No big stuff yet.

 

[seny]

rowingengineer,
thanks for the answer.

i take it as you implement requirements of the chapters 9&10 aci 318 into your modeling load schematic procedure for 3d analysis.

i agree - deflection is dark horse of the concrete design.
that is why chapter 9 has number of restrictions & regulations.
in my opinion min thickness requirement is the crucial one and coefficient ~2 for for long-term addition always seemed to me just like empirical factor of safety to guaranty of being on conservative side of the deflection check.

 

[rowingengineer]

9.5.2.5 ?? would be the ACI equivalent sightly better but still not as accurate as it could be.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[seny]

ACI 435R-95 gives variety of long-term multipliers by different authors and it looks like even ACI agrees (and this is trues) - nobody can predict for sure final deflection of not homogeneous material structure, so the goal is to make sure to be on conservative side of prediction. there are many sources with opposite opinions about this topic. personally i am comfortable w/ max ??=2 (9.5.2.5) as long as depth/span ration in range of industry typical standard. cantilevered conditions is little bit different story - that is were i like to have good amount of compression reinforcement even if aci min thickness requirement is satisfied.

 

[msquared48]

Some years ago, I believe it was the late 80's or early 90's, there was a high rise being constructed in Chicago I believe. It had been designed by a very reputable structural engineer in two directions. There was a special on TV regarding this some years back - perhaps the Discovery Channel?

Well, as I remember it, a graduate student in a major university ran the lateral analysis using a 3-D model and loading it 45 degrees to each of the two orthoginal axes. The result was that the building was in jeopardy of collapse in a high wind situation. The results were verified, and remedial measures undertaken to strengthen the structure as a major storm was approaching. Drama suitable for TV miniseries no less...

Point being... don't ever assume that only two directions are sufficient for the design. 3D analyses just help this process go much faster.

Mike McCann
MMC Engineering