Wind Base Shear vs. Wind Trib. to Diaphragm 1

[medeek]

When looking at the wind tributary to a diaphragm in order to calculate the shear wall capacities I normally resolve the wind forces to the roof and half the wall height (single story, four ext. walls for simplicity sake). This force is then applied to the shearwalls at the roof eave height.

However for overturning calculations it would seem more appropriate to include the forces on the lower half of the wall as well as resolve the roof forces at a height of the "mean roof height".

Does this seem logical? I've seen different calculations and spreadsheets that show the wind base shear as both of these numbers, one that includes the forces on the lower half of the windward wall and the other that is only the total forces tributary to the roof diaphragm. My feeling is that the wind base shear should be the total lateral force.

 

[medeek]

Going through the process to create this calculator has definitely been an education. It is interesting to note that I can sometimes miss all of the possible load cases and minimum requirements. That is why in my calculator for the lateral loads I have included tables with side-by-side comparisons. I can now analyze a simple gable structure for wind in about 2 minutes which once took me a couple hours of manual calculations (slightly exaggerated but you get my point).

I like to call it code "bloat". If you compare the current ASCE 7-10 and IBC with its predecessors back about 20 years ago you will notice that the level of complexity has greatly increased. Almost to the point that an engineer has a hard time getting a feel for the numbers. Most engineering today is efficiently done with programs such as this that take the actual engineering calcs out of the hands of the engineer and effectively make him a technician.

An excellent example of where this all leads to is the MPC truss industry. Yes, there are still truss engineers but by and large most truss packages are generated by technicians using software from companies like Mitek. The engineering is all automated and performed by the software. I've seen stamped truss documents from some truss manufacturers but I often wonder what exactly do those engineers do?

 

[CELinOttawa]

Maybe Kootk will chime in... That was his work in a previous life.

 

[KootK]

Sweet! I've never been summoned here by name before. Pressure to perform...

I'll start with a couple of general statements:

1) In my opinion, MPC truss design has evolved, in Darwinian fashion, to just the right level of complexity. It's working.

2) In a typical building employing MPC trusses, those trusses are usually the most thoroughly engineered components of the structure by a considerable margin.

Now for the cast of characters:

The engineer that the EOR wants designing his trusses

This is a knowledgeable, grey haired fellow with a graduate engineering degree and an SE license. He sits around skillfully designing your trusses one by one. He doesn't exist, and he never did. Back in the pre-computer era, fabricators had big books of canned designs that they adapted to specific situations. It was almost unheard of for a fabricator to have a P.Eng. on staff. I can tell you, with absolute certainly, that MPC trusses have never received more PE attention than they're getting nowadays.

The truss fabricator's engineer

In years gone by, this was almost unheard of. Nowadays, most good sized truss manufactures will have at least one PE on board. However, that PE is not designing your trusses. Rather, he's implementing QC and dealing with non-standard design issues. Even with a PE on board, most of the drawing stamping will still be done by an engineer working for the plate supplier.

The plate supplier's engineer

This truly is the guy that you'd want designing your trusses. he's as educated as you are and is the Fazlur Khan of his realm. Trouble is, he's way too valuable to be designing all of your trusses one by one. Instead, he stamps hundreds of MPC trusses every day that are sent to him already designed by the fabricators truss technicians. This engineer's primary responsibility is to check that truss design input has been entered correctly. As an EOR, I like this. Garbage in, garbage out is our usual software complaint. The plate supplier's engineer is the final gatekeeper preventing the garbage from getting in.

The truss fabricator's technician

This is the guy that takes a lot of flack around here. And he may be just as scary as you think. When I first started being this guy, I was an education major working part time at a truss plant. I started working in the shop where the 3:4:5 rule was a useful tool for squaring the jigs. With my arcane knowledge of SOH-CAH-TOA, I was able to dazzle my colleagues by extending the 3:4:5 rule to other situations like 5:8:9.43. It was great. I felt like Ender in Ender's Game. This is how I got the design gig when it opened up.

From the EOR perspective, the design technician does not usually know as much engineering theory as you'd like him to. However, in conjunction with the other professionals involved, he gets it done. More importantly, the technician is an invaluable member of the team. Among other things, he knows:

1) All of the code provisions applicable to MPC trusses.
2) How to optimize MPC trusses for fabrication.
3) How to manipulate software to address the important engineering issues associated with MPC trusses.
4) How to anticipate the needs of framers in a way that your average EOR does not.

A good truss design technician is an essential bridge between the EOR and the framers who will put the building together.

The software

Modern truss design software is very sophisticated. Truly, it's almost dummy proof. The important part is obviously the data input. The software is set up to carefully guide the user through the required input for environmental loads etc. Working through the input screens isn't all that different from reading ASCE7, just with better, more targeted formatting.

The real issue with most EOR concerns about the MPC truss industry is that the truss designers rely very heavily on their software. Personally, I'm fine with it. As I mentioned above, there are check and balances built into the system. And, on a more fundamental level, I think that's it's naive to think that our entire industry will have any choice but to rely heavily on our software.

Either we're able to trust our software or we'll have to stop using it altogether. Most of the output checking that gets done in practice is either fictional or woefully simplistic. I expect you'll object to that last statement CEL. You're one of the few engineers that I'll concede probably is checking computer output carefully. You're not a very representative data point however.

At many premier firms that crank out a lot of high-rise concrete work, you'll find a cadre of people who basically just run shear wall ETABS models all day long. I've worked with several that had graduate engineering degrees and years of experience but couldn't detail an unconventional concrete connection to save their lives. These people are also technicians. And like all technicians, they're efficient, useful, and require some supervision. My point is that the EOR world is every bit as plagued by the technician / software dilemma as the MPC truss world.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

Thank-you for your very informative response KootK, very well written.

I don't object to the use of software in engineering. However, when the code complexity almost requires the use of software to perform "basic" analysis it makes me wonder what is the future of this profession. 20 to 30 years from now how many volumes will the IBC and ASCE7 take up. Will a structural engineer actually be able to still perform his job in analyzing a full structure from top to bottom or will it have to be broken up into components and specialty fields (ie. truss, foundations, floors, walls etc...) and then performed using canned software (blackbox approach), subbing out certain portions to component manufacturers.

I don't think I am alone in these concerns. A recent article in Structure Magazine addressed this exact issue a lot more eloquently that I have:

How Code Complexity Harms or Profession

 

[woodman88]

And on the otherhand I have trained 5 secretaries on truss designing per a truss plate manufacturer software.

Also, when working in the industry, most of the engineers only checked that the truss design worked for the input. They never saw any other information on the project.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

@Medeek: I agree with your last post 100%. Codes are basically being written with the assumption that we'll be using software. I'm a pretty techy guy. Like you, I'm also a hobbyist programmer. Still, if I could push a button and un-invent computers (from our profession only). I'd push it. Economic pressure + ready access to shortcuts = trouble. Additionally, I'm an an awesome human computer. Society would value me more if I did't have to compete with machines.

@Woodman: yeah, there are horror stories to be told for sure. How did the secretaries manage? Seriously. It's also important to note that one of the people who should be checking the truss design input is the EOR. That's most of what I check when the truss shops show up.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Well while I can't say it makes me happy, I don't have much to say about woeful output checking by plate manufacturers... It is similar to all the left-over poor practice, like Part 9 construction details that give a competent engineer the willies; it will stay until there is a sufficiently egregious problem to cause change.

I hate computers in our profession, rather passionately. I have had three interns start working for me and quit within the week. To be fair, one simply asked to be transfered to another engineer. That happens with about half of them, and I suspect it has to do with the fact that their calculators away and hand them a training slide rule and a "How to use a slide rule" book. I then advise them they can have their calculators back once they can tell me the rough answers to a design discussion to the correct MAGNITUDE without pen or paper.

As an aside, I have another intern starting Monday. Let's see how this one goes...

 

[CELinOttawa]

I also take your pointing me out as a computer triple-checking Luddite as the highest of praise! *HUGE smiles*

 

[Ingenuity]

...and hand them a training slide rule and a "How to use a slide rule" book.

You are kidding right? Damn, I would quit after 1 hour if I had to work with you and you insisted on using a slide rule to demonstrate a point - and I have a slide rule too. Back-of-the-envelope calc, no problem. Slide rule 'know-how' and you are going to turn a grad engineer off engineering for life, IMO.

Insisting on using a RPN calculator is totally acceptable.

I am 'wedged' between the slide-rule-era engineers of yesteryear and the techno-gecks grads of today. Comfortable being in that position.

 

[medeek]

Never used a slide rule, never will. My HP48G purchased in 1990 is still running fine and I haven't found a calculator that I like enough to replace it with.

 

[CELinOttawa]

My money, My stamp, My rules.

 

[KootK]

Ha! Is that the new engineer's cube space? I'd like to come intern with you for sport. I wouldn't quit but there's a pretty good chance that I'd get myself sacked.

I must have that element sizing document. Chasing it down from IStructE now...

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[WinelandV]

CEL, do you do all your calculations with a carpenters pencil? At first I it was for the field, but I don't see any other pencils. Unless the yellow cylinder with the blue cap is one.

 

[CELinOttawa]

We're a couple of provinces apart, but so far from your posts I think you'd likely do very well for yourself here Kootk. I doubt very much you'd get yourself fired... I've only ever fired for being lazy or habitually late. Everything else we can work with. The will to learn and the desire to do good work without ever compromising the safety of the unknowing public are things which cannot be taught and you show in spades. If not yet a P.Eng., you are going to be an excellent one. I doubt there would be anything for us to teach you... If I'd been involved in your Internships, I would be proud to point you out to colleagues. I hope your mentors are.

All calculations are in pencil. Company policy. A single line-through is the "erase" we permit.

Estimation document attached.

 

[CELinOttawa]

Pen! Lol/Sigh... Calculations are all done in PEN! I am developing a special hate for my auto-correct... Any letter accidentally hit after 'pen' causes auto-correction to "pencil".

God but I AM a Luddite!

 

[CELinOttawa]

Oh, and drawings, sketches, etc, are all pencil... Hence the erasing shield. I just thought it would be nice to take him over to Wallack's on his first day and let him pick whatever pencils he'd like. More than pens, pencils are quite personal and, in my view, need to be chosen after trying a number of them.