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Wind Base Shear vs. Wind Trib. to Diaphragm

Wind Base Shear vs. Wind Trib. to Diaphragm

Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

When looking at the foundation, it would make sense to include the total wind shear (which is what our office does for foundation sliding). I don't believe overturning matters though as your force doubles but your moment arm gets cut in half.

Also you should visualize how the force gets into the system. The perpendicular walls take the shear force from the studs and the studs span between the roof and the foundation. Your shearwalls are then designed for the roof reaction. It's not necessary to design them for the foundation reaction as the studs transfer that load directly without the use of a shearwall.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

The rule of setting aside the lower half of the wall is due to the way in which the load is transfered. An element (stud, wall section, tilt up panel, etc) that spans vertically and transfers reactions to the floor plate above and floor plate below does not have the bottom half contribute to lateral shear loads if at ground floor. These "bottom half of base wall" forces do not contribute to overturning either.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
For simplicity sake lets ignore the roof for now. The picture below hopefully clarifies my question.



CEinOttawa you are correct in that the overturning and the shear loads are not affected by the loads on the bottom half of the wall.

However, if you consider foundation sliding then the full lateral load of "HW" is in play as Jdgengineer pointed out.

I guess it is just a matter of terminology, that I want to make sure I am getting correct. When we talk about the Wind Base Shear which is the correct value?

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

It still matter where you mean. In a side on of a whole house, your sketch is misleading. You cannot have the forces you show, since they are not on the part you have drawn.

If you have a solid block, your sketch is correct. For a building or house, your HW/2 is close, but the lower of the two is on a perpendicular section of foundation wall into (or out of) the page.

For your shear design, lateral analysis, detailing of connections, etc, etc, etc (all but the relatively obscure check of foundation sliding) this lower force is an out of plane force only.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
Redrawing the Sketch, I still pose the same questions about the Wind Base Shear:

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

The total base shear as a global action is indeed HWL, but it is meaningless for design with only rare exceptions. The actions that matter are the top half of the wall except for one or two checks in narrow circumstances such as a few hot climates and very narrow designs which don't need depth due to other causes (tornado/cyclonic, soil conditions, etc, etc, etc).

Re your sketches you're missing the next box, where your blue HW/2 is resolved into the foundation wall parallel to the UDL shown, and the base shear that matters is the HWL/4 for each wall element. In terms of overturning those forces are present, but on a lever arm at or near zero.

Your building's base shear only matters if your building is little more than a structure on shallow strip footings. Even for slab on grade this is never going to govern any part of the design. You can call this the base shear until the cows come home, and you're technically right, but it is still never going to matter especially when this gets extended to a full size structure and multiple walls.

The reality is already complicated enough with the components of shear that come from leeward walls, windward and leeward roofs, etc.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
Based on the preceding conversation and considering the following load pattern on a gable roof with wind in the transverse direction as shown below:



I then get the following loads applied to the roof diaphragm (plf):



From this we should be able to calculate the reactions on the diaphragm from the exterior shear walls (no interior shearwalls for this derivation).

We will then check the minimum load case where 8 psf and 16 psf are applied to the roof and walls respectively and also the case where the roof loads are neglected and only the wall loads are considered.

In this particular situation the roof zones 2 and 2E are negative so the load case where the roof loads are neglected will govern.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
In summary, the Wind Base Shear @ Load Case A (transverse) would be the summation of all of these distributed loads multiplied by their length of application.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
Interesting to note that for a gable roof only the wind forces on the walls contribute any lateral forces to the roof diaphragm and then to the shear walls for the longitudinal direction (wind parallel to ridge).

The breakdown of the forces would then be:

Transverse:



Longitudinal:



One could argue that there is a certain amount of wind force on the edge of overhangs but I'm thinking its insignificant.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

The overhangs have negligible facing loads. Much more important would be to not forget drag forces which, to be fair, are also typically negligible until you get a long structure. But they are more important than any contribution of the overhang, and they should be included.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
How do you consider the drag forces?

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

I've only considered drag forces for open structures with flat roofs. In those situations, standard wind load procedures give very small diaphragm loads so it seems prudent to bump them up a bit. Gas station canopies are the best example that I've encountered. I'm curious to know what the procedure is for handling drag loads myself. I've been simply loading these flat roofs as though they were mono sloped at 15 degrees. And that's just me making stuff up. I imagine the answer would depend on surface roughness and the behaviour of the stagnant air layer separating the surface from the fast moving air.

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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

While they are always present, and I always consider them if I feel the structure warrants them, strictly speaking drag forces only need to be addressed to meet the legal minimum of competent practice if they are sighted or included by the code you're applying. They matter more in areas that get high winds, but are present regardless.

Note that drag forces build up to the point of being more than negligible when the length (either dimension) of the structure is four or more times the mean height. That makes them reasonably rare.

The code I am most familiar with for "frictional drag" is AS/NZS 1170.2 "Wind Actions". In that code the procedure is:

Fz = fz * Az

where Az = Area of surface along which drag is calculated
fz = 0.5ρair[Vdes,Θ]2CfigCdyn

Cdyn is always 1.0, except where the structure is considered aerodynamically sensitive (fabric structures, for example)
Cfig for drag is equal to Cr, the frictional drag coefficient.

Cr = 0.04 (for surfaces with ribs across the wind direction)
= 0.02 (for surfaces with corrugations across the wind direction)
= 0.01 (for smooth surfaces without corrugations or ribs or with corrugations or ribs parallel to the wind direction)

Note that I have no idea what the US equivalent would, however the AS/NZS procedures are compatible with the Canadian codes and I do apply drag forces in my Canadian work where I believe they are warranted.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

P.S. All that to say that Kootk's instincts were right... Lol.

Medeek: If you are writing the kind of all-encompassing software you seem to want to be producing, I believe you should include drag forces and make note of where these are coming from in your code comments, or release documentation (etc).

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

Thanks for taking the time to set this out for us CEL. Is there an actual requirement in the Canadian code for drag? I'll confess that it's been some time since I've gone through it myself in detail.

4 x h will encompass a fair number of one story buildings in at least one direction I'd think. Also, based on the coefficients that you've quoted, it sounds as though the "stagnant boundary layer" business that I was taught is mostly BS.

One of the projects on my "probably never get around to it" list is to compare all of the English speaking codes. I sense that each code (us/can/au/NZ/bs) contains some pearls that aren't found elsewhere. Kinda wish I could read German too. They seem to kicking ass when it comes to wood and glass.

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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

No mention of drag in NBCC or OBC, at least not that I've found. The only minor nod is under free standing walls, where there is a transverse component to the loading.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
I don't see anything for drag in the ASCE 7-10, at least not in Ch. 28 Part 1 Envelope Procedure. I think I will add a footnote to the longitudinal lateral load diagram explaining that if the building length (L) is longer than 4 times the mean roof height that wind loading due to drag should be further investigated. There will obviously be some limitations of the software I'm developing since I can't program every conceivable configuration. I am trying to make this calculator comply with the US standards (ASCE 7-10, IBC 2012).

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
Wow, I think I have it actually calculating the base shear for both transverse and longitudinal directions (gable roof type only). The overhangs are so messy to calculate, much simpler without them. It is actually quite interesting to include them though so one can get a better sense at how much they actually contribute to the lateral forces on the building.

I think the next step is to apply the loading calculated and generate the reactions at each end of the diaphragm for the shearwalls.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
Shearwall reactions are now complete, assuming of course that only the exterior walls are acting as shear walls. This assumption is pretty useless for building with dimensions exceeding a certain limit since they will probably involve steel moment frames or multiple internal shear walls. I'm wondering if it might be useful to add some sort of advanced option which lets one specify an internal shearwall spacing or even a internal shearwall layout. The programming would get pretty complex.

I also added the ability to calculate truss/rafter uplift and horizontal load. Not entirely sure how useful this is either but I've personally ran into it enough times so I thought it might be of some benefit. The funny thing is after writing the code and creating the image for this section (3) I happened to look at a truss manufacturer's output for a garage I was designing a while back. I quickly noticed that the horizontal reactions and uplift were listed on the document so if you've already taken your design to get the trusses quoted you probably don't need this information calculated. Just out of curiosity I used the same parameters as the truss manufacturer used for their wind loads and after adjusting for the TC and BC dead loads both my horizontal reactions and uplift were within 0.5 lbs of their values. Nothing like a check nailing it so perfectly, that is why I love this stuff.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
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?

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

@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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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...

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

Quote (CELinOttawa)

...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. smile

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

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

(OP)
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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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!

RE: Wind Base Shear vs. Wind Trib. to Diaphragm

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.

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