Civil PE Exam - Structural Review Problem - Diaphragm Design Example

[@bonegrubber]

All these years I've just been pluggin it into the company spread sheet. Nice to see and understand the inner yet simple workings

[@Kestava_Engineering]

Matt its a breath of fresh air when that happens. I get that feeling all the time, slowly but surely. And then you can really start to push the bounds of your designs

[@Kestava_Engineering]

@@bonegrubber that spot on. Structural engineers don’t typically like to assume that moment can be transferred between vertical lateral elements do to the assembly of the diaphragm.

[@ziggaby4411]

I have a question that may show my ignorance, as I don't work with transverse loading typically: Why do we treat the design as simple span? My assumed answer is that diaphragms are typically comprised of individual wood sheathing that is not continuous between shear walls, so it's safe to assume a single panel can be treated as a simple span.

[@engrmattd]

I had no idea what diaphragms in buildings were. Now I can design them. Thanks!

[@josemurillo4179]

Could you do steel connection example please? This example was outstanding and very helpful. Thanks for creating a PE channel

[@Kestava_Engineering]

yes ill do a steel connection detail this weekend!

[@dontransue9843]

Great example for this; it's been over 14 yrs since I designed parking structures and I used to do these calcs frequently back in the day with Walker Consultants. Q: We took the 80' span to illustrate and come up with the largest chord force, right? Stayed away from the 60' span for that reason?

[@Kestava_Engineering]

spot on Don

[@tajudeenolusegun2304]

It's so understanding..Nice presentation

[@qureshisiddig9274]

Well explained , what we so the chord forecd after find it?!

[@M-vz8po]

Great explanation, thank you! Are diaphragms similar to floors, if yes, then why not just say floors instead of diaphragms?

[@Kestava_Engineering]

Great question Farid, "floors" are more of an architectural term, which could mean "flooring" or the type of finished floor that would be chosen by an architect (carpter, stone, wood). Floor does not necessarily mean a structural floor. Typically in the professional field we specify a "floor" as a "structural slab" or "deck" for the case of gravity elements, and then we describe the "floor" in the case of lateral elements as the diaphragm. There are so many systems to choose from that simply calling something a "floor" is difficult to express what you are truly referring to. I hope this helps somewhat!

[@arturzych138]

Thanks for asking this cause I had the same question. Also the answer was super helpful

[@rajashekar2023]

I have question how to calculate wind load on diaphragm I know you explained half the load from wall and other half to foundation. But I am curious do we need to consider negative and positive loads on the wall since other side of the wall experience negative pressure. I am glad of you answer this question. Thanks

[@Kestava_Engineering]

Hi Raja - with wind you combined the positive and negative wall pressures (if that is what condition you have) and then you compare that cumulative story force to your seismic story force (if you have it). but when designing walls for out of plane forces you have another set of equations to use to calculate your walls.

[@tonyantonio2]

understood this as a 4th year civil student, thanks so much ^^

[@Kestava_Engineering]

Nice! Well done Tony!

[@tonyantonio2]

@@Kestava_Engineering I watched the video a week before we learned the topic in timber design, I did well on the final exam :D

[@faisalm294]

How to find diaphragm shear,shear in wall and collector force?

[@vladimirmoreno665]

Thanks for the video, real easy to understand, a quick question, If the diaphragm is taking half of the load (other half taken by foundation), that load wouldn't be acting exactly at the diaphragm, but would be offset (located at 1/4 wall height from top), wouldn't it transfer some moments/ torsion on the diaphragm?

[@Kestava_Engineering]

Hi Vladimir, that bit of force would be transferred through the walls through shear (for walls parallel to the acting lateral load) and for walls perpendicular to the force they would resist the load in out of plane bending. does that make sense.

[@mohamedhaddat8307]

really great video, thank you! I have a question I was hoping to get an answer for plz, what's the criteria for a semirigid diaphragm ? I looked throu ASCE7-10 and I found flexible and rigid but nothing in between! thanks again

[@AntonioGutierrez-xl6nr]

Should the wind load which transfers to the diaphragm be 320/2? Since the loads on the bottom half of the wind-facing wall would transfer to the ground?

[@sugassuwag7513]

awesome explanation thanks

[@williamlee7119]

great video! Taking California seismic in a little over a week

[@Kestava_Engineering]

thanks William! good luck on your exam, knock that thing out of the park brother

[@gnidnoeled786]

Just to make sure that I understand it correctly. Is the flexible plywood diaphragm the horizontal element subjected to a wind load of 320lb/ft supported by the shear walls? Thanks in advance for the reply?

[@Kestava_Engineering]

That is correct

[@gnidnoeled786]

@@Kestava_Engineering Thank you once again.

[@zacharypeacock3597]

Great explanation!!!

[@Kestava_Engineering]

Glad it was helpful Zachary!

[@Speed001]

You did a great job explaining.

[@Kestava_Engineering]

THANK YOU SPEED! lets keep up that momentum

[@Rama-ry5vn]

Since you're make an analogy to deep beam, those 3 nos shear walls should acting as 3 nos pinned support in continuous beam, shouldn't it?

[@Kestava_Engineering]

the diaphragm is not assumed to act continuous over the middle shear wall. wood diaphragms are assumed to simply supported between all lateral supports

[@khaledmashal6853]

Very clear, thank you

[@Kestava_Engineering]

You are welcome!

[@raydellorenzo5171]

Hi. You divided the diaphragm in two simple supported beam. Why you don't considered as a continuous element. In that case your moment is not the isosttic moment. Any reference on why this is done in that way? Thx

[@Kestava_Engineering]

in wood construction, the individual plywood boards that make up the diaphragm may not be oriented along shear walls to resist any negative bending moment. the boards may be edge to edge along the shear wall and nailed, creating a simply supported boundary condition. therefore it is not wise to analyze diaphragms as continuous. instead you always analyze as simply supported spans from shear wall to shear wall and design the diaphragm for a moment of wl^2/8 which should be larger than a continuous condition.

[@andytakla3945]

These videos are excellent.

[@Kestava_Engineering]

Glad you like them! always let me and the Team know what specific problems you have Andy!

[@xiyuli9135]

I love your videos! They are all very informative and easy to understand. Sometime your quick jokes get me! Haha. Do you happen to work at a company too while making KZbin videos?

[@Kestava_Engineering]

thanks Xiyu! and yes i have a full time professional engineer at a structural engineering firm in the USA

[@mohammedabdulsami3774]

Sir, why don't you consider A,B and C as continuous beam and solve it for getting max bending moment value?

[@Kestava_Engineering]

Hi Mohammed, if we considered the diaphragm as a continuous beam then we would need to take into account negative bending moment at the shear walls which is not what actually happens. the flexible diaphragm is not fixed at the shear walls its a pinned condition so the diaphragm is free to rotate at the walls and no moment is able to transfer through the connection.

[@mohammedabdulsami3774]

@@Kestava_Engineering But Sir, in most standard software like ETABS and SAP2000, their are either rigid diaphragm or semi-rigid. Even in CRSI manual, they assume it to be rigid.

[@iwandesu96]

@@mohammedabdulsami3774 I think you can modelate the "floor" as a membrane type, since it doesn´t have the property to transfer moments.

[@gopinath.l6963]

Its continuous beam action seems but conservatively done for simply supported behaviour,so may I know d reason for y not done for less moment economically by considering continuous action

[@Kestava_Engineering]

a flexible diaphragm is not permitted to be analyzed as being able to translate moment through its supports laterally as the system isnot stiff enough. so you must look at it as simply supported spans

[@aus2tigran]

but if the diaphragm is a single piece, how can you assume it will act simply supported? The middle shear wall is basically an intermediate support therefore there should be a negative moment occurring there...

[@Kestava_Engineering]

Hi Tig, Each bay between shear walls acts as a simply supported beam. In this example, since the diaphragm is flexible, we know that the largest tributary width to each shear wall generates the largest chord forces. If we analyzed the entire structure then there would be two spans of simply supported beams, where each boundary condition is a shear wall location.

[@aus2tigran]

@@Kestava_Engineering thanks for your response, does this mean the diaphragm is to be built as two separate floor panels in the real world?

[@Kestava_Engineering]

@@aus2tigran diaphragms are built using numerous sheets of either plywood or OSB type boards all nailed to the framing system. When designing in the real world, we check the capacity of the wood boards themselves for shear, and then we check the nailing pattern used to fasten the boards to the wood framing as well (including chord forces). Most of the time the design of the nailing is what is most important (but not always). Because the diaphragm boards are nailed to the top of the shear walls, the nailed connection acts as a pinned connection because the nails do not have any ability to transfer moment through the connection. The boards simply rotate about the nailed connection to the shear wall, this is why we treat all shear walls as only simple spans, not continuous spans where you get negative moment.

[@aus2tigran]

@@Kestava_Engineering yes that makes sense. I would imagine if it was a whole single sheet spanning across three supports then, it would act as a continuous beam

[@Kestava_Engineering]

@@aus2tigran , i just realized i didnt mention that the diaphragm example i drew was an example of a diaphragm with a shear wall at each end. the little example does not represent the actual problem. So sorry about that! ill add a note to clarify. there is indeed a shear wall at the middle, but the static breakdown would still be two simple spans between shear walls. i hope this is starting to clear some things up!

[@adamalday01]

How do you know there are only two shear walls ? If there are four then how does the problem change?

[@adamalday01]

Just zoomed up. How would it change if there walls on four sides.

[@Kestava_Engineering]

it would only change if the diaphragm was rigid or partially rigid!

[@AndreasChristianto]

how about B1 sir?

[@Kestava_Engineering]

Hi Andreas, for B1 you would substitute in the 60ft bay for L and all the other numbers would remain the same!

[@chandrasajana2299]

thanks man!

[@jr3474]

Thanks....

[@Kestava_Engineering]

Anytime JR!