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
@@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.
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?
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.
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!
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?
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
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.
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?
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.
the diaphragm is not assumed to act continuous over the middle shear wall. wood diaphragms are assumed to simply supported between all lateral supports
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
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 RU-vid videos?
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?
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
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.
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
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
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...
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 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.
@@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
@@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!
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.
@@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.
