today I was reviewing the design of a graduate engineer and first thing I asked him was how he
would brace the structure and his answer was that the concrete suspended slab would take
the bracing loads so this is a misconception that students and graduates often have and I
understand where they come from because it's easy to imagine that as soon as a
concrete slab which is a quite stiff plate is installed the whole structure will get stable
however they're missing one of the most important concepts in engineering which
is understanding load paths so the concrete suspended slab cannot absorb any load it can
only transfer loads and I'm gonna show you an example here so I've done these two models in
Autodesk robot they're exactly the same except that one has a concrete suspended slab and the
other doesn't and I've applied a lateral load of 10 kilonewtons per meter on this bar on this
bar and a point load of three kilonewtons on the corner of the frames if we run the calculations
let's run the calculations here go to results diagram for bars and let's have a look at the
deflections so notice that the structure without the concrete without the suspended concrete slab
is deflecting 10 millimeters and the structure with the concrete floor is deflecting half of
that right so it's 10 millimeters it reduced to six millimeters and this is the reason students
and fresh Engineers think that the suspended slab is bracing the structure but in reality this lab
is only transferring the loads to stiffer elements right since we have three stiff columns here we
have three larger columns on this side and we've got only two little columns on the other side
the concrete slab allows the forces to be distributed to the stiffer elements remember
when you when you learn that that the stiffer elements will attract the loads that's what's
happening here and if you don't have a concrete slab you won't have this floor diaphragm to
distribute the loads to the bracing columns so let's turn the deflections off and let's check
the reactions here if we turn FY on there we go so here we can see that in this mod we've got
a force reaction of 3.13 kilonewtons on these two small columns however on the model with a
concrete slab we have half of that Force going to the smaller columns and what's happening is
this Force cannot disappear right this force is just it's only it's just being distributed
notice that on the bigger columns on this side we have 9.96 but on this side we have 11.42 so what
happens is we have more forces going to the larger columns right this column now is taking 11.42 and
before it was taking 9.96 this column here is taking 11.41 and before it was taking only 10.09
so it's a matter of distribution of the loads now to prove my point once again let's draw
a wall on the side with two columns so I'm going to go to polyline parameters cladding
I'm gonna do this point third point four point fifth point click ok
now let's do the same thing a wall
now let's make these two walls a concrete panel say 200 concrete panel
apply now we've got this side here where they choose more columns were a stiff side stiffer
than the other side or as stiff as so let's run the calculations and see what happens now
click here let's check the results diagram for Boss deformation exact information for Boss
and voila the deflection now is the same right without or with the concrete slab the structure
is behaving exactly the same and that's because we do have stiff bracings on both
sides of the structures and they're going to take the loads straight away they don't need
a floor diaphragm to distribute the loads to the stiffer elements because the stiff
elements are already where the loads are being placed so there we go there was just a short
video to explain this concept that many people who are starting on Structural Engineering get
wrong and if you still have any questions just drop a comment below and I'll see you next time