what if i told you that this is not a fixed connection well you might say that the end of
the beam is welded all around to the flange of the column therefore it is a fixed connection maybe it
is maybe it's not what about this reinforced concrete beam fully embedded into the column
is it fixed or pinned when i was at uni great times by the way my structural analysis lecturer
would give me a beam or a portal frame to draw the bending moment and shear force diagram um i would
get something like this a simply supported beam or a cantilever beam fixed at the end or a beam
fixed or pin to a column and i would happily draw the diagrams without giving much attention to how
those supports and connections actually worked however in real life you don't have your teacher
telling you if the beam or column is fixed or pain and it is your job as an engineer to decide the
most appropriate connection design it and detail it in this video i'm going to try to simplify the
way you look at these connections and show you how you can make sure a connection is rigid or
flexible in simple terms a fixed connection is able to restrain rotation and transfer moment
between the structural members that's when you get that negative moment in the column
a pin connection on the other hand will allow the structural member to rotate
and therefore cannot transfer any moment so the beam will take all the bending moment
let's focus on the ability of disconnections to transfer moment if we use this i-beam example we
know that the top fibers above the neutral axis are in compression and the bottom fibers below
the neutral axis are in tension so if we draw the stress distribution over the cross section we get
something like this with the maximum compressive stress at the top and maximum tensile stress at
the bottom of the beam this is not new for you you learn this in mechanics of materials my point here
is the flanges are taking the bending moment so if you're trying to transfer moment from the beam to
the column like a fixed connection does guess what you need to connect the flanges to the column it
makes sense right the same principle applies to reinforced concrete beams but now instead of a
flange you have the you have the reinforcing bars a paint connection also known as shear connection
will require only the web of the beam to be connected to the column or sometimes the bottom
flange as well and if you use the same train of thought you know that the web takes most of the
shear and that's why you only need to attach the web of the beam to the column and remember this
has nothing to do with welding or bolting there is a misconception among students and fresh engineers
that if there is weld it's a fixed connection and if there are boats it's a paint connection um you
can weld a cleat blade to the web of the beam and it's still a paint connection the boats and weld
do not define the type of connection they they're only elements of the connection as a whole a
welded paint connection can resist higher moments loads than both at once um maybe i think maybe
that's where this misconception comes from i would assume i'm going to walk you through um a couple
of examples of sheer connections as well as moment connections let's start with sheer connections
this is a cleat plate welded to the flange of the column and bolted to the web of the beam
this one is a beam to bend connection
you can have an angle bolted to the flange of the column and voltage to the web of the beam
you can also have a welded angle sit with bolts to the beam
and for the moment connections this is a welded connection between beam and column with stiffeners
this one is a bolted moment and plate
this is an apex connection used a lot in portal frames
this is a bolted splice connections when you need to extend the beam or column
and reinforced concrete structures are typically designed to transfer moment
as the structure is monolithic and there's negative and positive reinforcement as well
the next point i'll touch upon is the stiffness of the elements
stiffer structural elements will attract more of the internal forces a very stiff beam will take
away the internal forces from a more flexible column i'll show you in a practical example
let's consider a 250i beam supported by two 250 columns and the columns are fully fixed to the
beam so here we are expecting some negative moment at the column under gravity loads
as you can see we've got 111 kilo newtons meters positive bending moment and 89 kilonewtons made
it negative therefore the beam is transferring moment to the column as expected from a fixed
connection now let's change the size of this beam to a much stiffer cross section like a 610 ub
now you can see that even though i haven't changed the connection
and the column is still fully fixed to the beam there's almost no transfer of moment we went from
89 kilo newton's meter to 9 kilonewtons made it and this happens because now you have a stiff beam
support by a relatively flexible column in any structural system the internal forces are
distributed among the structural elements according to their relative stiffness you
can imagine a reinforced concrete beam supported by a column made of gummy bears
even though it's a delicious column and the column is fully fixed to the beam the gummy bears
are too flexible and will not be able to take any moment and that's what happened to or 610 ub
supported by a 250 ub the 250 ub is like a gummy bear if compared to the 610 ub
it is important to keep these ideas in mind when designing a beam without taking into
account the stiffness of the support we often simplify our designs by replacing the column for
support like this and if we were to analyze this beam like that we would get a bending moment
we would get a bending moment diagram completely different from this structure
here we're getting 135 kilonewtons meter while here it is only nine kilonewtons made it
this support here is an idealized perfectly rigid support and there's no perfectly rigid support
in real life now going back to that first image that i showed you in the beginning of the video
it definitely looks like a configuration of a fixed connection however
if the flange of the column is not stiff enough it could deform and the beam could rotate
which would invalidate or assumptions of a fixed connection
um that's why we use stiffness sometimes and you gotta be mindful of these details this also
applies to a base plate you can have your four bolts call them welded to the base plate but if
your base plate is not thick enough it can deform and therefore it's not a fixed connection anymore
i'm going to stop for here now because this video is getting too long but if you have any
questions or you would like just to add some more information just leave a comment below make sure
to like the video and subscribe to the channel thanks for watching and i'll see you next time