the design of steel connections is literally the most important thing that you can look at in
any structural steel design it can literally make or break your structure
and many people have overlooked it and has led to disastrous consequences so I'll be going through
the basics of steel connection design to make sure that you can make the right decisions and
things that you should look out for my name is Brendan your structural engineer now let's
get into it so what type of connections can you have in a steel structure so you can either have
welded connections or bolted connections well the connections are quite easy to understand
you put two members together you have some sort of plates and you weld around them to connect them
sometimes this welding because it doesn't allow for any slip may lead to unwanted forces
and welding can also be highly variable based on where you're doing it so typically if you're
on site you'll have a lesser quality weld than if you're doing it in a shop and because you've got
giant steel structures you can't just put a whole welded frame together from the factory
plop it down on your side there's also two main types of welds that you may be looking for either
got a full-strength butt weld which is FSBW or a continuous fillet weld CFW so the full strength
butt weld you've got two bits of steel you made a chamfer and you butt weld them together to get
a full strength connection whereas a fillet weld you're potentially connecting two plates together
or two areas together and you just fill it in locally in that area so you can have different
sizes of phillip weld so you're gonna have a 6 cCFW or 4 CFW 3 CFW10 20 whatever size
fill up weld you need when we're looking at the full strength butt weld as it was in its name
is a full strength connection where a philip weld has a lower strength so six phillip welds roughly
achieves about one kilonewton per millimeter so if you've got a hundred kilonewton load you need
a hundred millimeters of weld if you're looking to resist high forces you can just put a bigger
phillips weld so it ain't fill it well ten filler world or even bigger there's a couple of problems
with that and if you go more than about an 8 or 10 world you potentially need to do it in
multiple passes so the cost can be exponential and also you're limited by how big a phillip weld you
can connect two members together as if you go too big it can overheat the area and cause
local damage so you won't achieve the strength that you're looking for this is roughly about 80%
of the smallest plate so if you've got a 10 mil plate connecting to a 10 mil plate it will
need to be a maximum of an 8 mil throat of the weld any bigger and you're potentially looking at
additional damage much like steel design is all about the connections don't forget to bolt down
the LIKE button not only does it help my channel out but also allows to get this video out to more
people and as we're saying that welding heats up the element there's also another problem here as
we know that when we heat up steel it's expanded so if we got a really long section that needs a
long weld to join them together if you just weld along them the structural bow is essentially the
steel is heating and strength it's expanding and as it expands you're welding it together you're
welding in that expanded state and when it cools down it will deform and bow based on which one it
expanded more as it's shrinking back to its normal state so a way to get around this sometimes is
if you do need to weld a member continuously along the whole section you do it in a hit miss fashion
so you go for the first section you might hit the first 300 then miss a 300mm gap hit another
300 and rinse and repeat until you weld it all the way to the end then you wait a day or so or
even just the same day but wait for that member to cool down once it's back to room temperature you
come and repeat that same process filling in the gaps that you've left out this means you haven't
overheated the element and you haven't locked in those outer balance forces causing the member to
work and bow but as I was saying that welding is potentially problematic as you'll be taking it to
site it can get quite expensive as you need to get welders there potentially you won't achieve the
same strength as they don't have the same level of quality as they're trying to weld up in an area
which is not efficient for that welding process to occur one of the most common methods of connecting
steel structures is bolting them together and this allows you have the fine control that you have
in the factory putting the holes in the precise locations you've also got manufactured the bolts
in a factory setting so you got more higher controls so it's more likely to lead to a better
result it's also quicker as well potentially sometimes you'll put it up and you just bolt
it together it's already in place where if you're welding something together you may need to wait
when we're talking about bolted connections they can both achieve a pin or a moment connection so
you have that versatility there's also a number of different bolts that you may need to consider
so you either got 4.6 8.8 they're going to be connected snug tight slash S they can either
be bearing type slash TB or friction type which is TF we're talking about the different types of
bolts that 4.6 or 8.8 they both have their unique capabilities and used in their unique situations
well first start off the primary difference between 4.6 and 8.8 is its strength so the 4.6
is roughly about 300 mpa where the 8.8 is roughly around 600 mpa so we can see with the numbers the
4.6 is about half the strength of an 8.8 but just doesn't stop there do you describe the highest
strength ball it's not always the for the right situation they're also a little bit more expensive
so the problem with the 8.8 is the fact that you can't weld it it's a high strength grade of steel
if you weld it the steel becomes brittle so potentially won't achieve the strength
that you need so if you're welding around it you can't use that 8.8
where the 4.6 is a mild steel so it allows you to weld it so if you're using it in situations
such as welded studs for whatever reason you can't bolt all the way through and you need a
welded stud there you can bolt it on and have it with a 4.6 if you've got members that don't need
that high strength such as bolting to timber or to purlins so they don't have really big moment
capacities or loads that they need to resist the 4.6 is better it's a little bit more versatile
you don't need to worry about that overheating issue and it's cheaper well typically if you're
connecting steel to steel connections you need to make sure it achieves the exact strength
that 8.8 is typically used for steel beam or steel column connections 4.6 on the other end as you can
weld it allows you to place bolts more precisely so for whatever reason you've got tying down bolts
and you want to make sure the template is accurate typically you use that 4.6 in the tying down bolt
situation spacing of bolts is super critical when you're looking at how you're detailing
your connections there's a number of rules that you have to abide by to spacing those bolts out
in the correct locations first up is spacing them enough so it allows for the installation to occur
so if we've got the bolt there we also need to put a ratchet gun or socket over the top so we
need to make sure we've got enough distance around it we need to make sure that we're at least 55 to
65 clear of any flat plates to allow the ratchet again to go on to achieve the torque that we need
the other concern as well is that make sure that you've got enough gap between those two bolts
so typically when you're spacing out your bolts you'll make sure there's at least
50 mil clear between each of the bolts that is your bare minimum that's for your smaller type
bolts but when your bolts start to go up in size you need to have a bigger and bigger gap
which is roughly about two times the bolt diameter the reason for this is it needs
to transfer shear through that connection so you need to make sure you've got enough steel
after you take it all those holes to transfer the shear that that connection is trying to transfer
back to the structure as we were saying there's really two types of connections that you can add
we'll first go through the moment connection now moment connection that's bolted together can
either come in two forms the first one is where we've just got shear plates and top and bottom
plates now these plates have different functions so we've got the central web plate is where you
get most of the shear transfer and that's similar to when you're designing a steel beam so you have
your essential steel plate they'll be typically welded on one side to limit the amount of work
that you need to do and bolt it on the other the purpose of this central cleat plate is only to
transfer the shear in the location the critical nature of this single plate onto the web is really
there just to transfer sheer so typically you only need to make the bolts nut tight as you don't mind
a little bit of movement in them but it allows for that shear transfer similar to what you do
in steel beam design where if the top and bottom plates you're bolting through the connection
and that's allowing for either transfer the compression or tension force this is really for
your moment fixity so the top and bottom plates need to transfer those high forces and typically
you will have TB bolts there as you wanted to grab a little bit earlier so we can have a more
stiffer connection as a softer connection there may be a little bit flexible talking about TB
bolts on the top and bottom of the plates there's also another one that we talked about which is
tf which is friction type bolts typically you try and limit or try to not need friction type
bolts wherever possible it's a really expensive connection essentially you need to rough on the
surface you need to prep it also the bolts need to be torqued to a higher load so not only is it more
expensive due to all the preparation action it mean they have a lesser capacity to resist shear that
slip is only there for service moment it doesn't limit slip in an ultimate situation so you only
put this in situations where it's highly sensitive to movement one pro tip when you are putting those
connections in the ones on the side plate are quite easy you can put them in either direction
but the ones on top and bottom always make sure that the nut is on the bottom the reason for this
is if it ever becomes loose you'll still have the bolt through there providing that shear transfer
so you can come back and bolt it up later where if it's up the other way the bolt will fall through
you'll have the nut resting on top and the nut resting on top really doesn't help you at all the
next type of connection is an end plate connection these occur in a number of different locations
so you can either have this butting up towards a column you need to have this button up towards the
wall or you can have this as a base plate so there's a number of different situations that
you can put them in depending on the stiffness of your plate will depend on whether that's a
moment connection or a fixed connection typically your thicker plates allow for less movement so it
will provide the fixity where if you just need it for a base plate such as a column you want to make
sure your base plate isn't too thick so you don't provide the fixity that you may not be looking for
with these type of connections or whether you're just beam to beam beam to column or even on that
slab potentially there's a problem with prying which is an amplification of the force there's
a number of ways that you can deal with this first up is trying to increase that A to B ratio to at
least 0.75 to make sure that A is smaller than the B to make sure you're not getting those priming
actions to occur next one is making sure you're at least spacing your bolts more than 90 millimeters
apart to make sure they're not too close to cause that coupling action to occur the next
one is also making sure the thickness of your base plate is thick enough so if you've got a really
thin base plate it can cause those prying actions really easily for this you want to make sure that
the plate is at least 1.2 times thicker than the diameter of your bolt if you stick to these rules
you don't need to worry about prying actions another problem that you can have when you're
trying to locate say a moment connection fixity to a core is potentially your oversized the hole so
it allows you for the have tolerance in there then you weld a 10mm washer on the top to lock it in
place good in theory as potentially you're allowed for the tolerance that you need on site to be able
to build it however what happens is the fact that you've got your 10 ml washer you'll have a
clear gap between where you're trying to transfer that load that gap is causing a moment connection
in the bolt so the bolt needs to transfer not only shear which is over here which is trying
to transfer over here but as we have a lever arm it's also transferring moment and bolts
typically especially your smaller bolts only have a limited amount of shear that they can resist
in that combined bending and shear action the main type of steel connection that you'll see
is that fin plate connection which is that pin connection and as the name suggests
pretty much just a fin with a series of bolts in it typically these bolts as you can allow for some
slip and slide they will be the snug tight and they're only really transferring shear forces but
as they're not transferring any moment you need to be careful as potentially you've got a connection
that's offset from the centroid of the beam that's supporting it that offset could continually induce
a moment that the supporting structure needs to resist if you're interested in learning more about
steel design and bring your steel design to the next level I've got linked to my golden rules of
Steel design that goes into all aspects of steel design to bring your designs to the next level
and if you want to support this channel I've got links to my Patreon in the below description
where through a small donation you can help make this content possible
watch like these many members here as always stay safe keep learning and I'll see you next week bye