Connections in steel structures can be either, shear connections (simple connections) or moment connections. The name stems from the force that is transferred from one member to the next via the connection and therefore what the connection is required to resist. We are going to look at the different types of connections, both shear and moment resisting connections, and some details to show you what they look like.
Just a small note… The names of connections vary from region to region and even between fabricators, these constantly change but the general details rarely do.
Connections cost between 5 – 20% of the overall steel cost due to the fabrication and erection costs. Engineers seldom get the opportunity to develop sufficient knowledge to design connections efficiently, to get to this level it can take years of expose and patience. With an understanding of connection design, engineers can design more economical frames and structures.
Through experience, the best approach to designing a connection is having a full understanding of the load path that is transferred through the structure globally but also locally in the connection. Think about how the wind is transferred, where the load needs to end up etc. With this understanding, we can break down each element of the connection and analyse the resistances of each members. I like to think of a connection as a set of mini puzzles.
First of all, we will look at different forces in a connection and then the elements that resist them. From there we will look into the connection details. We are not going to cover any actual calculations here because this is one of the times in engineering, I personally believe that theory trumps practical experience.
Forces In Steel Connections
Three forces can be found in a connection, shear force, axial force and bending moments. The type of force is extremely important when it comes to designing the elements and deciding the load path the force will ultimately take.
Going forward it’s good to remember that tension and compression is taken in the flanges of a beam and shear force is taken in the web of the beam (an I-Beam).
Shear force acts parallel to the weld (transferred in the web) and perpendicular to the bolts. No other forces are induced from shear force, unless it is eccentric.
For moment connections, bending, tension and compression forces are the driving factors behind the design. If for example, we are designing a beam to column connection, the tension and compression needs to be transferred into the flanges, as this parts of the column then take the load downwards. On top of all this, don’t forget that shear will also co-exist.
Eccentricity creates a moment in a connection due to the fact that the shear force has to ‘travel’ a certain distance before being able to be transferred through into the next member.
Elements of Connections
Let’s break down all the elements you would typically see in a connection, these are, welds, bolts, end plates, toe plates, cap plates, stiffeners, and top/bottom plates.
It is good to think of welds as glue, the strength of the weld is determined on the thickness of the root. The strength of the weld can only be as strong as the connected member. In simpler terms, and the way I personally like to think about a weld, if the connecting member is 10mm thick, an effective weld will be a maximum size of 10mm. Weld strengths can be calculated and due to the consistency can be found in data tables such as ‘The Blue Book’. See the diagram below for a cross section of a weld.
Something else to consider is the fabrication process of the welds. A 6mm and 8mm fillet weld can be completed in one run, where as a 10mm weld will take 10mm runs. As you can imagine beyond these the number of runs required increases significant along with cost.
Bolts, these transfer the force through the connection and are obviously an important component that cannot be ignored. For site work and steel erection, bolted connections are faster and can be completed in all weather conditions (unlike welds), therefore it is cheaper and preferred. As a designer, please try to minimise all on-site welding. Bolts have set grades and high yield and tensile strength, it is easy to calculate the shear and tensile capacity of bolts and similar to welds, data tables exist showing the capacities. Another tip, try to use only one or two bolt sizes in the whole project, it is surprisingly easy for bolts to get mixed up on site, causing a lot of headache!
End plates are the plates that are connected to the end of the member.. Yes the name says it all. These are normally welded to the member that is to be connected. For example, a beam that is going to be bolted to the column. The major check for end plate is the bending capacity, various yield line patterns exist due to the effects of the bolts transferring the force through them. A lot of research and guidance is provided for the design of end plates. P358 and P398 are great books for guidance in accordance with Eurocodes for simple connections and moment connections respectively.
A toe plate is very similar to an end plate, this plate however not directly attached to the connecting beam or column. Further down we will see different arrangements of the connections and label them up to avoid confusion.
Stiffeners are generally welded (but can also be bolted) between the flanges of a member. The purpose of the stiffeners is to transfer the force from the connection into the area of the next area that will then continue carrying the force. Think of a beam to column moment connection, the aim is to transfer the compression force from the beams flange into the columns flange, from here the force is then sent down the column. Stiffeners as the name suggests, stiffen the member to allow for transfer of force. This in itself can stop local buckling of the member due to the force.
A cap plate can be found on a bracing end detail, the top of a column (which is also a stiffener) or even the base of a column connecting to another beam.
Top and bottom plates can be found on splice connections. The idea of these plates is to allow force to transfer through the spliced member as if it was one. We will look at a splice connection in more detail below.
5 Types of Shear Connections
Now we are going to look at typical steel connection detailing practises. These are from the experience gained in the United Kingdom at a large steel fabrication company. Therefore, names of the steel connections may vary as well general detailing practises, the good thing to note is, although they might look slightly different, the analysis and thought process behind is very similar.
Small note, for clarity in the details, no welds are shown. Next to each description will be a list of elements to consider in the design.
SC01, the fin plate, also called a fish plate. This is used to connect a beam web to a column flange or web.
- Fin Plate
- Column Flange Weld
- Column Flange Capacity
- Bolt Capacity
SC02, the end plate, the end plate connection, used to connect a beam to a column web or column flange.
- End Plate
- Column Flange Capacity
- Bolt Capacity
SC03, base plate connections. These connect the column at the base to the foundation or slab. Rarely they will be connected to a steel member, although these can also be called cap plates.
- Base Plate
- Bolt Capacity
SC04, braced end connections. One way of connecting a bracing member such as a flat plate or hollow section (square, rectangular or circular) to a plate which will then be connected to another plate, the gusset plate.
- Tab / Cap Plate
- Weld Between Cap / Tap Plate
- Weld Between Cap / Brace
- Bolt Capacity
SC05, gusset plate. The final one covered for simple shear connections. This connects the bracing end to the member. Commonly found on base plates, columns and beams. These can be bolted using an angle or welded.
- Gusset Plate Shear Capacity
- Gusset Plate Tension Capacity
- Gusset Plate Buckling Capacity
- Connecting Member Flange
4 Types of Moment Connections
Let’s look at some moment connections, the analysis of these is much more demanding and software does not go as far to aid designers for these. General understanding of connections, forces and structural design is a prerequisite for these types of connections. That said, diving straight into these will also provide all of the above!
MC01, the haunch connection. An end plate connection into a column flange with a few additions. Haunch connections are used when a large moments needs to be transferred. The haunch increases the lever arm in the connection, thus reducing the tension and compression forces that are transferred into the column. These types of connections are found commonly in portal frames. The haunch make up can either be from plates or cut directly from steel section. It is important to ensure the capacity of the flange and web of the haunch.
MC02, toe plate/end plate connection. One of my favourite connections (no idea why). The only reason these are classified as moment connections is due to the eccentricity produced in the connection. Therefore the bolts need to be checked in tension. A good connection for understanding.
MC03, moment transferring base plate. In apparent can actually be identical to the shear only version of the base plate but additional checks are required. Due to the bending in the flanges, bolt tension and plate bending are some extras required. It is not uncommon to see stiffeners to increase the effective length for the bending resistance. Off-set base plate design is also considered in these.
MC04, spliced connection. This connection holds a special place for me personally, it is the connection that a lot of my previous understanding ‘click’. Naturally by calculating the forces and resistance of the elements in this connection, an understanding of all the others is naturally inherited. The connection allows for members to be cut into two separate pieces and connected back together on site. Due to logistical and erection difficulties these can save a lot of money.
It can be seen that a lot of the moment connections very similar to the shear connection varieties with additional elements to resist the other forces that need to be transfered.
Plenty of research articles and design guidance is provided all over the web for the design of connections. See the extract below from Eurocode 3 – Design of Steel Structures – Part 8 – Design of joints. I have not included all of it as the table is large but it explains the mode of failures for different elements with reference to the location in the document.
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