Civil's Guide
Steel Connections
Types of Connections
Connections can be defined as rigid, semi-rigid or flexible which allows rotation. The type of connection chosen will affect the analysis of a steel member.
A rigid and semi-rigid connection can transfer moment between steel members. An example is a portal frame structure, which commonly uses haunches (rigid joints) with end plates which gives fixity between the column and beam allowing the transfer of moment.
Flexible connections do not resist any moments and allows rotation, which is usually analysed as pin support connection. There will always be a small amount of moment transfered but this is a neglible amount and assumed to be zero. In construction, this is achieved through using fin plates and flexible end plates. The plates are usually welded to a column or welded to another beam and connected via bolts. However, the bolts will not provide any resistance to rotation.
In general, beams are designed as simply supported (flexible connection) as moment connections can become expensive, especially complex connections. Moment connections provides stability to a structure, but stability can be achieved via cross bracing. The use of cross-bracing in a design can help avoid the use of these complex connections.
Overall, the engineer/designer needs to determine the nature of the connection, is it a flexible connection or moment connection?
Welded connections
| Leg Length s (mm) | Throat thickness a = 0.7s (mm) | Longitudinal capacity \(F_{W,L},R_s\) (kN/mm) | Transverse capacity \(F_{W,T},R_s\) (kN/mm) |
|---|---|---|---|
| 4 | 2.8 | 0.7 | 0.88 |
| 5 | 3.5 | 0.88 | 1.09 |
| 6 | 4.2 | 1.05 | 1.31 |
| 8 | 5.6 | 1.4 | 1.75 |
| 10 | 7.0 | 1.75 | 2.19 |
| 12 | 8.4 | 2.10 | 2.62 |
| 15 | 10.5 | 2.62 | 3.28 |
Refer tata steel blue blook on fillet welds
\(F_{w,Ed}\leq F_{w,Rd}\)
where \(F_w,Ed\) is the design value of the weld force per unit length and \(F_w,Rd\) is the design weld resistance per unit length
\(F_w,Rd=f_{vw.d}a\)
\(f_vw.d\) is the design shear strnegth of the weld
\(f_vw.d=\frac{f_u/\sqrt{3}}{\beta_w\gamma_{M2}}\)
\(\beta_w = 0.9\), \(\gamma_{m2}=1.25\) and \(f_u = 410 N/mm^2\)
Welding is used to join steel members together via a process called metal arc welding. There are many different welding techniques but the welding process uses an electric arc to generate heat which melts the base metal. A seperate filler material is also used help this process to joining the metal pieces together. The arc generates heat and the filler materal is usually welding wire or stick electrodes.
Bolt capacities for non-preloaded bolts - Class 8.8 hexagon head bolts with S355
| Diameter of bolt, d (mm) | Tensile stress area \(A_s\) \(mm^2\) | Tension resistance \(F_t,Rd\)(kN) | Shear resistance | Bolts in tension \(t_{min}\) (mm) | |
|---|---|---|---|---|---|
| Single Shear \(F_{v,Rd}\)(kN) | Double Shear \(2\times F_{v,Rd}\) (kN) | Min. thickness for punching shear | |||
| 12 | 84.3 | 48.6 | 27.5 | 55.0 | 3.7 |
| 16 | 157 | 90.4 | 60.3 | 121 | 5.5 |
| 20 | 245 | 141 | 94.1 | 188 | 6.8 |
| 24 | 353 | 203 | 136 | 271 | 8.2 |
| 30 | 561 | 323 | 215 | 431 | 10.1 |
Bolted Connections
End plate connections are a common design in steel structures and invloves using a steel plate connected to a steel member via bolts. There are different Types of bolts and classification which affect its strength and properties. The most common bolts used are hexagonal head bolts and countersunk bolts.
There are minimum and maxximum spacing requirements as mentioned in BS EN 1993-1-8.
| Distances and spacing | Minimum | Maximum | |
|---|---|---|---|
| steel exposed to the weather or other corrosive influneces | Steel not exposed to the weather or other corrosive influences | ||
| Edge distance \(e_1\) | 1.2\(d_0\) | 4t+40mm | |
| Edge distance \(e_2\) | 1.2\(d_0\) | 4t+40mm | |
| Spacing \(p_1\) | 2.2\(d_0\) | The smaller of 14t or 200mm | The smaller of 14t or 200mm |
| Spacing \(p_2\) | 2.4\(d_0\) | The smaller of 14t or 200mm | The smaller of 14t or 200mm |
\(d_0\) = Hole diameter
The table above is table 3.3 from BS EN 1993-1-8
A clearance holes is a hole through a material which is oversized so the threads of a screw or bolt can pass through but not the head. For bolts which are below 24mm, a 2mm clearance hole is required. For exmaple, an M12 bolt will require a 14mm clearance hole. Any bolts larger than 24mm will require a 3mm clearance hole.
Bolt capacities for non-preloaded bolts - Class 8.8 countersunk bolts with S355
| Diameter of bolt, d (mm) | Tensile stress area \(A_s\) \(mm^2\) | Tension resistance \(F_t,Rd\)(kN) | Shear resistance | Bolts in tension \(t_{min}\) (mm) | |
|---|---|---|---|---|---|
| Single Shear \(F_{v,Rd}\)(kN) | Double Shear \(2\times F_{v,Rd}\) (kN) | Min. thickness for punching shear | |||
| 12 | 84.3 | 34.0 | 27.5 | 55.0 | 2.6 |
| 16 | 157 | 63.3 | 60.3 | 121 | 3.8 |
| 20 | 245 | 98.8 | 94.1 | 188 | 4.8 |
| 24 | 353 | 142 | 136 | 271 | 5.7 |
| 30 | 561 | 226 | 215 | 431 | 7.1 |