Steel connections are the details that let beams, columns, braces, and plates act together as a structural system. A beam-to-column connection must transfer loads safely while also being practical to fabricate, erect, inspect, and maintain. Small choices such as bolt spacing, weld size, plate thickness, and edge distance can control the strength and stiffness of the whole frame.
Understanding connections helps engineers turn an idealized structural model into a buildable steel structure.
Key Facts
- Connection shear demand often comes from the beam end reaction, V = wL/2 for a simply supported beam with uniform load w.
- Bolt shear strength is checked against the shear force per bolt, V_bolt = V/n when load is shared equally by n bolts.
- Bearing stress on a connected plate can be estimated by f_p = P/(t d), where P is bolt force, t is plate thickness, and d is bolt diameter.
- A fillet weld effective throat is a = 0.707w, where w is the weld leg size.
- Moment transfer in a beam-to-column connection is commonly modeled as a force couple, M = F d, where d is the distance between tension and compression resultants.
- Slip-critical bolts rely on friction at the faying surfaces, while bearing bolts allow small slip before force is resisted by bolt bearing.
Vocabulary
- Shear connection
- A connection designed mainly to transfer vertical shear while allowing little or no moment transfer.
- Moment connection
- A connection designed to transfer bending moment by resisting tension and compression forces across the beam depth.
- End plate
- A steel plate welded to the end of a beam and bolted to a column or another member to transfer force.
- Slip-critical connection
- A bolted connection in which clamping force and friction between plates are used to prevent slip under service loads.
- Weld throat
- The effective shortest distance through a weld that is used to calculate weld strength.
Common Mistakes to Avoid
- Treating every bolted connection as slip-critical is wrong because many standard bearing connections are allowed to slip slightly before the bolts bear against the hole.
- Ignoring eccentricity in a bolt group is wrong because a load that does not pass through the bolt group centroid creates an additional moment and uneven bolt forces.
- Assuming a shear tab is a moment connection is wrong because a simple shear tab usually transfers beam reaction through the web and is not detailed to develop large flange forces.
- Checking only bolt shear is wrong because plate bearing, tear-out, block shear, weld strength, prying action, and column local limit states may also control the design.
Practice Questions
- 1 A simply supported steel beam carries a uniform load of 24 kN/m over a 6 m span. If the end reaction is transferred by 4 identical bolts in single shear, what shear force is carried by each bolt assuming equal load sharing?
- 2 A fillet weld has a leg size of 8 mm and a total length of 200 mm. Using a = 0.707w, find the effective throat thickness and the effective throat area of the weld.
- 3 A beam-to-column joint must resist both a vertical shear reaction and a large end moment. Explain why adding only more web bolts may not create a true moment connection, and describe which connection elements would be needed to transfer the moment.