Structural Steel Design / module 14
Column bases and anchorages
Where steel design stops and concrete and anchor design begin. A base plate cannot spread pressure further than it can span, the tension side is a T-stub you have already met, and the shear is not carried by the bolts.
What you will understand
- Explain the assumptions and geometric limits behind enhanced joint bearing strength.
- Derive the effective bearing area from the plate's own bending capacity
- Recognise the tension side of a base as a T-stub
- Explain the assumed shear-transfer mechanism at a column base and the checks it requires.
- Identify viable shear-transfer paths and the strength, slip and compatibility checks each requires.
- Relate rotational stiffness to a stable structural model without assuming a pinned cantilever has finite sway.
Lessons
The base plate, and the concrete under it
The concrete is stronger than you expect and the plate is weaker. Both facts come from the same place: how far a plate can reach before it runs out of bending capacity.
Tension, shear, and how fixed a base really is
The tension side is a T-stub. Shear may be transferred by verified friction, a shear key or a designed anchor arrangement. And the base is neither pinned nor fixed, which matters more than either.
Module summary
- Bearing strength requires verified concrete, grout and supporting distribution geometry.
- The effective projection c follows the stated cantilever-strip resistance model; it does not locate an actual contact boundary.
- With assumed fy = 345 and fjd = 40, the 30 mm plate gives 3965 kN against 3800 kN applied.
- Whole-area coverage requires a clipped union calculation and thickness-appropriate material strength.
- Plate plan dimensions and thickness depend on bearing, bending, anchors, edge distances and erection requirements.
- A T-stub mechanism does not establish anchor or joint rotation capacity.
- Less axial load means MORE bolt tension: 148 kN at 800 kN, 421 kN at 150 kN
- Shear transfer may use verified friction, a key or designed anchors; compatibility and all governing failure modes matter.
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
A 400 mm square plate sits on a 1600 mm square pad. What is the concentration factor, allowing for the cap of 3.0?
Practice
A 20 mm plate in S355 bears at fjd = 30 N/mm². What is the effective projection c, in mm? Take γM0 = 1.0.
Practice
A base plate's thickness is doubled. By what factor does the effective projection increase?
Practice
A base has N = 500 kN and M = 120 kNm, with the compression centre 120 mm from the axis. Is there a tension side? Give the eccentricity in mm.
Practice
A base carries 900 kN of axial compression. With Cf,d = 0.2, what shear can friction transfer, in kN?
Practice
A base must transfer 100 kN of shear by friction alone at Cf,d = 0.2. What axial compression does that need, in kN?
Check yourself
Which load combination usually governs the holding-down bolts of a column base?
Check yourself
Why is assuming a pinned base safer than assuming a fixed one, when the truth is somewhere between?