Structural Engineering & Statics
where the load goes
Concept

Anchor bolt — where it appears

A bolt cast into or fixed in a concrete foundation to hold a steel column's base plate down against uplift and overturning. Its free length between the concrete and the nut sets how far it stretches under load, and so how stiff the base is and how much preload it keeps.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

What the frame notices. An 8 m by 4 m portal whose columns stand on bases of every rotational stiffness, in units of the column's EI/L on a logarithmic scale: solid, the portal's sway under a load at the knee as a multiple of the fixed-based portal's; dashed, the columns' effective length factor in sway (fixed bases 1.13, pinned 2.26). Bolts sleeved 300 mm, 5.3 EI/L: 1.45 times the fixed portal's drift, an effective length factor of 1.30 and 75 per cent of its buckling load; as detailed, 9.1 EI/L: 1.28 times the fixed portal's drift, an effective length factor of 1.23 and 83 per cent of its buckling load; rigid by EN 1993-1-8, 30.0 EI/L: 1.09 times the fixed portal's drift, an effective length factor of 1.16 and 94 per cent of its buckling load.

The fixed base the bolts decide

A column base drawn as fixed is a spring, and the component method says what it is made of: the grout squeezed under the flange, the plate bending under the bolt row, and the anchor bolts stretching over their free length. For a 500 mm plate on two M24 bolts the bolts are three fifths of its flexibility, so the base comes out at nine times the column's EI/L — under a third of what EN 1993-1-8 calls rigid — and no thickness of plate can lift it above fifteen. Leave the bolts free in sleeves for erection tolerance and it falls to five. The frame on it drifts 28 per cent more than a fixed-based one and buckles at 83 per cent of the load.

connections · Base plate Retightened after every storm, the bolts stretch in every storm. The permanent stretch in the worse bolt row of a 500 × 400 × 20 mm base plate with two M24 holding-down bolts in each of two rows, accumulated over ten storms of four cycles to ±170 kN·m, with the nuts retightened after every storm to 200 kN, to the 150 kN they were first given, to the 124 kN the first storm left, or left alone. Left alone, the bolts stretch 0.12 mm in the first storm and not at all after it. Retightened to 150 kN they add 0.12 mm in every storm, 1.25 mm after ten; to 200 kN, 0.43 mm a storm. Retightened only to what the first storm left, they stretch no more than if they had been left alone.

The nut turned after the storm

A preloaded column base that has been through a storm has lost some of its preload, and the obvious repair is to retighten its bolts. Left alone, the bolts stretched once in the first storm and never again; the base settled. Retightened, they are given back the preload the storm could not live with, and the next storm takes it away again by stretching them again. Retightening after every storm turns a base that settles into one that ratchets, a tenth of a millimetre at a time, and the count of storms it has in it is set by the bolt's elongation.

connections · Base plate The preload a storm can live with, raised by a spring under the nut. The largest preload the bolts of a 500 × 400 × 20 mm base plate with two M24 holding-down bolts in each of two rows can be given and come out of a storm without stretching, against the stiffness of a stack of disc springs under each nut, for storms to ±165, ±170, ±175 kN·m; dashed, the same for the bare base; dotted, the bolt's yield of 226 kN. To ±170 kN·m the bare base keeps at most 128 kN; with a stack of 500 kN/mm, 150; 100 kN/mm, 187; 20 kN/mm, 214; 5 kN/mm, 223.

The spring under the nut

Retightening a column base after every storm makes its bolts ratchet, because a storm can only live with 128 kN of preload and the spanner keeps putting back 150. A stack of disc springs under each nut changes what the storm can live with. Soft enough, the bolts never reach their yield: the compression side keeps its preload, the bed's resultant moves out to the plate's edge, and the moment rides on lever arm rather than on bolt force. A stack as stiff as the bolt itself keeps 150 kN through every storm. Much softer, and the storm the bolts no longer feel lands on the grout.

connections · Base plate

Named alongside it

The objects these essays reach for when they reach for this one.

Base platePreloadRatchetingShakedownComponent methodCyclic loadingEffective lengthGroutJoint classificationKernPryingRotational stiffness

All concepts