Anchor Bolts

Anchor bolts, also called anchor rods or holding-down bolts, are steel rods cast into the concrete foundation. Each column's base plate sits over a group of anchor bolts and is fixed with nuts and washers. The bolts hold the building down against wind uplift, transfer horizontal shear into the foundation and locate the columns precisely. If they are in the wrong position, nothing else fits, so anchor bolts are the single most important item to get right before the steel arrives.

Types of anchor bolt

• Headed or plate-anchored bolts: a straight rod with a nut and plate, or a forged head, at the bottom. They provide reliable pull-out resistance and are preferred for uplift.
• Hooked bolts (L or J shape): the bend provides anchorage. They are common for light loads but have lower pull-out capacity than headed bolts.
• Post-installed anchors: chemical or mechanical anchors drilled into hardened concrete. They are used for corrections or small additions, not normally for main frame columns unless designed for it.

Reading the anchor bolt plan

The anchor bolt plan, released after drawing approval, gives everything the foundation contractor needs:

• grid lines and the steel line of the building;
• the position of every bolt group relative to the grid;
• bolt diameter, length, grade and projection above the concrete;
• base plate dimensions and grout thickness;
• the column reactions for each load case, used to design the footings.

Projection matters: too short and the nut cannot be fully engaged, too long and the bolt may clash with the floor finish.

Setting anchor bolts accurately

1. Make a template for each bolt pattern, drilled to match the base plate.
2. Fix the bolts to the template with nuts above and below, at the correct projection.
3. Set out the grid lines from a fixed reference and position the templates; check diagonals.
4. Fix the templates rigidly to the formwork so the bolts cannot move during the pour.
5. Check positions and levels again just before and after pouring the concrete.
6. Protect the threads from concrete and damage until the steel arrives.

Industry practice, such as the AISC Code of Standard Practice, sets tight tolerances: typically about 3 mm between any two bolts in one group and about 6 mm between adjacent groups. Your project specification may set different values; follow it.

Uplift and foundation design

The bolts can only resist uplift if the foundation is heavy enough or anchored deeply enough. The foundation designer uses the uplift reactions on the anchor bolt plan to size footings, which on light buildings in windy areas may be governed by uplift rather than by bearing.