Without them the load path stops at the bottom plate
Frame a wall, sheath it, put a roof on it, and you have built something with a great deal of surface and not very much weight. Wind pushing on one side tries to slide the whole thing. Wind moving over and around a building lifts it, and it lifts hardest at the corners and along the roof edges. All of that force runs down through the studs into the bottom plate, and at the bottom plate it has nowhere left to go.
The anchor bolt is where it goes. A bolt through the sill plate and down into the concrete makes the foundation part of the structure instead of something the house is merely standing on. The usual one is a J bolt, a length of threaded rod with a hook bent into the buried end so it cannot pull straight back out of the concrete. Without the bolts, a frame is a large, well built object sitting loose on a slab.
The layout has to be settled before the forms go up
Anchor bolts are not placed where they look about right. Diameter, spacing, how deep they reach into the concrete, how far they sit from the end of each plate, and where the heavier hold down hardware lands all come off your drawings. We install what the drawings schedule. Sizing structural hardware is the engineer's job and we do not do it.
That means we want the framing plan in hand before the forms go up, with every opening marked on it and the hold down schedule attached. If an opening shifts on the plan after the pour, the bolts stay exactly where they were, and the frame has to be built around that. If your plan is still moving, tell us and we will form the following week instead of guessing and being wrong in concrete.
Pour day: they go in while the concrete is still soft
On a monolithic pour the sill hardware goes into the thickened edge while the concrete is still plastic, which is a window of an hour or two on a warm afternoon and longer on a cold morning. On a wall, the top of the form gets screeded to the line and then the bolts go into the top of the wall inside that same window.
Setting one means more than pushing it in. Each bolt has to stand high enough above the finished concrete that the washer and nut land on top of the plate rather than sunk into it. It has to be plumb, so the plate drops over it instead of fighting it. It has to be on the layout line, so nobody is drilling out at the edge of a plate. And the concrete has to be worked back around it so the mix closes on the entire buried length. A bolt dropped in and left leaning is a bolt the framer has to solve for.
- Right height above the finished surface for the plate, washer and nut
- Plumb, so the plate drops on rather than getting driven on
- On the layout line, and clear of where the studs and the openings land
- Concrete worked back around the full embedded length, not just the top
Hold downs are a different job from the bolts between them
Ordinary anchor bolts hold the plate down along its run. Hardware bunches up at the corners and on each side of a wide opening, an overhead door or a long span of glass, and there the plan usually schedules something heavier: a hold down anchor tied into the steel in the concrete and bolted to a specific post or stud in the frame above it.
Those are doing something the plate bolts are not. A wall with a big hole in it resists sideways force in the narrow strips of wall left beside the hole, and those narrow strips want to rotate and lift at their ends. The hold down is what pins that end down. It is scheduled by location, not by an even spacing, which is why the framing plan matters here far more than any rule of thumb about how far apart to put bolts.