Foundation Bolt

What are foundation bolts (hold down bolts)?

A foundation bolt is cast into the concrete with its threaded end standing up out of the footing or slab. The bend at the bottom (the cog of an L-bolt) keys it into the concrete, and a base plate, bracket or column goes over the thread and is held down with a nut and washer. On site they are called hold down bolts, cast-in bolts or L bolts.

Hobson lists its foundation L-bolts as pre-cast or cast-in anchors, for attaching structural steel elements to concrete foundations, and for foundations and retaining walls built with concrete blocks. They have a long thread to allow for different plate and membrane thicknesses, and Hobson has test reports for them.

Hobson publishes no load capacity, embedment depth, edge distance or spacing for any bolt on this page. Holding down steel columns, portal frames, sheds and carports is structural: the bolt size, length, embedment, spacing, edge distance and number of bolts come from the engineer's design, and the bolts are cast in as their drawings show.

L-bolts, starter bars and J bolt kits

• •Foundation L-bolts (hot dip galvanised, class 4.8 mild steel, M12 to M30): the cast-in hold down bolt, with a long thread for the plate, washer, nut and any membrane, and a cog at the bottom.
• •L bolt starter bars, hot dip galvanised mild steel (M12, M16): shorter L bolts with a 65mm thread and cogs from 40 to 140mm. Hobson lists them as starter bar L bolts with no data sheet.
• •L bolt starter bars, zinc plated class 8.8 (M12 to M30): one length per size, with the same thread lengths as the L-bolts of that size. Hobson lists them in its high tensile range with no data sheet. Zinc plating is not enough for a cast-in bolt: see the finish section below.
• •J bolt kits, zinc plated mild steel, 1/4 inch BSW: small J bolts that Hobson sizes by NB (nominal bore, the size system used for pipe) from 1/2 to 1-3/4 inch. They are not cast-in hold down bolts; Hobson lists them as J bolt kits with no use or rating. We do not carry cast-in J-shaped hold down bolts: the L-bolt is the cast-in bolt in this range.

Foundation L-bolt sizes

Every L-bolt we carry. The thread length and cog are the same for every length of a size. Select a row to open that size in the selector.

Starter bar and J bolt sizes

J bolt kits

Zinc plated 1/4 inch BSW J bolt kits, sized by NB (nominal bore). Hobson publishes no use, dimensions drawing or rating for them. Sold in packs of 100.

Galvanised or zinc plated?

The part of a hold down bolt that has to resist corrosion is the thread, nut and washer above the concrete. Hobson rates hot dip galvanising as high corrosion resistance and recommends it for long term outdoor use; it rates zinc plating as low corrosion resistance, for indoor and non-corrosive environments.

Hobson's corrosion guide (written for metal connectors) puts the minimum for anything exposed to the weather, such as carport and pergola posts, open sheds and decks, at hot dip galvanising or stainless. Even sheltered work in a very low or low hazard area needs galvanised Z275 at least; zinc plating meets the guide's minimum only for enclosed work in very low and low hazard areas. The hot dip galvanised L-bolts and galvanised starter bars meet the minimum outdoors. Zinc plating is not enough for a cast-in bolt, so for casting into concrete use the hot dip galvanised L-bolts or starter bars, not the zinc plated starter bars or J bolt kits. In the sea spray zone the guide calls for stainless, which we do not carry in this range: ask the engineer.

Setting cast-in bolts

Hobson publishes no installation instructions for these bolts, so follow the engineer's drawings for position, embedment and projection. The points below are general site practice, not Hobson instructions:

1. Order the size and length on the drawings. The thread length (T in the tables) must cover the base plate, washer, nut and any grout or membrane under the plate, which is why Hobson makes the L-bolt thread long.
2. Hold the bolts in position with a template that matches the base plate holes, so they stay at the right spacing, height and plumb while the concrete is poured and vibrated.
3. Keep the thread clean during the pour, for example with tape, so the nut runs on afterwards.
4. Check the projection and spacing before the concrete sets, and do not bend or heat the bolts to correct them afterwards; ask the engineer if a bolt is out of position.
5. Fit the washer and nut to the specification on the drawings. Nuts and washers are sold separately.

Hold down bolts in Adelaide

Allmat supplies foundation bolts, nuts, washers and post supports to builders, concreters and shed and carport builders across Adelaide and South Australia, along with the timber and steel fixings. Send us the bolt schedule from your drawings and we will price the lot.

Questions