Foundation Stud

What is a foundation stud?

Hobson describes its foundation studs as threaded rod in lengths ideal for use in building foundation applications. They are DIN 976 allthread: threaded end to end, so a nut and washer can go on wherever the job needs them. Hobson's bolts and screws article defines a stud as a fastener with no head, threaded at one end, both ends or all the way, used with one or two nuts.

On site, foundation studs are set into concrete footings, piers and slabs to hold down steel columns, base plates and other structure: cast in to the engineer's detail during the pour, or fixed into hardened concrete with an anchoring system the engineer specifies, such as a chemical anchor. Hobson's data sheet for its hot dip galvanised class 4.6 rod (the ASMSGCM codes these studs belong to) lists concrete anchoring and cyclone tie-down rods, says installation as a tie-down rod should be carried out according to AS 1684.3, and publishes no capacity, so any such use is to the engineer's specification.

Hobson publishes no load capacity in concrete, embedment depth, edge distance, spacing or anchor plate detail for these studs. How a stud holds in concrete is part of the structural design, so the class, size, length, embedment, spacing, edge distance and any nut or plate at the bottom come from the engineer's drawings.

Foundation studs and engineering studs: which is which

• •Foundation studs, class 8.8, hot dip galvanised (DIN 976): Hobson's Sampsonrod and foundation stud sheet lists them for building foundations. M20 to M36, 1,200 to 1,600mm.
• •Foundation stud, class 8.8, plain: M30 x 1,200mm with no coating. See the finish section before using plain steel anywhere it will see the weather.
• •Foundation studs, class 4.6, hot dip galvanised (DIN 976): the mild steel option, M24 to M48. Hobson's Did You Know sheet for the M36 studs notes NATA reports are available for ultimate tensile strength.
• •Engineering studs (HEC 938): short studs with a thread at each end. Hobson's sheet for the 316 studs calls them tap end studs and says the shorter thread (T1) equals the diameter and screws into an existing tapped hole, and the longer end takes the part being fixed; it lists engine blocks and cylinder heads. The M16 x 100 has T1 16mm and T2 76mm, the M20 x 110 has T1 20mm and T2 80mm. The plain class 4.6 studs are made to the same HEC 938 specification; the documents we hold do not give their thread lengths. They are not foundation studs.

Foundation stud sizes

Every foundation and engineering stud we carry. Select a row to open that class, finish and size in the selector, then pick the length.

Strength: proof load of the rod, not a hold-down capacity

Hobson publishes minimum proof loads for its threaded rod. They describe the steel of the rod only. They are not a safe working load, and not the capacity of a stud set in concrete, which also depends on the concrete, embedment, edge distance and spacing, none of which Hobson publishes for these studs. Use them only as the engineer directs.

The two classes are not interchangeable: at each size, class 8.8 has about 2.7 times the minimum proof load of class 4.6 (211.8kN against 79.4kN at M24). Never substitute class 4.6 where 8.8 is specified.

Galvanised or plain: finish for the site

Hobson's sheet for its hot dip galvanised class 4.6 allthread says it is galvanised to AS 1214 to a minimum of 40µm and an average of 50µm, for external applications, and its finish comparison rates hot dip galvanising as high corrosion resistance, recommended for long term outdoor use.

Hobson's corrosion guide covers atmospheric exposure only (enclosed, sheltered and exposed); it does not cover steel cast into concrete or buried in the ground, so the durability of the embedded part is set by the engineer. For the thread, nut and washer above the concrete, the guide's minimum where they are exposed to the weather (its examples include decks, pergolas and open sub-floors) is hot dip galvanising or 304 or 316 stainless, and in the sea spray zone stainless only. Our foundation studs are galvanised or plain; for coastal sites ask the engineer.

The plain class 8.8 stud and the plain engineering studs have no coating and are supplied lightly oiled to inhibit rust in the packaging. Plain steel is not a minimum for any exposure in Hobson's corrosion guide, so use plain only where the engineer's specification calls for it.

Setting foundation studs to the drawings

Hobson publishes no installation instructions for these studs. The points below are general site practice, not Hobson instructions:

1. Order the class, size and length shown on the drawings, and check the class on the label: class 4.6 and 8.8 studs look alike.
2. Cast-in studs: hold them in position with a template that matches the base plate holes, so they stay at the right spacing, projection and plumb while the concrete is poured and vibrated, and keep the projecting thread clean.
3. Post-installed studs: drill, clean and set them with the anchoring system the engineer specifies, following that product's installation guide for hole size, embedment and curing.
4. Fit the nuts and washers to the specification on the drawings. They are sold separately.
5. Need the paperwork? Hobson's sheets say test certificates and reports can be downloaded from its website.

Foundation studs in Adelaide

Allmat supplies foundation studs, foundation bolts, nuts, washers and chemical anchors to builders, concreters and steel erectors across Adelaide and South Australia. Send us the stud schedule from your drawings and we will price the lot.

Questions