Small components that hold a building together Sag rods and foundation bolts are among the cheapest items on a steel building and among the most consequential. One stops the roof structure deforming; the other is the only connection between the whole frame and the ground. Both are easy to under-specify because they look like commodity hardware, and both are difficult and expensive to correct once the building is up. Sag rods Purlins are strong in the direction they are designed to carry load and weak sideways. On a sloping roof, part of the load acts down the slope rather than through the purlin, and that component tries to bend and twist the purlin laterally. A cold formed Z or C section is particularly prone to this because the shape is not symmetric. Sag rods, also called purlin rods, run between purlins at right angles and tie them together. Each purlin is then braced by its neighbours instead of resisting the side load alone, and the whole roof plane acts as a connected system rather than a set of independent members. Symptoms of missing or insufficient sag rods are visible from inside: purlins that have rotated so the flange is no longer horizontal, and sheeting that has distorted along the run as a result. Since the fix means working above a roof already covered in sheeting, it is far cheaper to get right at erection. Rods are supplied threaded at both ends, or fully threaded where site adjustment is wanted. On longer runs the number of rows between frames follows the span and slope rather than being one row by default. Foundation bolts Foundation bolts, also called anchor bolts or holding down bolts, cast into the concrete and hold the column base plates. Everything the frame does under wind and load passes through them. The critical case is usually uplift rather than compression. A light steel building under wind can experience net upward force on the frame, and the foundation bolts, with the concrete they are embedded in, are what resists it. That is why embedment depth and the anchorage detail at the bottom of the bolt matter as much as the diameter. Bolts are supplied threaded at the exposed end with the embedded end formed to anchor: bent to an L or J, or fitted with a plate or nut to bear against the concrete. The anchorage is what develops the bolt’s capacity, so substituting a plain straight bar because it is easier to source changes the behaviour completely. Setting out is where jobs go wrong Foundation bolts are cast into concrete before the steel arrives, which means an error is set in concrete before anyone can check it against the actual frame. The two common failures are position and level. If the bolt group is out of position relative to the base plate holes, the options are all bad: enlarging holes in the base plate, which reduces its capacity, or breaking out and recasting. A template holding the bolts in the correct relative position during the pour prevents this, and it is worth the time on every foundation. If projection above concrete is too short there is not enough thread for the nut and washer; too long and the exposed length may need cutting, which damages the thread unless done carefully. Levels should be checked after the pour and before the steel is delivered, when there is still time to react. Threading and coating Threading quality decides whether the nut runs freely on site. Rolled threads are stronger than cut threads because the material is displaced rather than removed, leaving the grain following the thread form. The exposed portion of a foundation bolt sits at the base of a column, which is exactly where water collects, dust builds up and floor washing ends up. Galvanised bolts are worth specifying in any building with a wet or washed floor. The embedded portion is protected by the concrete; it is the projection and the nut that corrode. What to send us For sag rods: purlin spacing, span between frames, roof slope, and the purlin section. For foundation bolts: the base plate detail with hole positions and diameters, the required projection, and the number of columns. A foundation layout drawing covers all of it. Where anchorage capacity has been calculated by an engineer, send the embedment depth and anchor type so we supply to the design rather than to a standard pattern.
