Steel erection components

Steel erection involves the use of lots of different components and connectors. This page has all you need to know about roof trusses, purlins, girts, bolts, nuts and washers.

Roof trusses

Before lifting roof trusses

Here's what to check before attempting to lift roof trusses into position:

• the mass (may have to be calculated)
• details of the lifting points provided by the manufacturer of the roof trusses
• visually inspect all rigging equipment and lifting points for defects
• the truss points should be properly bolted, welded or riveted.

Lifting medium and large size truss

Lifting medium and large size truss

Manufacturers should provide advice on lifting connections. This is because lifting puts a force on a roof truss that is the opposite to those the truss was designed to take.

During a roof truss lift

Do not leave roof trusses or lattice girders standing unsupported in an upright position. They should be secured by lashing to a building column or another suitable anchorage.

Lift in supporting steel work to stiffen the structure.

The bottom chord is only designed to take tension. When lifting by the top chord, the bottom chord is in compression and can buckle and bend. To stop this from happening, strengthen the bottom chord by use of a spreader beam to stop the compression.

Trusses should always be lifted at the panel points as per lifting plan.

Make sure that the slings are protected by packing to stop them being cut by the sharp edges of the gusset plates.

How to safely install purlins and girts

High pitched gable trusses should be slung directly at the apex or as per the lifting plan.

Flat pitched gable roof trusses should be lifted using a spreader beam and slung with a central vertical sling. The mass should be evenly distributed to the slings.

Fit temporary guys to the apex of the first truss. Four guys should be fitted if the truss exceeds 10m in length. Do not remove the guys until the wind bracing is fitted.

Finish the first bay completely before continuing erection of the rest of the building.

Purlins should be added after all the structural steel is installed.

Fit the bottom girts first. This is to allow fewer obstructions from above as you continue to install the girts.

Types of fastenings

There are lots of different fastenings available for structural connections. The rigger must know which fastenings have been specified by the structural designer for the various members. This involves carefully reading all the drawings and specifications.

Riggers must also be able to recognise different grade, diameter and length fasteners. They should be able to do this by visual inspection.

Bolt, nut and washer markings

Bolts

If the wrong bolts are used, the strength or stability of the steel structure can be seriously affected.

Structural bolts are generally categorised by their nominal diameter, overall length and thread length.

So what is nominal diameter? The nominal bolt diameter in millimetres is designated with the letter M followed by a number. For instance, an M 16 bolt has a nominal diameter of 16 mm.

High strength structural bolts have their heads marked "8.8" (for the steel property class). They also carry three radial lines and the manufacturer's identification or trademark.

Nuts

High strength structural nuts can be identified by the three arcs indented or embossed on the non-bearing faces. With some brands of double-chamfered nuts, the markings may appear on both faces.

Washers

Washers can be either flat round or square taper. Flat round washers designed to be used with high strength structural bolts and nuts are manufactured with three protruding nibs around their perimeters.

Square taper washers are used less frequently in modern structural design. But they are still generally available. They are usually manufactured with taper angles of either:

• 5° (for use with taper flange channels), or
• 8° (for use with taper flange beams or RSJs).

More information on the specifications and allowable tolerances for structural bolts, nuts and washers can be found in AS1252:2016 and EN 14399.

Tensioning bolts and nuts

The fitting of extra bolts and the correct tensioning of all bolts should be done once the structural members have been set in place and field-bolted. The torque sequence should be followed when specified.

Tools used during the consolidation of the structure include:

• tension wrenches
• impact wrenches (referred to as rattle­ guns).

Impact wrenches

Precautions should be taken when using impact wrenches. Make sure to follow the recommended operating procedures and use correct PPE.

When using impact wrenches, follow these rules:

• torque capacity/strength of the tool can vary with air pressure or battery charge level
• the socket and tool should be kept in line
• be aware of hand clearances during operation
• do not hold the socket
• do not use a worn socket.