20 September 2016
Insufficient understanding of the distinction between torque and tension in structural steel bolting can mean the structural reliability of steel buildings is compromised over time by loose bolts. Prominent engineer, Grant Roe, explains why this problem occurs and how it can be prevented or fixed if detected.
The construction of steel buildings in most industrialised nations is regulated by standards. In Australia, AS 4100 sets out minimum requirements for the design, fabrication, erection, and modification of steelwork in structures. AS 1252 specifies requirements for the high-strength steel bolts and nuts used in steel structures. The bolt tension specifications for certain joints are critical to the performance of the steel structure. However, in the gritty, real-world conditions of a typical construction site, these exacting specifications are not always achieved.
“During inspection of steel buildings we often find structural bolting that is nowhere near the degree of tension required,” said Grant Roe, BE(Hons) MEngSc MBA MIEAust CPEng, NER managing director of Costin Roe Consulting. “Any steel building with critical friction-grip joints that are held together by loose bolting is potentially dangerous. Moreover, in connections with tensile loads, loose bolts can lead to larger than anticipated beam deflections.”
It might be difficult for building owners to imagine how such lapses in precision could occur in construction, given that some steel buildings found with insufficient tension on structural bolts are relatively new warehouses and industrial complexes.
Structural bolts are typically tightened by construction workers using an impact wrench, a power tool driven by electricity or compressed air, to deliver high turning-power with minimal human exertion but imprecise control.
Bolts are specified as either ‘snug-tight’ or ‘fully-tensioned’. Snug-tight is when the bolt is tightened sufficiently to bring the steel plies (or plates) of the connection close together. This type of bolting is used in connections where the load is primarily carried by the bolt working ‘in-shear’.
In certain situations, where additional capacity or strict movement control is required, bolts are specified to be fully-tensioned. These bolts are tightened further – beyond snug-tight. The process stretches the bolt and creates a tension force in the bolt. The resulting tension force then acts to clamp the plies firmly together, adding greater strength and stiffness to the connection. “It is interesting and important to note that in these types of connections, the applied load is carried by the clamping force, not directly by the bolt,” Grant said.
Diagram of high-strength friction grip bolt.
The structural performance of the connection is fully reliant on the clamping force. Thus it is critical that the required tensile force is generated in the bolt. This can be achieved only when the bolt is properly tightened.
AS 4100 specifies minimum bolt tension that must be created in the bolt for the joint to have sufficient strength and stiffness. The required tension is set to be at the bolts “proof” load, or just below the point where the bolt will yield. This is why the term ‘fully-tensioned’ is used. The bolts are tightened until they begin to reach the limit of their tensile strength.
Torque does not mean the same as tension
How does the construction worker know when the bolt has achieved the required tension? This is a very important question. Insufficient tension leads to low clamping force and poor joint performance. Too much tension can break the bolt during installation, wasting time and resources. Careful control is required therefore to achieve the specified result.
“Torque is not the same as tension,” Grant said. “Torque is the effort it takes to turn something, whereas tension is force in the bolt that creates the the clamping force. In structural terms, it is the clamping force which resists the load, not the shank of the bolt. For the most critical joints in a steel structure, fully-tensioned high-strength friction grip joints, the clamping force must be within a precise bandwidth of compliance to deliver the required performance.”
The pin clutch of an impact wrench in action.
“In controlled environments such as mechanical workshops, where parts are in pristine condition, a torque wrench can be used to tension bolts to the required specification, which is relative to the size of the bolt,” Grant said. “The torque wrench disengages when the degree of torque reaches the point theoretically required to produce the required tension. Torque can translate into tension with sufficient precision in controlled conditions. However, torque wrenches are not suitable for use in structural bolting because workshops and building sites are very different environments. In construction projects, working with dusty, superficially-corroded bolt assemblies is more the norm.”
Bolts are exposed to dirt and moisture when placed around a construction site in open kegs or boxes for access by workers. The ‘white rust’ seen on galvanised steel when exposed to the elements rapidly causes the surface of the bolt to become coarse and irregular. Dust and debris can accumulate around threads on the bolt. The more irregularities on the surfaces of the bolt assembly, the greater friction when the nut is turned into position, and the greater disparity between measurement of torque – the resistance to turning – and the tension produced by turning.
This creates a significant variation in the torque required to achieve the specified tension in the bolt. It is for this reason that AS 4100 specifies minimum tension, not torque.
The base unit of a Skidmore-Wilhelm Bolt Tension Calibrator.
“In controlled demonstrations using a torque wrench and bolts that were somewhat dirty and corroded, as found more often than not on construction sites, the tension delivered is only a fraction of the minimum standard required. What we’ve found in the field with our testing of some steel buildings is a great variance in tension between bolts, or series’ of bolt fastenings, with some tension readings so low as to be non-compliant with Australian standards by a wide margin.”
To achieve the minimum tension, AS 4100 specifies two methods: either by a part-turn of the nut, or by measuring tension using a direct tension indicator device (“DTI”) or washer.
“What we have observed in the field is a general assumption that using an impact wrench to torque-up bolts is sufficient. There is a misunderstanding of what is required and as a result, bolts are not properly tightened.”
“To get bolts properly tightened, the only acceptable methods are part turn of the nut, or using a DTI. The additional tightening to achieve fully-tensioned bolts should be completed as a separate and controlled process that can be audited and checked for compliance. This approach will lead to bolted connections that can achieve the required performance.”
Costin Roe offers testing for structural bolting
“Costin Roe Consulting can assist clients by inspecting steel bolting for compliance during construction stages,” said Grant. “We can also assist with testing the structural bolt tensions on completed buildings should the owners have any concerns.”
“In addition, we can assist with establishing procedures on-site that calibrate and control bolt-tightening procedures that are compliant with the Australian Standards.”