Commentary

Dux Hot Water, Moss Vale NSW

Some owners and occupants of new high-bay warehouses are concerned to find the pristine appearance of floor surfaces becoming blemished by black marks that resist cleaning. Grant Roe, BE(Hons) MEngSc MBA MIEAust CPEng NER, explains why this common problem is often misunderstood and prescribes the appropriate solution.

Dux Hot Water, Moss Vale NSW

The floor surface of a new warehouse looks pristine but can soon become blemished by vehicle traffic leaving ‘nasty’ black marks.

There can be technical misunderstandings behind trade-level advice as to what causes black marks to accumulate and persist on the surface of new warehouse flooring. Due to these technical misunderstandings, ineffective remedies can be recommended with well-meaning intentions but unsatisfactory results.

Costin Roe Consulting is one of the world’s leading engineering firms in high-bay warehouse design from both civil and structural engineering perspectives, and winners of the ACSE NSW Award for Excellence in Structural Engineering for work including the high-tech concrete pavement at Veolia complexes in Woodlawn and Banksmeadow. Europe has led the world in high-bay warehouse design and automation. Each year, the firm’s Managing Director, Grant Roe, spends time in Europe examining the latest developments in high-bay warehouse engineering, and in Australia, he is acknowledged as an engineering expert engaged with numerous successful high-bay warehouse design and construction projects. A recent Q&A session with Grant Roe, on the topic of warehouse flooring, delivered the following explanations and recommendations for warehouse flooring maintenance where cleaner appearances are required.

Cause and effect determined through physics and logic

The curing compound used in the concreting process, to achieve greater precision and efficiency, is often blamed for the ongoing accumulation of undesirable floor markings. In fact, the curing compound is formulated for rapid break-down and dissipation. “Any effects from the curing compound are gone after six to twelve months. So, whatever its contributing factor may have been for a brief period following construction, people are finding the black marks keep accumulating and resisting all the usual attempts at cleaning and preventative treatment,” Grant said. With the curing compound dismissed as the prime suspect, the discipline of engineering looks objectively at the forensic evidence and the science at work behind evident factors.

“When rubber tyres are rolling normally over warehouse floors there is no problem. However, new warehouse flooring offers less friction for tyres to maintain traction and keep rolling normally. When reduced friction causes loss of traction, the tyres slip and slide, and the rubber can heat to the point of burning. The slipping and burning actions leave behind carbon and rubber residue, appearing as black marks on the floor,” said Grant. “The problem has become more apparent in recent years because forklifts have been modified to become more powerful, the wheels are smaller in diameter, and more energy is being applied at the interface. It’s a worldwide issue, as similar concerns have been raised and solutions sought in other countries for some time.”

Grant went on to explain that white tyres had been tested as an alternative but the white rubber was found to be not as durable. Since black tyres last much longer, and perform better for heat absorption, changing to white tyres for cosmetic reasons would not make sense commercially. Yet, for some warehouse operators, aesthetic appearances will matter almost as much as performance. This is where engineers can more accurately identify the cause of problems and prescribe the most efficient means of achieving the desired aesthetic result.

Floor densification treatment the answer for increasing friction

The answer to persistent black marks on new warehouse flooring, as explained by Grant Roe, is simple, practical, and readily available in Australia.

“Floor densification is a good solution, combined with controlled surface grinding. Although this approach may sound counterintuitive, increasing the friction, and thereby minimising the loss of traction, are the keys to resolving the problem. When properly executed, the grinding modifies the surface and combined with the densification treatment, the friction properties of the floor surface are changed. The densification treatment also serves as a surface sealer, providing easier removal of any tyre markings that may occur. This combined grinding and densification treatment is used quite widely in Europe but not so much in Australia where the products and trade-skills are available but the prescriptive expertise is still emerging,” said Grant. “The answers I’ve given today are intended to assist local building owners and maintenance service providers.”


For specific assistance with new warehouse flooring projects and any remediation or maintenance issues, call Costin Roe Consulting on 02 9251 7699.

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.

Structural steel bolt tension testing“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’.

Tensile strength diagramIn 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.

High strength friction grip bolt

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.”

Torque wrench

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.

Bolt tension calibration service

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.”

 

High-bay warehousing

The development of high-bay warehousing facilities as distribution centres means greater risk to business in the unlikely event of structural failure. Should the owners of distribution centre properties take additional measures to hedge against this greater concentration of risk?

In contemporary supply chain management there is a strong movement away from the traditional network of warehousing facilities, staged at various locations, to having goods and resources centralised in the one national or regional facility, as with what is commonly called a “distribution centre”. With centralisation comes economies of scale, and the huge gains in efficiency delivered by consolidated investment in sophisticated and extremely costly warehouse automation systems.

High-bay warehousing

High-bay warehouses used as distribution centres

While the business case for the development of distribution centres is compelling, centralisation also presents building owners with a more critical risk-management scenario than the decentralised distribution model. With decentralisation, the risk of exposure to threats like fire, earthquake, ground subsidence, tsunami, or the consequences of freak weather is confined to individual warehouse structures separated from each other by topography and distance. To better manage the unique risks associated with centralisation, an improved understanding of structural reliability and risk-informed decision-making is needed.

Why major structures in Australia will (rarely) fail

It is very rare for any major building in Australia to suffer structural failure – so rare that if significant structural failure occurs, no matter what the causative circumstances, it can make prime-time news and send virtual shock-waves throughout the broader community – as if structural failures may never happen here. This perception is allowed to prevail only because Australia, like most of the world’s more developed nations, has its own building codes and construction standards based partly on ISO 2394 (International Standard #2394).

In the rare event of catastrophic structural failure in Australia, such as when a freak hail-and-ice storm swept through the Eastern Creek area of Sydney on Anzac Day in 2015, causing at least eight warehouse buildings to collapse, the question is raised as to whether or not Australian standards should be changed to ensure such destruction could not happen again.

ISO 2394 optimisation principle

Illustration of the optimisation principle for the maximisation of benefits – from ISO 2394:2015 (E), page 19.

ISO 2394, titled “General principles on reliability for structures”, defines in empirical, theoretical, and sociological terms how standards for building design, construction, maintenance, and de-commissioning are optimally formulated. The usual engineering-related computations of building resistance, and responses to actions, are refined by application of the theory of probability, and the principles of uncertainty. Consideration of the interpretation of data, performance-modelling over time, environmental factors, societal needs, and commercial realities is also included. This leads to risk-informed decision-making processes to arrive at an optimal balance between ideals and feasibility, called the “target performance level” or “safety index”.

To quote from ISO 2394: “The appropriate degree of reliability shall be judged with due regard to the possible consequences of failure, the associated expense, and the level of efforts and procedures necessary to reduce the risk of failure and damage.”

Human safety is prioritised, of course, but it is not feasible for the reliability of all structures in all locations to provide maximum protection against all potential hazards, especially when the probability of any specific type of event occurring at any particular place can be extremely remote. For the owners of warehouse buildings in the Eastern Creek area, it may not be practicable to increase the structural reliability of properties by the magnitude required to protect against damage from a type of storm so rare that it is statistically unlikely to reoccur anywhere in the area during the building’s lifespan. Neither is it in the community’s best interests for building owners to take extraordinary precautions against improbable risks, as all costs incurred along the supply chain are ultimately borne by the consumer. Such risk may be more effectively managed by the insurance industry. Insurers understand risk and premiums reflect the insurer’s exposure to risk, based on probability. This can be implemented at much less cost to the business than large-scale structural fortifications which could prove unnecessary.

Are Australian building standards adequate?

Building design and construction standards are varied in requirements between regions. These variations depend on factors including the known frequency of particular hazards occurring in any given area, and the likely potential intensity of those hazards should they occur. For example, a structure with an expansive roof, designed for an area that’s routinely subjected to cyclonic winds and torrential rain, such as in Northern Queensland, would differ in required performance from the same type of structure in an area where there is more likelihood of seismic activity, such as in Newcastle, and differ again from where it would need to bear occasional loads of snow and ice, such as in Orange, NSW.

The rarity of significant structural failures in Australia makes it self-evident that Australian building design standards are sufficient for Australian conditions, and in accordance with societal expectations regarding public safety and the longevity of building service life. – Grant Roe BE(Hons) MEngSc MBA MIEAust CPEng NER

ISO 2394 relevant hazards-structural-risk

Overview of relevant hazards for risk assessments and structural safety – from ISO 2394:2015 (E), page 95.

Where buildings are of great strategic importance, structural reliability is increased. Hospitals, utilities, and all structures that house essential services need to withstand a broader range of potential hazards of greater intensity than typical commercial, industrial, or residential buildings. In the event of a major disaster, for example, the societal priority is for essential facilities like hospitals and relief-effort command centres to remain safe and operational throughout the post-disaster and recovery period.

When exceeding standards is justifiable for business

“The supply and distribution of goods in Australia is increasingly dependent on transportation to and from distribution centres of massive size, serving vast geographical areas,” said Grant Roe, managing director of Costin Roe Consulting. “The value of structural reliability to a distribution centre is exponentially more than the replacement cost of the building itself. Even with a relatively minor structural failure, such as damage to part of a roof, there is risk of human injury, loss of stock contents, damage to extremely valuable automation equipment, and disruption to building function which could go on for months or even years while remediation is completed. The value to people of a continuous supply of products throughout cities and regional areas is incalculable. These factors pose a question to facility owners as to whether increased structural reliability is necessary for their individual situation.”

It is in the design stage of distribution centre development that building owners can most effectively consider the costs and benefits of opting to exceed Australian standards to achieve greater structural reliability. The objective is to improve building performance and business continuity should the building be subjected to some more severe hazard. Somewhere in this assessment is the optimal balance of structural reliability combined with sufficient insurance and contingency planning to allow the business to continue operations effectively in the unlikely but possible event of structural failure or other major calamity.

The basis for risk-informed decision-making

“Costin Roe Consulting engineers have the expertise and experience to successfully undertake a comprehensive analysis of any distribution centre design, proposed for any site, to report on the feasibility of increasing structural reliability in sufficient magnitude to hedge against the increased concentration of risk,” Grant said.

Costin Roe Consulting uses 5D BIM technology to model the proposed building components and attributes in their entirety, including costs that recalculate in real-time when variable changes are made, and can demonstrate and examine the effects of potential or probable impacts using other advanced technology such as 2D TUFLOW flood modelling.

“With the data we have available and our in-house modelling technology, what Costin Roe Consulting can provide to the building owner is the basis for risk-informed decision-making in relation to their planned development,” said Grant.

The video below, produced by the Australian Department of Industry, Innovation and Science, explains the “Structural Reliability Verification Method” used as the basis for the National Construction Code in Australia.

 

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