BY CLAIRE SMITH
Selection of a structural anchor should be based on the performance needed for each application but how much is the decision process driven by previous specifications or brand?
NCE joined forces with anchor specialist Liebig to look in detail at the issue.The variety of off the shelf anchors and fixings available to structural engineers has grown significantly in the last decade and, beyond these ever more complex designs, there is also the option of bespoke solutions too.
Knowing what’s out there can be just as much of a challenge as making the right design choice for the problem at hand – and few would dispute the possibility of specification changes on site have become an everyday occurrence.
“Engineers will commonly go through a technically approved route for design but the selection is all too often based on personal preference and previous experience rather than the unique parameters posed by each new application,” says Liebig specification sales Paul Papworth.
Site specific details can be key to selecting the right solution“
There are quite a few reports from the Standing Committee on Structural Safety (SCOSS) that point to poor selection and poor design of fixings, as well as specification change. The problem with using previous designs is that the application criteria may not be exactly the same, so an in-depth check and some form of calculation is essential. Just because it worked on the last job and the fact that it hasn’t fallen down is no means for confirming its suitability or that it remains the most cost-effective solution available.”
To understand the factors driving the choice of anchor in the UK market, NCE worked with Leibig’s parent company EJOT to carry out an industry survey.The results showed that many respondents favoured brand loyalty over the type of anchor, despite the use of a number of approaches to design.Design tacticsMost respondents – 45% – use a mix of hand calculations, software and technical guides to aid their deign but 40% prefer to use software as their only approach to design. None of the respondents use technical guides from manufacturers as their only source of information for design.
However, the availability of technical guidance and support from manufactures and calculation software are the biggest factors affecting the choice of solution but proof of suitability through onsite testing and case studies are also key factors.One respondent told NCE that although design guidance from manufacturers can be very detailed, it can make it difficult to find what you need. “On the other hand quick reference guides rarely contain enough detail,” said the engineer, who preferred to remain anonymous.
Nonetheless, Papworth believes that manufacturers data should in most cases be sufficient for many standard design requirements such as an anchor that isn’t close to an edge, not located close to another anchor and in conventional concrete that has a prescribed compressive strength. It is when you go outside the standard application parameters that additional calculations should be essential.
“The anchor should be designed and tested to European Technical Approval (ETA), so it should therefore be possible to compare different options from different manufacturers even without proprietary software,” explains Papworth.
“However, some engineers maintain that using traditional concrete capacity design methodology that predates ETA, offers more flexibility where the application cannot be met by the ETA approach alone.”No-second-chances.-Anchors-in-nuclear-dockyard-application-225×300.jpg
Some projects demand the right solution is fitted first time with no second chances to find an alternativePapworth says that some manufacturers still use concrete capacity design data – or even data expanded beyond the ETA approach in their software so users need to be aware of how the designs are being evaluated and generated.
“As with any proprietary software engineers need to verify the output using their own calculations,” he urges though.
Many respondents agreed with this call and one said: “Complex designs can be challenging to assess and often call for engineers to use hand calculations to check the design and interaction with other adjacent fixings too.
“Designers need to engage with anchor manufacturers with a more holistic approach from concept through to installation. This would ensure that the correct technology and the most cost-effective solution is identified, correctly designed and equally as important, correctly installed.”
Mechanical or resin?
Few respondents report getting the design right first time though – 19% of respondents said that it never happens, while 64% say that it does occasionally happen.Resin bonded anchors are the main solution that designers go for with 57% calling that their preferred option. 40% use mechanical fixings as their “go to” solutions. Undercut anchors were the least popular option with only 6% describing that method as their preferred option. The preferred fallback was mechanical fixings with almost 43% saying this was their second option and 38% opting for resin bonded fixings as their fallback.
“The use of resin fixings has increased significantly in recent years – there is definitely less reticence to using them now – but there will always be a strong preference to go for a mechanical fixing and the choice is largely dependant on specific application and performance demands. Resin anchors are unable to immediately take up the action load due to curing times and the higher risks associated with their reduced tolerance for installer error are also considerations,” says Papworth.
One respondent told NCE: “People often specify resin fixings but there is a good place for mechanical fixings too. There is no real logic for the mentality to choose one over the other.”If the desired result isn’t achieved through the initial design, most respondents (47.5%) said that they would look again at the design or change the type of fixing (56%) rather than explore bespoke or non-standard options.
Lead times, absence of technical data and stock availability do not seem to be factors in driving designers away from bespoke solutions. Perceived cost and even lack of awareness that the option is even available seem to be the main issues here.“Bespoke solutions come into play when the design and application parameters do not allow for a standard off the shelf solution to be used,” says Papworth.
“Around 5% to 10% of applications lend themselves to a bespoke design. The perception that this carries a high price tag and long lead time puts designers off from exploring this route – and some don’t even know the option exists – but given the modular design of many high performance mechanical anchors it is no longer the case. By raising awareness manufacturers can create the demand by delivering what the customer needs rather than simply focusing on what can be produced in volume. Involving a manufacturer in the early stage of a design can save a significant amount of design and installation time, reduce material costs and increase safety.”
Moving onto siteOne respondent told NCE that careful specification can go out of the window once work moves onto site when contractors may use an alternative anchor on grounds of costs or buildability.
“On site, some will use past experience rather than design and applicability,” they said.
Papworth adds: “The approach tends to be design-led until cost is factored in and that is often when changes in specification can occur.”
On site testing is sometimes essential to prove the designProduct availability is the main factor driving changes to specification according to more than 47% of respondents, while a further 31% said that cost is a driver too.Installation can also bring challenges and Papworth believes that more training is needed to avoid issues on site.
“Designer error does happen but the major cause of failure is errors in installation and the problem can be amplified by change of specification” he says.The Construction Fixings Association (CFA) has published guidance on this subject with procedures to avoid problems that are a knock-on of specification change.
According to Papworth, specification change is often an inevitability and using CFA guidelines can bring benefits. “It is also part of the anchor manufacturer’s role to support these guidelines and in doing so be able to contribute positively to the industry,” he adds.
Nonetheless, having a qualified engineer is critical to interpreting the performance requirements into the design and specification change often occurs without the designer being involved in the decision making.
“You may have an anchor that performs the same in published data for tensile and shear load but when it comes to the seismic qualification, the ability to perform close to the edge of concrete or in a fire load condition, they may be quite different,” explains Papworth.
“Resin anchors were seen as a problem solver 20 to 30 years ago and now it is just part of the toolbox. The development was driven by poor base materials as it doesn’t create the expansion forces typically associated with mechanical expansion anchors but they are simple to use.
“They can, however, bring their own problems in terms of things going wrong – it is essential that the fixing hole is clean and dry otherwise significant reduction factors have to be considered at the design stage. Mechanical anchor holes also have to be cleaned out but it is less critical compared to resin since they do not rely on adhesion.
“All anchors, whether resin or mechanical, need to be correctly set using a torque wrench which is a crucial element of how an anchor performs but often overlooked at the point of installation.
”Papworth says that in some instances anchor manufacturers are even moving away from providing standard anchoring design software and moving towards providing whole application software, giving the option to design baseplate and anchor within the same package.
“I feel that it can unnecessarily complicate the design and add to whole application costs” he adds.While there are many different approaches to design and many more solutions available on the market, it is more essential than ever that designers understand what they are trying to achieve before starting on the specification choice. According to Papworth, once the specification is set there needs to be greater governance to ensure this is followed through onto site too in order to achieve the best outcome.