PART 1
By Spencer Erling,
Education Director, SAISC
BACKGROUND AND HISTORY
South Africa has a long tradition of specifying and using hot dipped galvanized bolts for steel structures with generally great success.
The SAISC has recommended for a long time that bolts to class grade 4.8 bolts and class 4 nuts (in general because of our relatively small market class 8 nuts are made for use both with grade 4.8 and 8.8 bolts) should be used for minor applications only (e.g. purlin connections, hand rails treads etc). For this reason and to ensure that steel erectors did not erect grade 4.8 bolts in a connection that required grade 8.8 bolts we recommend that the maximum diameter of 4.8 bolts should be limited to 16mm.
We further recommended that for all bearing/shear structural connections that grade 8.8 bolts and class 8 nuts 20mm diameter and bigger be used. All of the above bolts were made in South Africa to the appropriate DIN or ISO specification (which really only differed marginally on the across the flats dimensions, the thickness of bolt heads and nuts and the length of the thread on the bolts). These grades of bolts and nuts all hot dip galvanize well with very few, if any, problems. The small differences in dimensions (between the ISO and DIN specifications) have no impact on the structural strength of the bolts as far as designers are concerned.
When it comes to dynamically loaded connections our recommendation was to use grade 10.9 bolts with class 10 nuts. The most readily available 10.9 bolts were those made for HSFG applications (10.9S). We recommended these bolts be used not only for friction grip connections but also for any application where a pretensioned bolt would be required, such as for fluctuating dynamically loaded structures and non-slip applications.
From a bolt manufacturer’s point of view, un-worked steel having the mechanical properties required for 10.9 grade bolts and nuts is available. The process would be based on using a wire with a suitable chemical composition (often called a Boron steel) that could be heat treated up to the hardness and ultimate strengths required to satisfy the 10.9 grade requirements (as specified in ISO 898 parts 1 and 2, recently reviewed in 2009).
It is common knowledge that when it comes to the heat treatment and hot dip galvanizing of grade 10.9 material it is very important to adhere to good practice recommendations such as not pickling grade 10.9 in acid to remove rust before galvanizing. It was found that excess pickling could lead to hydrogen embrittlement. It was thus recommended to blast clean the bolts before dipping in the zinc. Current European developments do permit acid dipping with a suitable hydrogen inhibitor (technology that has not yet been fully developed in SA). Two types of failures (occasionally) occurred.
The broad term hydrogen embrittlement, and also hydrogen induced delayed stress corrosion cracking (the finding of an extensive research programme) are often mentioned to be the cause of failure of these bolts. What is not mentioned is the impact of site procedures and abuse of the bolts (see part 2 of this article in the next issue of Steel Construction).
Bolt failures associated with incorrect hardness (i.e. too brittle or too soft) have also occurred. The latest version of the ISO 898-1 specification very clearly defines hardness ranges to ensure that this problem does not occur. A further recommendation (not in the code) is that ultimate tensile strength should be in the range 1040 to 1170 MPa (and should not exceed this number). It should be noted that there is a close relationship between hardness and ultimate tensile strength so the latter instruction may not necessarily be of value to bolt manufacturers. Such failures were few and far between and to the best of the writer’s knowledge were all picked up before completion of the project and did not result in any collapses.
Most failure cases, usually caused by the heat treatment or galvanizing process failing to adhere to good practice, could most often be traced back to a request by a (steelwork) contractor to his favourite bolt supplier to do him a special favour and to deliver the bolts in a hot dipped galvanized finish by the following morning. This resulted in rushing the process and failing to adhere to good practice recommendations ending up with bolt failures.
Generically one could say that as a result of the split responsibility between bolt manufacturer, heat treater (if applicable) and the galvanizer, no one party took overall responsibility for the quality of the product delivered. SANS EN 14399 suite of documents addresses this issue.
SANS EN 14399 PARTS 1 – 10 HIGH STRENGTH BOLTING ASSEMBLIES FOR PRE-LOADING
In 2011 SANS adopted the EN 14399 suite of documents covering high strength bolting assemblies for pre-loading.
Pre-loading in this instance has the same meaning as the term previously used in South Africa viz. pre-tensioning. Clearly this set of documents therefore replaces the 10.9S bolts as described above.
The great emphasis is now on the word ‘assemblies’ in the title. The term ‘bolt assembly’ in this context means the combination of bolt, nut and washer(s). The bolt manufacturer takes responsibility for and certifies the adequacy of an assembly to be suitable for pre-loading (all as defined in the various parts of the code). Irrespective of whether the responsible bolt manufacturer buys in from others items such as the hardened washers (there is no washer manufacturer in South Africa) or subcontracts out hot dip galvanizing and/or heat treatment, the bolt manufacturer takes total responsibility for the whole assembly and coating and certifies it to be suitable for use as a preloaded bolt assembly. This in itself should go a long way to eliminating hardness and/or embrittlement issues of the past.
It should be noted that there are a series of very stringent tests to prove both the mechanical properties required by ISO898 and suitability of the assembly as required by SANS EN 14399. Only once the manufacturer conforms totally to these requirements can he certify the assemblies suitable for pre-loading.
There are some requirements in the documents relating to the shape of the bolt head and the radius between the bolt shank and the bolt head that are specifically intended to lower the possibility of hydrogen embrittlement (during hot dip galvanizing). One of the EN 14399 requirements is that nuts be tapped only after the galvanizing process (i.e. no re-tapping of nuts after galvanizing is permitted).
Clearly the technical issues to achieving the requirements of ISO898 and EN 14399 have not been without their difficulties.
Further requirements are that all bolts be traceable back to source material, that they be supplied in sealed containers as delivered by the responsible bolt manufacturer. This means that small quantities will not easily be available ‘off the shelf’, but not impossible, from bolt stockists. It also means that adequate lead times will be necessary to allow the bolt manufacturer to manufacture, test, and coat in a controlled manner to achieve a good and suitable end product.
The bolt manufacturer can also be requested to supply (galvanized) bolts for pre-loading in a lubricated form, where the friction coefficient of the lubrication is known.
Part 2 of this article will be published in the next issue of Steel Construction which will look at the possible impact of chemical composition of the steel on hydrogen embrittlement, how bad site practise and bolt tightening procedures could be the cause of some of the failures as well as some thoughts and recommendations for HD bolts.
