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Engineering in detail – M20 Grade 8.8 black bolt

27th February 2019

As a business PDMA Consulting Engineers is involved in projects large and small, from temporary works for the redevelopment of New Street Station in Birmingham to permanent formwork on the River Thames as part of the Thames Tideway Scheme.

And whilst work of this nature is a testament to the solutions we engineer for clients on large, national civil engineering projects, personally I’m a details man.

It’s not all big picture, blue-sky thinking for me, but a consideration of even the smallest detail, like the bolts we recommend for connecting steel members.

A love affair started in the eighties   

In the 1980s, when I was at Loughborough University undertaking a degree in Civil Engineering, the standard method of connecting members seemed to be grade 4.6 black bolts.

These were used when we learned about connection design and the specification of these bolts was easy to understand. The colour black, referred to the fact the steel used to make the bolts was not finished with any corrosion protection, although zinc plating is a common finish for durability.

The grade 4.6, relates to the strength of the bolts. The first digit relates to the ultimate strength of the steel and the second represents the ratio of yield stress to ultimate strength.

So, in the case of 4.6 bolts, the ultimate material strength is 400N/mm2 and the yield stress is 60% of the ultimate strength.

Riveting lectures make the case

By his time, rivets were regarded as old technology (much lamented by certain lecturers!) and although High Strength Friction Grip (HSFG) bolts had been around for a few years, they appeared more difficult to use and there were many stories around of them not being properly installed.

Following graduation, whilst spending time on site I entered the design office to do my ‘design year’, which at the time was part of the requirement for a chartered engineer.

It was during this year that I was introduced to grade 8.8 bolts. It was immediately apparent that these were significantly stronger than the grade 4.6 bolts I was familiar with and had none of the drawbacks of HSFG bolts.

The impressive grade 8.8 bolts appealed to my love of detail and have an ultimate material strength of 800 N/mm2 with a ratio of yield stress to ultimate strength of 80%.

A good big one beats a good small one

By the time I was familiar with all the strengths of the grade 8.8 bolts, I had worked on site through several cold winters and at times I had struggled with cold hands when setting up the theodolite and with other tasks on site requiring a degree of dexterity.

I’m sure that this is the same with steel erecters handling cold steel, who will quickly find their touch deserting them as the temperature drops.

So, whenever possible, I specify M20 grade 8.8 bolts, because the steel connections need the strength and the steel erectors need the size of the bolts, which are easier to handle, especially with cold hands – the devil is always in the detail.