Fire protecting steel work

[Gus Potter]

Here is a bit of a good news story. 

 

There are two common ways of fire protecting steels. We can box it in with steel angles and say Fire Line plaster board or we can paint with intumescent paint. 

 

But BC often ask up front for a specification on the paint system. I wrote this morning to Rawlins Paints the following: 

 

Dear technical Department.

 

I'm seeking assistance with a paint specification for steelwork fire protection and certificate / data sheets for a building warrant submission. Attached is a drawing showing the proposed steelwork.

 

The project is a single story domestic house extension with a pitched concrete tiled roof in Scotland. 

 

The requirements are:

 

1/ Level of fire protection required 30minutes ( short duration).

 

2/ Section sizes are 178 x 102 UB19 S275 or S355 and 152 x 89 UB16 S275.

 

3/ The load ratio for the 178 x 102UB 19's is 35% and the load ratio for the 152 x 89 U16 is 50%

 

4/ Exposure to fire: Three sides are exposed; bottom flange and sides. The top flange has a 145 x 45 timber wall plate shot fired to it to support timber rafters. 

 

5/ The beams are orientated in the vertical plane (top flanges upper most), loaded vertically downwards about their major axis.

 

6/  Quantities:The steelwork lengths are shown on the drawing.

 

7/ Steel design code is BS5950 part 1: 2000

 

8/ Exposure to weather: All steelwork is within the weatherproof envelope of the building. Thus dry conditions. No chemical exposure is required.

 

9/ Application of paint is to be on site, ideally brush applied. This can be done before fixing of the wall plates.

 

By the close of business today I had a paint specification and an undertaking to supply the certification certificate provided we use their product of course. That is a fantastic service!

 

Now for folk on BH. I'm putting these steels in awkward places, thus to box them in is going to be very labour intensive and that is very costly. 

 

On a technical note in item 3. I refer to what is called the load ratio. The steel sizes on this project are sized on how much they deflect in service. That stops cracking in ceilings for example. But in a fire we just want to make sure they don't fall down. The load ratio is the load on the steels in a fire compared with the steel beam ultimate strength / buckling strength. Long steel beam design is often governed by buckling, it twists and distorts first. That is why we tie floors into the steel beams for example to prevent the twisting in normal use.  But during a fire that contribution can get lost as the floor / roof can burn away to the extent that it stops restraining the beams. Thus the load ratio is based on the strength of the beam when say part of the restraining floor or roof burns away in less than the required fire protection time. 

 

But even so using a paint system can be very cost effective.

 

If a beam is not fully loaded up to it maximum capacity during a fire then the intumescent paint system can be of real economic advantage. To explain roughly. If a steel beam is loaded up to it's maximum capacity then it will fail at a lower temperature. A beam can still carry a bit of load at a higher temperature and that is where the load ratio comes in.  

 

The paint acts like an insulating blanket that slows the rate the steel heats up.. and that gives us the fire duration requirement we see in the building regs. 

 

The above it intended to give you a bit of a template if asking about intumescent paints. 

 

 

 

 

 

 

 

 

Edited 2 hours ago by Gus Potter
typos