Intumescent fire protection of beams with web openings
Beams with Circular Openings
Since the 1980s, beams with web openings have been fire protected using a rule derived from a series of tests carried out on castellated beams protected by boards and sprays. These tests demonstrated that castellated beams heated up more rapidly in fire than the sections from which they were cut.
The widely accepted guidance developed from the tests was that the fire protection thickness for the castellated beams should be that which was appropriate for the parent section plus 20%. Over time, this rule was accepted as being appropriate for cellular beams and also for intumescent coatings.
At the present time, cellular beams have almost entirely replaced castellated beams and the great majority are fire protected using intumescent coatings.
During the period 2001-2003 UK’s leading manufacturer of plated girders, carried out seven loaded cellular beam tests with both circular and rectangular openings. In addition it also carried out twenty-six assorted 1m and 2m short section beam tests.
These tests showed that areas around openings in the web get hotter than a plain web of the same dimensions. Structural failure can therefore occur by a number of mechanisms such as vertical shear, bending, web post horizontal shear, vierendeel bending or web post buckling.
The Steel Construction Institute (SCI) subsequently undertook an analysis of the data, which showed that use of the ‘20% rule’ for intumescents was unconservative in many areas beams with closely spaced openings, slender webs and rectangular openings were found to be particularly vulnerable.
During 2003 the SCI produced an Advisory Desk note, AD269, which alerted the construction industry to this issue as well as providing some guidance on how to assess thicknesses of intumescent coatings for cellular beams. This was supported by a technical report, RT 983 which provided more detailed guidance on a procedure for beams with cellular holes. Subsequent work by the SCI on behalf of the dominant producer of cellular beams manufactured from rolled sections in the UK, extended the scope of this guidance in RT1006. These were updated in 2006 to produce RT1085.
The Association for Specialist Fire Protection (ASFP), the representative organisation for fire protection manufacturers and applicators in the UK, then made it clear that it no longer supported the ‘20% rule’ for intumescent coatings on beams with web openings.
The ASFP then took the initiative with the help of SCI, to set up a working group to devise a fire test programme for cellular beams with circular openings. The expectation was that this test programme would enable all intumescent manufacturers to develop rules for determination of protection thicknesses for cellular beams.
The key to the test programme was to be simplicity. The ASFP membership wanted to be able to assess their products with the minimum of testing.
In October 2004 the ASFP/SCI fire test protocol for cellular beams with circular openings was agreed and circulated to the membership. The test specimens included a loaded beam section and a number of short sections, all with circular openings in the web, with a range of web post widths and slenderness ratios. The test specimens did not include rectangular openings.
Essentially the fire testing provided a comparison of web post temperatures to bottom flange temperatures over a range of web post widths and slenderness ratios. The temperature ratio for each web post tested provides a web post modification factor for that web post width. The test laboratories then provided a report that included a plot of modification factor vs web post width. The ratio of these temperatures for different web post widths gave SCI sufficient information to provide manufacturers with tables of product specific, limiting temperatures for a wide range of beam designs. These temperatures are determined using the web post plot in conjunction with the SCI structural model described in RT1187, published in June 2009.
The intumescent materials must have a multi-temperature assessment so that the correct protection thickness can be determined for each beam design. Where there are a number of web post widths in the same beam then the lowest limiting temperature i.e. highest coating thickness is applied to the whole beam.
Full details of the test protocol are available in the ASFP ‘Yellow Book’ Fire Protection For Structural Steel In Buildings
by Bill Allen, Director of Innovation, Leighs Paints
Read the second half of this paper about rectangular beams.