Beams with Rectangular Openings
Why are rectangular openings different from circular openings?
To date, there is little detailed fire resistance test evidence in the public domain of how rectangular openings behave. It is generally accepted that the tests performed in 2003 showed that the web posts between rectangular openings became hotter and were structurally less stable (and weaker), than web posts between circular openings.
This result is not surprising. It can be seen in the diagram that, if a web post between rectangular openings is compared with a web post of the same width between circular openings, it has a smaller area and is more likely to twist. Even if spacing between adjacent openings is large, there is still an increased tendency for instability with rectangular openings because of the higher temperatures and the opening shape.
"Circular" web post is larger and more stable
There is also an increased tendency for failure to occur at a rectangular opening in vierendeel bending. This is the mode of failure in which the opening distorts to form a lozenge shape.
Vierendeel bending failure on a rectangular opening
Although the behaviour of web posts between rectangular openings would be taken account of by the structural engineers for normal (cold) design, it is generally not possible to take account of the behaviour in fire because of lack of knowledge. To quantify the additional heating and possible instability, and then subsequently correctly fire protect beams with rectangular openings, a fire protection manufacturer would have to have access to information from fire resistance tests.
The ASFP and SCI have recently developed a test protocol to extend the structural model to include rectangular openings. This project is industry funded and will commence in the last quarter of 2008.
Until this project is complete the ASFP have published interim guidance using 450°C as the default limiting temperature for beams containing rectangular openings – provided the design engineer is satisfied that this will be conservative for all modes of structural failure.
The only other method of determining limiting temperatures and protection thicknesses is to use proprietary software available from a small number of major cellular beam manufacturers.
At present, apart from the approaches mentioned above, no simple method of assessing the fire protection requirements of beams with rectangular web openings exists.
However there is always the possibility of finite element analysis calculations to design individual beams with rectangular openings in fire, which would have to be linked to a multi temperature analysis of a specific intumescent product.
by Bill Allen, Director of Innovation, Leighs Paints
Back to Technical Information