TR9 is based on BRANZ technical recommendation TR9 Design of light timber framed walls and floors for fire resistance.
The studs in a light timber-framed wall, when subjected to fire, are modelled as axially loaded columns. The effect of fire is simulated by the loss of timber cross-section in the studs and a calculation of the residual loadbearing capacity. The secant formula is applied to what is assumed to be eccentrically loaded studs (in a wall) and has been modified to include the effects of charring of the studs and face loading due to lateral forces.
A prototype fire resistance test on a loaded wall is required to establish the performance of the lining system and timber frame in terms of the criteria of stability, integrity and insulation. The test result will also enable the charfactor (degree of damage to the wall) to be assessed.
The charfactor, as the measure of fire damage, is applied to the new design, which may be required to be a different height and/or loadbearing capacity. A new stud size is selected to support the applied load at the required height, thus meeting the stability criterion. If the wall remains intact the integrity criterion is also likely to be satisfied, as the curvature of the new wall cannot exceed that of the tested wall due to the imposed limitations preventing reduction of either stud dimension. The insulation criterion as established in the prototype test will also be met, provided the space between linings is not reduced.
The result of a fire resistance test on a loadbearing floor system may be applied to a floor/ceiling of similar construction. The basis for comparison is that the calculated stress induced in critical areas of the extrapolated design is not greater than that in the test specimen. The span of the floor may be increased if required, and the structural adequacy of the joists is subject to the critical stress in the prototype test not being exceeded. The load on the extrapolated floor may not be increased above the load applied to the prototype, as there is a danger of the flooring membrane being penetrated, causing an integrity failure.
A simply supported beam is modelled by the software. The parameters for the prototype test are entered and the stress at the bottom of the beam (joist) at mid span is calculated. The required parameters for the extrapolated floor are entered, and for the same induced stress, the minimum depth of joist is calculated.