In order to calculate the section factor (A/V) of a cellular beam section, the ‘I’ beam should be split into two sections: ‘T’ shape above the web opening and ‘T’ shape below the web opening. This allows for the section factor of the top and bottom half of the cellular beam to be calculated separately. The below diagram shows the required dimensions:

If the values in the above diagram are obtained, these can be sent to Promat Technical Services who can calculate the section factors of the cellular beam sections. 

The overall section factor for the cellular beam is then the highest value of: 

• The section factor of the ‘Top T’
• The section factor of the ‘Bottom T’
• The value calculated from 1400/tw where ‘tw’ is the thickness of the web in mm

This value can then be used to specify the correct board thickness for the desired fire resistance period.