How is Wood Beam calculating Fb* and Fb’ for glulam beams? |
  
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How is Wood Beam calculating Fb* and Fb’ for glulam beams? |
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The calculation of Fb* and Fb’ depend upon CD or Lambda, so the discussion should always focus on a particular load combination.
2018 NDS defines Fb* as follows:
Some glulam beams have two different values for Fb, depending upon whether the bottom or the top is in tension. This means it is theoretically possible to calculate two different values of Fb* leading to two different values of CL, depending upon which surface is in tension. And this could change along the length of a beam.
ENERCALC SEL uses a conservative simplification for the calculation of Fb*. It uses the smaller of the Fb values to calculate just a single value of Fb* for each load combination, resulting in just a single value of CL for each load combination.
But when it comes to the calculation of Fb', the program does actually consider the location of the tension. So if the tension is on the bottom at a location of interest, ENERCALC SEL calculates Fb’ as follows:
Fb'(bottom in tension) = (Fb*) * (lesser of CV and CL) * Cfu
And if the tension is on the top at a location of interest, ENERCALC SEL calculates Fb’ as follows:
Fb'(top in tension) = (Fb*) * (lesser of CV and CL) * Cfu
