flexibility factors and stress intensification factors of bending and tee are defined in B31.3. It is obvious that stress intensification factor is used for calculating the bending stress, but what is flxibility used for ?

Hi Liyun
SIF use to multiply axial, bending & tensional stress with that factor not only bending stress.
Flexibility factor is used for bend, elbow, mitre bend.
It is experimental data that pipe elbow do behave much better that structural angle beam. elbow when it is exposed with force at one side at centre point it form an oval shape ( not remain circular) so that it offer more flexibility.
That's why in a very stiff model it we add one more elbow we get far better result.
If flexibility factor is say 10. Adding one elbow will serve same purpose rather than adding 10 times of equivalent pipe length. Constraint in piping layout sometimes make it difficult to add that much huge pipe length. Some times adding elbow solve the problem.
It's obviously experimental data for elbow, bend.

Regards

Habib

Hi, habib
thank you for your answer.
Would you like list the equations in B31.3 or other piping design code about how to use SIF and Flexibility factor. I only find equation(17) to equation(20) in B31.3 contianing SIF.

Read this:
http://www.paulin.com/WEB_Markl_SIFs_ASME_VIII_2.aspx
http://www.paulin.com/WEB_when_a_sif_is_off.aspx
http://www.paulin.com/WEB_SIFandFESIF.aspx
http://www.paulin.com/WEB_USING_FESIF.aspx
if you search for this forum you can find many interesting discution about SIF

Nice Collection thanks

Mariantrifan.