Dear mohammad
actually we are working in vendor side. we are GRP pipe manufacturer . u can tell me ur all question .

actually allowable stress of the GRP pipe will come from HDB . HDB value will come from 10,000 h test result. our HDB value value is 132 MPa .
allowable stress in axial direction when u r using code UKoOA code is f1*f2*f3*HDB/2
where f1= 0.77 (vendor data)
f2= 0.67 (sustain codition)
=.83 (operating condition)
=.89 (occ. condition)
f3=.85 for above ground
=1 for underground

span length for supporting are mentioned in the UKooa code and ISO 14692
the thermal expansion of GRP is more higher than the steel so span length is less.
all rigid element has to be supported properly .

if still u have some clarification u can come back to us