Table 4-4. Steel strains calculated using experimental data
Beam Load Steel strain layer 1 Steel strain layer 2
 (kips) (tp-) (ts)
 Control 21.9 248 141
 43.7 550 324
 65.6 850 502
 87.3 1150 676
 109.0 1452 840
 148.0 2008 1059
 Tube 22.4 418 264
 45.4 812 522
 67.4 957 575
 89.2 1199 707
 110.2 1442 831
 152.3 1829 942

 The strain in steel layer 1 at peak load was approximately 2010 Es for the control

beam and 1830 Es for the tube beam. This represents a difference of approximately 9%.

Both values were close to the yield strain the 60 ksi steel calculated based on Young's

modulus as 2070 gE.

 In reality the CFRP grid tubes reduce the rotations of the tube beam which also

means a reduction in the curvature of the beam. The lower curvature obviously causes a

reduction in the compressive strains as well as the tensile steel strain.

 Three strain gages (ST1, ST2 and ST3) on the top of the beams were used to

determine transverse expansion of the compression zone due to Poisson effect. The rate

of strain increase was relatively small up to a load of 120 kips and increased after that

(see Fig. 4-19). The rate was higher for gage ST2 located at mid-span. Very close to peak

load the transverse strain at the mid-span location increased very rapidly an indication of

an imminent concrete failure. This was typical behavior for both the control and tube

beams.