Load Cases

The structural condition of the post-tensioned concrete roof is important to assess if corrosion or other forms of deleterious phenomena have affected its integrity. In general, as a post-tensioned structure ages, its post-tensioning system experiences normal losses induced by relaxation of the steel tendons and by creep and shrinkage in the concrete. In addition, the concrete may experience cracking of some degree, which in turn affects its stiffness. Significant deterioration of the PT tendons due to corrosion would manifest as a loss of effective prestress and the roof s overall ability to carry load.

There were several variations of the structural model and load cases that were examined for comparison purposes with field measurements and to assess the sensitivity of various parameters on the roof behavior. Table 1 summarizes the various analysis cases that were examined.

Table 1. Summary of Analysis Cases.

Case

No.

Description

Column

Inertia

Long-term

Modulus

DL

(psf)

LL

(psf)

1

Base case.

Ig

No

56

12

2

Considers cracked columns. Added 3 psf due to foam roof material.

0.5 Ig

No

59

0

3

Considers time-dependent effects on concrete modulus.

0.5 Ig

Yes

59

0

4

Determine LL to produce 6yjf tension fiber stress.

0.5 Ig

Yes

59

Varies 0 to 19

5

Determine effect of 10% loss of prestress due to deterioration.

0.5 Ig

Yes

59

0

6

Determine effect of 25% loss of prestress due to deterioration.

0.5 Ig

Yes

59

0

7

Determine effect of 50% loss of prestress due to deterioration.

0.5 Ig

Yes

59

0

Base Case

This model assumed that the structural elements were uncracked, and therefore, assumed gross section properties.