Load Combinations [LRFD ART. 3.4]

Total factored load shall be taken as the following:

Q = η Σ γ i q i

LRFD Eq. 3.4.1-1

where

η	=	a factor relating to ductility, redundancy, and operational importance [LRFD Art. 1.3.2.1] For the purpose of this example: [LRFD Table 3.4.1-1] η = 1.0
γi	=	load factors
qi	=	specified loads

Investigating different limit states given in LRFD Art. 3.4.1, the following limit states are applicable:

Service I: to check compressive stresses in prestressed concrete components. [LRFD Table 3.4.1-1]
$Q = 1.00 (D C + D W) + 1.00 (L L + I M)$
Service III: to check tensile stresses in prestressed concrete components. [LRFD Table 3.4.1-1]
$Q = 1.00 (D C + D W) + 1.00 (L L + I M)$
Strength I: to check ultimate strength. [LRFD Table 3.4.1-1,2]
Maximum Q = 1.25 (DC) + 1.50 (DW) + 1.75 (LL + IM)

Minimum Q = 0.90 (DC) + 0.65 (DW) + 1.75 (LL + IM)

Note: Minimum load factors for dead load (DC) and future wearing surface (DW) are used when dead load and future wearing surface stresses are of opposite sign to that of the live load.

Calculation of Check Points In Precast/Prestressed Girder

The checkpoints at which results are obtained are tenth points on the precast beam measured with respect to the left bearing centerline.

\Pr e c a s t O v e r h a n g = \frac{\Pr e c a s t l e n g t h - B e a r i n g t o B e a r i n g}{2} = \frac{32.70 - 32.40}{2} = 0.15 m

Checkpoint At 0.20L:

0.20 L on Precast = (0.20) (32.70) = 6.54 m measured from end of precast

The same point on the beam measured from Centerline of Bearing is:

= 6.54 – Precast Overhang = 6.54 – 0.15 = 6.39 m

Checkpoint at Transfer Length:

Transfer length = 60 (Strand diameters) [LRFD Art. 5.8.2.3]

= 60 (12.7) = 762 mm

Transfer length, bonded = 0.762m measured from end of precast

The same point at transfer length measured from Centerline of Bearing is:

= 0.762 – Precast Overhang = 0.762 – 0.15 = 0.61 m