Ultimate moment capacity of prestressed beam 2

[piglord]

HELLO!
Recently, I've dealt with a 243-meter-long prestressed bridge using RM bridge.
After analysis, I found that the ultimate moment capacity after 30 years is greater than the capacity after 28 days.
Does it make sense?
It is consistent with what I expected(after 30 years, the steel relaxation causes the lower initial stress in tendon. Hence the stress state of tendon becomes far away from rupture point, As shown in the picture).

I told many engineers this result, they have another opinion but i didn't get it.(may be i don't think i am wrong)

Hope anyone can help me!Thanks!

 

[rb1957]

so it's telling you that over time the pre-stress erodes (something like creep) ?

so if you design with a certain value of pre-stress, when does your design assumption become invalid ?

another day in paradise, or is paradise one day closer ?

 

[piglord]

YES, Creep compliance and Relaxation modulus have a relation in viscoelasticity.

Design assumption becomes invalid? because these calculation procedures are only approximations, i think they are always valid.

if it is possible, i want to explain that phenomenon by simple intuitive concepts.

 

[rb1957]

creep ... over time tension stresses decay as the material stretches, reducing the tensile strain

another day in paradise, or is paradise one day closer ?

 

[BAretired]

The only way it would make sense is that concrete gains strength over time so the compression block required at ultimate load becomes smaller and the distance from steel to compression force becomes larger.

Relaxation of the steel over time does not alter the ultimate strength of the steel. Prestressing steel does not gain strength over time.

BA

 

[piglord]

http://mucc.mahidol.ac.th/~egpcp/Handout406/40610 Prestressed2.pdf

it said there is no prestress in ultimate state

 

[BAretired]

The article states (in blue):

* At ultimate of prestressed concrete beam, the stress in steel is
somewhere between yield strength f_py and ultimate strength f_pu

* Stress is lower for unbonded tendon because stress is distributed
throughout the length of the beam instead of just one section as in the
case of bonded tendon

* At ultimate, the effect of prestressing is lost and the section behaves just like an RC beam

If you prestress a steel bar with tensile force T, then start applying tension to the prestressed bar, the prestressing force will be lost when the applied force equals or exceeds T.

In the case of prestressed concrete, the stress in the steel at factored load f_ps is given by the expression in the article. Its value does not change as a result of loss of prestress over time.

BA

 

[Ron]

Agree with BAretired. Analysis likely assumes 28-day strength is somewhere between 90 and 95 percent of ultimate with an ultimate over tolerance of a few percent based on historical concrete strength gain curves.

 

[rapt]

If anything, the compression zone should soften over time due to creep, resulting in the neutral axis depth increasing, the lever arm reducing and ultimate capacity reducing slightly.

RE the loss of prestress at ultimate, logically when the strain in the concrete at the level of the prestress reaches the pre-strain in the prestress steel, the steel will no longer be "prestressed" compared to the concrete around it, but it has no effect on the design. It is a meaningless statement!

 

[gwynn]

No, while creep and shrinkage will reduce the depth of the compression zone, and the stress in the tendons, in unloaded conditions, this does not change the ultimate capacity. Ultimate capacity depends on the compressive strength of the concrete and the tensile strength of the tendons/strands. Creep changes neither.

It will effect serviceability states, but not ultimate.

 

[piglord]

i am studying these calculation procedure..!