Effective depth

[slickdeals]

In calculating effective depth "d" for a prestressed concrete beam with additional non-prestressed reinforcing, the code states that "d" is calculated as the distance from the extreme compression fiber to the centroid of prestressed and nonprestressed reinforcing.

In doing this calculation, is it correct to convert the areas based on their stresses (fse and fy) just like doing transformed areas and then finding the centroid of this group?

I know this is like splitting hairs, but I want to make sure I am understanding the provisions right.

 

[JAE]

No I don't think so. Transformed sections are "transformed" based on different modulai? of elasticity, E.

Not Stresses.

Since most steel hovers around 29000 ksi, there's no transition required.

 

[slickdeals]

I feel stupid now, yes you are right. They are transformed based on modulus of elasticity.

 

[BAretired]

Don't feel stupid. It is a good question. I'm not so sure that effective depth has anything to do with the transformed section.

It seems to me that you have two effective depths, one for the prestressed steel and one for the nonprestressed steel. However, you have only one compression block.

BA

 

[slickdeals]

I was actually trying to run through some numbers to find the value of "d" to use in calculating shear capacity of a partially prestressed beam and did not want to use the code allowed 0.8h. I had some high shears near the supports, with post-tensioning cables being continuous over the supporting column.

 

[rapt]

slickdeals,

Thechnially, it is based on the actual stresses under the load condition being investigated, except for shear design.

In shear design, which is a fairly empirical methodology, it is treated as an RC section from decompression plus the initial PT effect. So all steel is based on its area, not stress in the steel for shear design and the extra force in the PT is added as an extra effect. So yes, for mshear it is based on the centroid of areas of the steel

Remember that the first section you are looking at is d from the face of the support for the effective depth, so the tendon will have dropped significantly from its peak height.