Hand Calculations for two-way post-tensioned slab

[structuralengr89]

Does anyone know of any good sources for hand calculations for a two-way post tensioned slab? I have PCA Notes 318-08 and a time saving design aid I found online.

I'm trying to do a quick check of the capacity of an existing building. It has bays 30' o.c. in both directions, 8.5" slab, column caps are 3'-8" square and is used as office space. The existing plans show 24 tendons about every 30 feet in the exterior bays and 20 tendons about every 30 feet in the interior bays.

The examples in pca show the same number of tendons in the interior and exterior bays. I was wanting to do a quick check of tendon stresses, flexural and shear capacity. Was not planning on looking at deflection.

Anyone know of a good source?

 

[strucguy]

Post Tensioning institute has various publications. Check out the link below.

http://www.post-tensioning.org/category/x_cY2lk4PT/Design

 

[dcarr82775]

If you know the tendon final effective forces and the drape you can easily calculate the load being balanced. Then a quick little equivalent frame will get you what you are looking for.

 

[structuralengr89]

dcarr82775

The P effective is 26.6 k/tendon
Drawings show 8.5 inch slab with 1" to c.o.g of strands at top and bot of slab
Bays are all 30' x 30'
f'ci=3000; f'c=4500
Lo=60psf
DL=106psf slab + 25 for part+MEP

The design examples I have from PCA show the same number of tendons at the exterior and interior bays. The drawings on this job show:
24 stands at the Exterior Bays
20 strands at the Interior Bays
For 4 bays across the building one way I have: 24 | 20 | 20 | 24 tendons

How does the differenent strands in the int/ext bays effect my calculations?

Thank you!

 

[BAretired]

The moment in an exterior bay is larger than that of an interior bay, so in your case, the designer used an extra 4 strands to provide for the larger moments. Those four strands should be anchored well beyond the first interior column in order to develop negative moment at the column. Usually, there is a mat of unstressed reinforcement over each column in addition to the strands to make up the rest of the required reinforcement. Your calculations should take into account both stressed and unstressed reinforcement.

BA

 

[structuralengr89]

thanks BA. Do you know of a source/example for doing handcalcs when there are different number of strands at the end bays?

 

[BAretired]

No, but I don't understand the problem. The strands in the end bays have 20% more GUTS (Guaranteed Ultimate Tensile Strength) than those in the interior bays. That means, with the same effective depth they will resist 20% more moment (neglecting the minor difference in depth of compression block) than the 20 strands in the interior bays.

BA

 

[slickdeals]

Usually you end up with more strands in an end bay compared to a interior bay of similar span. The reason is that the tendons exit the slab at mid depth. As a result, you lost the drape or "eccentricity" that helps to lift the slab up. In an interior bay with 8.5" thick slab, the top and bottom have 1" to CGS, which means your eccentricity is (8.5-2)=6.5". However, the tendon profile in your end bay is 7.5" - 1" - 4.25" (slab edge). More tendons are added to compensate the reduced drape.

 

[dik]

I often use the top kern point as point of anchorage to increase the sag slightly in the end spans...

Dik