PT slab bottom cover: ibc 2003 table 721.2.3(2) and 720.1.1(1) which governs

[Once20036]

Hi,

For a 2 hour rating PT unrestrained slab bottom concrete cover: ibc 2003 table 721.2.3(2) specifies 1 5/8" and table 720.1.1(1) --4-1.1 specifies 1 1/2", which one should govern? Thank you.

 

[gte447f]

Good question. I have always used 1 1/2". I did not know there were two tables with two different values. Can anyone explain why there are two tables?

 

[slickdeals]

This has been a point of debate. My boss used to argue that end bays of PT slabs are restrained and you could use the .75" cover. This was based on ASTM E119.

Incidentally AISC allows end bays of steel beam with pinned ends to be treated as restrained.

PTI's official position is that end bays are unrestrained and you should use the higher cover. With tendons exiting at mid depth and use of highe cover, you will need to jack up the number of strands in your end bays.

 

[gte447f]

slickdeals, I understand your points, but I tink the question is that the two tables referenced by Structural20036 appear to give two different values for cover for the same scenario (i.e. unrestrained PT slab).

 

[hokie66]

I don't know, but just a thought...perhaps one is to the duct, the other is to the strand?

 

[woodman88]

In depends on how you are designing the fire resistance of the PT slab. Section/Tables 720 are for the Prescriptive and Section/Tables 721 are for the Calculated method.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dcarr82775]

Slickdeal,

Did you mean the opposite, where the end bay is unrestrained and the interior bays are restrained? PTI recommends end bays be considered unrestrained and interior bays restrained.

Woodman is right, it is different methods of calculating resistance. I think you always end up with more cover when using 721. I just picked 721 and always use that one.

 

[slickdeals]

Dcarr,
I said end bays are unrestrained per PTI, but my old boss used to argue they are restrained per ASTM E119.

Also if end bay is unrestrained, how do you justify first interior bay being restrained. How does the thrust get developed?

Woodman,

It would seem prescriptive method should be more conservative than calculated method. Wonder why prescriptive method is higher.

 

[dcarr82775]

Slickdeals,

Bosses....what can you do

As for the next bay in, I really have never studied in-depth the tests and their controlling assumptions. If the fire is somewhat localized to the first interior bay I can envision it being restrained by the diaphragm in the exterior bay. If the fire is a large portion of the floor plate I can visualize the expansion in one bay being offset by the equal and opposite expansion in the next bay. No matter the scenario, in an end bay there is also expansion on the free edge for a fire in that bay. I can also see some funky scenarios where the above idealizations fall apart and you could have unrestrained interior conditions, but those scenarios seem less likely to me.