fredPE
Structural
- Apr 10, 2007
- 25
I am relatively new at designing elevated slabs (both PT and Conventional), and am working under a senior engineer who has a lot of experience, but says and does some things that I don't always agree with. So I am in a trust but verify mode with this supervising engineer, which leads to the following questions, which all deal with unbonded flat plate construction in the Western US:
First: The code requires that the minimum negative moment steel be placed within the 1.5h each side of the column. In addition to this minimum steel, additional steel may be required for the ultimate moment. According to my supervisor, since the code doesn't explicitly tell you to put this additional steel into the same (3h + column width) region, that you can space it out farther away from the column. He does this to somewhat mimic what happens in a conventional slab column strip, and to avoid congestion problems over the column with the banded tendons.
The code doesn't specifically tell you what to do with the steel in excess of the minimum required steel, but it seems that all of the commentary tells how important it is to get that steel right at the column. So, any thoughts on this? Also, if you are required to put all of the steel within the 3h+column region, and the loads require too much steel that will not adequately fit and allow placement of the concrete, does it point that you have too thin of a slab? Or at the least you need to add drop panels?
Second: Similar to the first question, if you need a certain amount of tendons to make the stresses work out, but their end anchorages (2-1/2" wide by 5-1/4" tall, typically) do not fit within the 3h + column width region, is there anything you need to do to account for that?
Third: Balance loads at end bays: It seems to be standard detailing procedure to place the end anchorages at mid-depth of the slab, but if you have a large end bay span, is there any problem with putting the tendons an additional 1" higher than the mid-depth? I realize that you would have to check the anchorage zones, and there would be a moment induced at the perimeter of the slab that you would have to resolve . . . I guess I am just curious if some of the experienced engineers have ever pursued this detail, or if that eccentricity is something you should avoid at all costs.
Fourth: Max skews on banded tendons where columns are not in line with each other. I have heard 15 degrees (i.e., a 1' in 5'/6' offset), but curious if there are any special considerations that need to be reviewed in order to resolve the forces from sweeping the tendons. (We do use a detail with #3 hairpins into the slab on sweeping banded tendons).
I have a few others, but I should cut it short or no one will want to answer my questions. Thanks for any guidance people can provide. Feel free to point me to a seminar or a book reference that may answer my questions as well. I have the ACI, the PTI manual (yellow book), and a few Post-tensioning texts from college, but they don't cover these more specific details.
First: The code requires that the minimum negative moment steel be placed within the 1.5h each side of the column. In addition to this minimum steel, additional steel may be required for the ultimate moment. According to my supervisor, since the code doesn't explicitly tell you to put this additional steel into the same (3h + column width) region, that you can space it out farther away from the column. He does this to somewhat mimic what happens in a conventional slab column strip, and to avoid congestion problems over the column with the banded tendons.
The code doesn't specifically tell you what to do with the steel in excess of the minimum required steel, but it seems that all of the commentary tells how important it is to get that steel right at the column. So, any thoughts on this? Also, if you are required to put all of the steel within the 3h+column region, and the loads require too much steel that will not adequately fit and allow placement of the concrete, does it point that you have too thin of a slab? Or at the least you need to add drop panels?
Second: Similar to the first question, if you need a certain amount of tendons to make the stresses work out, but their end anchorages (2-1/2" wide by 5-1/4" tall, typically) do not fit within the 3h + column width region, is there anything you need to do to account for that?
Third: Balance loads at end bays: It seems to be standard detailing procedure to place the end anchorages at mid-depth of the slab, but if you have a large end bay span, is there any problem with putting the tendons an additional 1" higher than the mid-depth? I realize that you would have to check the anchorage zones, and there would be a moment induced at the perimeter of the slab that you would have to resolve . . . I guess I am just curious if some of the experienced engineers have ever pursued this detail, or if that eccentricity is something you should avoid at all costs.
Fourth: Max skews on banded tendons where columns are not in line with each other. I have heard 15 degrees (i.e., a 1' in 5'/6' offset), but curious if there are any special considerations that need to be reviewed in order to resolve the forces from sweeping the tendons. (We do use a detail with #3 hairpins into the slab on sweeping banded tendons).
I have a few others, but I should cut it short or no one will want to answer my questions. Thanks for any guidance people can provide. Feel free to point me to a seminar or a book reference that may answer my questions as well. I have the ACI, the PTI manual (yellow book), and a few Post-tensioning texts from college, but they don't cover these more specific details.
