PT Podium Design - Applying Loads using Equivalent Frame Method

[MJC6125]

I have a few questions for how you apply loads to a PT podium slab from the load bearing walls that sit on the slab. I have Adapt PT that I can use for design, so these questions would be specific to using the equivalent frame method of analysis. See plan below for reference, which has boxed numbering that corresponds to the question numbering below (where applicable).

1. When you check the slab design are you always using strips that run along the column lines and are sized based on how far it is to adjacent column lines? Or do you ever check the uniform cables on a plf basis or a strip size that you choose based on loading (i.e. a 4' strip that has a load bearing wall running parallel and over the top of it)? This second option would be analogous to a one way slab with your banded tendons acting as a beam that supports the one-way slab and its loads.

2a. If you have a load bearing wall running directly on top of a column line in the uniform direction, does that load need to be applied to the slab design in the banded direction? (maybe it wouldn't matter if it was because the load will go directly into the support and won't affect the slab bending?)

2b. Same as 2a, but with the wall in the banded direction?

3a. If you have a load bearing wall running parallel to the uniform direction but at the midspan between two column lines, Do you put half of that load into each adjacent column line?

3b. Same as 3a, but with the wall in the banded direction?

4a. If you have a load bearing wall running parallel to the uniform direction but at some random point between two column lines (i.e. a 1/4 point), Do you put of portion of that load into each adjacent column line based on a ratio of how close it is to each column line?

4b. Same as 4a, but with the wall in the banded direction?

5. Do point loads work the same way as linear loads in terms of how it is incorporated into the design of different strips?

 

[BAretired]

If the bearing walls are masonry, I would consider deflections very carefully before carrying them on a slab and may well end up with a different framing system.

If the walls are lightly loaded wood or metal studs, a PT slab may be a reasonable choice.

Before deciding on the other questions, I would want to know the direction of span for the upper floors. If the upper floors and roof span N-S, I might consider a thickening on Gridline 3. In that case, 2a, 3a and 4a are not really bearing walls.

BA

 

[MJC6125]

The walls are wood. The attached plan was only provided as a reference, and doesn't actually match the wall layout of my project. I thought it would be simple enough in getting my questions across, but maybe it's not always so clear without seeing the framing above. This picture here shows the actual upper level framing layouts (floor and roof framing runs the same direction).

 

[rapt]

It depends on whether you listen to some PT pseudo experts or want to do it properly. I have seen some who suggest that all loads on a panel can be carried by all tendons in the panel for the distributed direction. That is blatently illogical.

All of your questions can be answered by the basic requirement that the tendon layout provide a load balancing tendon pattern, i.e. a pattern that will carry all loads to the supports.

In the "distributed" direction, that means that where loads are not simply UDL's, for each linear or point load there should be a strip of tendons that carry those loads to the supports. So the distributed direction is not necessarily uniform.

In the banded direction, if a load is on the band support line, then it is carried by the band only.

If cracking is allowed at service stresses in your slab (based on the real moment pattern allowing for concentrations of moment/stress) you cannot base your deflection design on the assumed average stress pattern which may show it to be uncracked. The slab does not understand the assumption! The deflections could be significantly worse when local cracking is considered.

 

[MJC6125]

rapt, when you are designing the tendons in the distributed direction, how do you determine your panel sizes for analysis. For example if I have a load bearing wall running parallel to the distributed direction and it is 1/4 of the way between two column lines, how wide is your design section of PT in that distributed direction going to be? Are you still using the column line as a panel and then distributing those total tendons within the panel based on some rational analysis or are you checking multiple strips more like one way slabs (i.e. an 8' strip with the load bearing wall and then the remainder without the load bearing wall load)?

 

[Enhineyero]

Looks like you need to hire an experienced structural engineer for the task.

 

[Ingenuity]

In Bondy and Allred's "Post-Tensioned Concrete - Principles and Practice" book (which is based upon 2D equivalent frame analysis) they state the following on page 207:

Modelling the Loading of the Structure Above

Typical residential wood framing results in numerous distributed bearing walls that occur approximately 12 to 14 feet apart. For this type of framing most structural engineers will use uniformly distributed dead and live loads in the podium slab analysis. However, when longer spans are used creating larger line loads (such as when trusses are utilized in the wood framing) the analysis may require that the line loads and point loads be modeled.

Using equivalent frame, I would use what rapt has stated. I have not designed a podium slab with wood framing using banded/distributed tendons. Principles are similar regardless of framing type, except wood framing and gyp board is more 'forgiving', compared to say masonry bearing walls, etc.

I have done transfer slabs with concrete framing above, using 2D equivalent frames. For transfer line loads I often used 6 to 8 times slab thickness to arrive at a suitable effective width. We provided transverse reinforcement to mobilize/distribute the line loads over such an assumed width.

My involvement with unbonded PT podium slabs has been limited to the repairs a 12" thick PT flat plate with cold-formed steel framing above - the existing PT was all uniformly-spaced distributed tendons, and it is fair to say that is the standard North American-practice, right, wrong, or in-different.