PT Design and Deflections

I've been see-ing the note on the "Ram Concept News Channel" each time the program boots up. And i've seen the heading about defelctions for the longest time now.

Today i looked at the post, and followed the link to learn more about it. I downloaded the PDF of the slides onto the server.

They talk about PT deflections like it should be something of note in our design considerations. I am not sure that we get too concerned about the absolute deflections reported by RAM Concept - we tend to look at overall deflections and compare them to other areas of the same slab, maybe trying to limit deflections to about 1/2".

The article talks about 3 methods for evaluating deflections:

1. 2D Equivalent frame
2. Spreadsheets considering cracking, creep and load history
3. 3D Finite element

In the article they review several completed buildings, and compare actual deflections with predicted.

The following results were apparent:

1. 2D Eq Frame methods significantly UNDERESTIMATE deflections
2. Spreadheets overestimated the deflection - dependant on actual cracking levels
3. 3D FE yielded reasonable predictions that generally underestimated the real world slabs.

My take-away:

• For our typical Mixed-Use type projects with the 16/24/16 spans that we prefer, these deflection issues do not seem to come up much at all.
• On some of the larger or unusual spans, we may pay closer attention to the deflection - or when you have unusual; loading (like Pete did on his Athletic club - had large Dead load Relative to live load). In these cases it may be wise to use larger deflection multipliers for creep and shrinkage than normal, or consider "tightening up" the deflection range
• In talking with steve and others, it seems that 1/2" long term deflection is a common number to aim at for long term deflections on most span conditions.
• Be aware that creep and long term delfections may be a factor in unusual or long-span conditions.