Dimensioning when designing Concrete Structures.... 3

[oengineer]

I was taught it is best to round up to the nearest even inch for dimensions when designing concrete structures.

However, I have seen some drawings using increments of 3 inches (i.e. 3", 6", 9", 12").

This may be splitting hairs, but I am curious. 3" and 9" are not even numbers & I assumed the reason for rounding to the nearest even inch was to make it easier in the field with form work for contractors.

What approach is the most optimal/good practice to take? Even inch or increments of 3 inches?

Comments/suggestions are appreciated.

 

[ggcdn]

I imagine every region or even every design office has its own little quirks about this.

I round to nearest inch on most things. Never heard of the 3 inch increments.

But then there are also numbers we avoid, for no theoretical structural reason really. For instance, we wouldn't typically call out a 13" wide column, or 13" rebar spacing. Same with 11",17",19",23",etc.

I had one engineer tell me he will never ever specify 15" rebar spacing and all the contractors will laugh at us if we do, even though that is closest to 0.2% in a 10" slab (eg 15M@15"). We're weird....



-JA
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[WinelandV]

I think 3" increments is nice (especially for a grain bin foundation), because it's 0.25'.

 

[jayrod12]

I've always likened it to framing lumber, keep it on the 12" or the 16" or some version of that. I too would never specify 13 or 15" spacings, it would be 12" or 16". I also am not a huge fan of odd number spacing anyway in slabs, especially two-way slabs. Because when I need to add additional bars over columns or beams, then they are at some decimal portion spacing instead of 6" or 8" etc. My standard two-way gym slab design has #5 @ 16" everywhere with additional #4 or #5 at 16" (net @ 8") over columns/beams. Makes it nice and simple. Not saying it's drastically different from having them at 15" and then 7.5", except that 7.5" just looks wrong.

I'm currently reviewing an existing building that has 5" wide x 16" deep concrete joists spaced at 25" o/c. Not sure why, but the 25" o/c bothers me. It doesn't work well for the bay spacings or anything, just seems like 25" was a common joist spacing measurement in the day.

 

[dold]

It depends on the formwork the contractor has / plans to rent, but forms like Symons panels go in 2" increments. Ditto for form ties. If you spec an odd wall thickness (or the architect DEMANDS that your wall be 7-5/8" thick...) or pilaster dimension, etc, the formwork will (probably) have to be stick built and/or form ties will have to be cut and re-welded. (this applies to mostly vertical concrete - walls/columns/pilasters/etc). Most of your flatwork is all stickbuilt anyway.

And what sort of architects do you guys have the luxury of working with to be able to suggest rounding off building dimensions to 1", much less 3"...? I haven't worked on a building with round/even/logical dimensions in years (thanks, Revit...)

As for rebar spacing I don't see anything wrong at all with odd spacing. A tape measure and a can of spray paint works the same when extended 15" or 16". I'm all for round, even numbers, but rounding a bar spacing down 1" just to satisfy your OCD or avoid the possibility of catching a funny look from a rodbuster could cost the owner a not-insignificant amount of money.

A firm I worked for had enveloped footing schedules based on bearing pressure and concrete strength that would go on every project. Just select the one with sufficient bearing area and it's already designed otherwise. The schedules spec'd footing thicknesses in 1" increments. I.e., you might end up with a 19" thick footing. Seems odd, but when you have a massive warehouse with 100+ footings, 1" of thickness can add up. I eventually came around to the odd numbers.

I'm currently reviewing an existing building that has 5" wide x 16" deep concrete joists spaced at 25" o/c. Not sure why, but the 25" o/c bothers me. It doesn't work well for the bay spacings or anything, just seems like 25" was a common joist spacing measurement in the day.

Could be the pan joist formwork modules they used?

 

[GC_Hopi]

For concrete structures, sometimes it nice to round off to 3" inches because it is easily divisible with common rebar spacing (3", 6", 9", 12").

dold: you need to work in industrial design then there is very little architectural say.

 

[BridgeSmith]

It depends on the formwork the contractor has / plans to rent,

That is the key. In bridge design/construction, 15" is a common thickness for retaining walls, etc. Not as common as 12" or 18", but we rarely, if ever, use 14" or 16".

As for rebar spacing I don't see anything wrong at all with odd spacing. A tape measure and a can of spray paint works the same when extended 15" or 16".

In bridge work, we commonly call out up to 12 bars at equal spaces, for a total length that may be any oddball dimension. Typically the contractors have a piece of stretchy rubber tubing with equally spaced marks on it, that they stretch out to the overall dimension and use to mark the intermediate bar locations.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Retrograde]

It's time you adopted the metric system.

 

[dold]

dold: you need to work in industrial design then there is very little architectural say.

But then I would never get to design yet another cheap midrise wood apartment building!!!

Typically the contractors have a piece of stretchy rubber tubing with equally spaced marks on it, that they stretch out to the overall dimension and use to mark the intermediate bar locations.

Thats pretty damn clever.

 

[3DDave]

I saw a video of a ruler that was designed like that stretch tubing.

https://www.youtube.com/watch?v=zeCslpB-JVU

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