Concrete Economy Brochure 2

[KootK]

Old dogs,

A fellow I used to work with had a great little brochure on economical concrete design. It told the story of an eager beaver 50's looking engineer who detailed a concrete frame so that every beam size and reinforcing bar was optimized to save as much material as possible. Then his wise manager went about explaining to him that economy was largely a function of formwork costs and therefore repetition etc. It was great. And funny.

Anyhow, I've got some junior engineers working with me and I'd love to have this brochure handy to share with them. Unfortunately, I'm no longer able to get in touch with my former colleague. Does anybody recognize the document? Know the title, publisher, or approximate year of publication? If I remember correctly, the brochure was about 6" x 11", fold out style. I know, it's a long shot...

Thanks,

KK

 

[Ron]

Try the Portland Cement Association (PCA) and ACI...both have produced many educational brochures over the years.

 

[TXStructural]

CRSI has downloadable Tech Notes on economical concrete construction. Visit crsi.org and click on the link to the WebStore - registration requires your basic contact info, but the Notes are free. Specifically, look at ETN-C-2-09 Efficient and Economical Reinforcing Bar Details, and ETN-C-1-10 Economical Reinforced Concrete Construction.

We also present a 2 hour seminar on this exact topic. (Free of charge)

As a basic rule, formwork represents about 50% of cost for a cast-in-place concrete structure. Members which are similarly situated should be sized and reinforced equally. The insignificant savings to reduce 2 inches of concrete width to a single member, or reduce the amount of reinforcing by a few pounds per foot are more than offset by increased labor and formwork costs.

On two to ten story buildings (most of what is built), size regularly spaced columns the same. Keep congestion in mind; for non-seismic, 2% steel in columns is ideal. Use larger bar sizes, and keep column dimensions and reinforcing consistent floor to floor. Reduce bar count before reducing size, then change bar size as needed.

For commercial floor design, choose uniform live loading at 80-100 psf (meet ASCE 7, just avoid complex loading which makes the design unnecessarily complicated.) If you use less than 80, you have to add partitions back in. Applying live load to match architectural plans makes it more complicated, not more economical. Use uniformly spaced and loaded slabs and beams, keep consistent formwork and reinforcement. Always use some top steel in gravity-loaded flexural members, since it is needed for continuity and stirrup support.

The cost of the structure is seldom the place to reduce the cost of the finished building, and trying to save a little here and there by making the construction complicated will not actually save money.

 

[dcStrucEng]

TX, thanks for the summary, very helpful. What is your opinion on varying f'c for the columns? For example, in a 10-story building, is it more economical to keep the same column size throughout the height of the building and increase f'c in the lower 1 to 2 lifts or keep the same f'c and increase the size of columns in the lower 1 or 2 lifts?

Another econ question: if you have a 24x24 column, what is more cost effective from a material and labor standpoint: 4#11 bars or 8#8 bars?

Thanks.

 

[Qshake]

Ron - what are you doing answering a post that starts off with "old dogs"?!

Kootk - back on topic, I'm aware of that brochure and have it somewhere around my office. I'll see if I can find it and post it.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[TXStructural]

Based purely on economics, four bars at the corners will typically be less expensive, simply because bars at the middle of the face will require additional ties (due to spacing of the bars.) Where bars are spliced at each level, you can also consider the lap lengths.

It is common in tall buildings to increase strength of concrete in columns, and it is frequently economical in these systems. For 10 stories and fewer, and small floor areas, it might not be. In the columns, it works well enough, but the design might require puddling of high strength concrete in floors at columns. When placing the floor using one pump or crane, you can end up with cold joints if you don't place the two concrete strengths simultaneously. In tall buildings, it may also be economical to use Grade 75/80 rebar.

Varying column size is perfectly acceptable, but it should be done to minimize the different sizes. For instance, it can be economical to make all columns the same for the entire height up to 3 or 6 stories. It might prove to be more economical to have several sizes is a 6-10 story building. The economy is lost when there are 6 sizes on the first floor, which change to 6 different sizes on the 2nd or 3rd floor, and change again every couple of floors. Hold the column consistent until you can make a significant change (24x24 to 18x18). If you prefer, change one side only as an intermediate step (24x24 to 18x24 then to 18x18, as you go up), since the unchanged side is reused. This gives three sizes for the formwork cost of two sizes.

Offsets greater than about 3 inches require the use of dowels in lieu of offsetting bends for bars to be lapped.

 

[KootK]

@Qshake: If you've got a copy that you could post for me, that would be wonderful. Thanks so much. I want it for the nostalgia of it as well as the concrete economics.

@TXStructural: thanks for all the useful information. I download the publications that you mentioned. They're great. I went to the CRSI website to try to find that 2hr seminar that you mentioned. I was unsuccessful I'm afraid. Could you post a link to it?

 

[TXStructural]

No link, contact me directly and I'll see about hooking you up. It is a presentation I do at SEA/ASCE chapters and in engineering offices.



John Turner CSP PE
CRSI Greater Southwestern Regional Manager

 

[KootK]

Hey QShake,

Any chance you managed to locate that brochure?

Thanks,

KootK

 

[csd72]

Here are a few uk ones:

http://www.civl.port.ac.uk/rcc2000/pdfs/concrete.pdf

http://www.construct.org.uk/bpg/BPGFlat_Slabs_Eff.pdf

 

[KootK]

Thanks CSD. That first document is fantastic.