Most Economical Reinforced Concrete Beams and Columns 1

[RLC32681]

I have a question guys. Is there any combination of f'c and Fy that would give the most economical section and reinforcement for reinforced concrete beams and columns? If not, How to design the most economical beams and columns for multi-storey buildings? What are the factors to be consider?

 

[JAE]

f'c and Fy don't affect the economy that much - in fact usually in the US you have f'c = 4000 psi and fy = 60,000 psi on the steel. Sometimes higher f'c values are specified, especially for columns.

The economy in concrete construction is usually affected more by the beam depth, beam/joist spacing, slab thickness, forming layouts of beam widths, etc. than the material strengths.

 

[hokie66]

My approach is to use the same size columns as much as possible, limit the reinforcement quantity to 1% or a little more, and adjust the concrete strength according to the load. In Australia, we customarily use concrete strengths of 32, 40, 50, 60, and 70 MPa for building columns. Beams and floor slabs are normally 32 or 40 MPa.

 

[csd72]

RLC32681,

You must be a student, otherwise you would most likely realise that this is not a meaningful question. No offence intended.

Steel and concrete are both only available at certain grades.

A higher steel stress grade would be of advantage for strength but would be no help for deflection/shrinkage. It therefore often does not make much difference on the overall steel volume.

Concrete grades tend to be dictated by a other considerations such as durability.

Material costs are only about a third of the total cost for concrete construction. At best you could shave 5 or 10% off the material costs it would be much more productive to focus your efforts on constructability where potentially larger savings can be made.

 

[RLC32681]

thanks for your opinion guys, but i have a follow-up question. assuming i am free to choose the sizes of the beams and columns, that is no predefined sizes as dictated by architectural considerations, how is the procedure to yield the most economical reinforced concrete beams and columns. most economical considering cost (materilas, labor, equipment.

 

[hokie66]

RLC,

Sounds like the same question to me. I suggest you get this type assistance from your mentor. Consulting engineering firms typically have guidelines for preliminary design.

csd,

I think it is a valid and meaningful question. However, he may be a student, as this is his first post.

 

[csd72]

RLC32681,

Apologies for my very opinionated post.

Column sizes are generally dictated by fire rating requirements with a minimum size of 300mm(1 foot) being the minimum that I would recommend for any column due to compacting and detailing issues.

The actual cost of the columns is not really an issue (generally being only 1 or 2% of most projects) but the loss of usable space often is. For this reason most columns are kept as small as possible.

For taller buildings the concrete strength is generally the issue, so you should keep it as small as possible using standard strength grades even if the lower columns are at a different strength than the upper ones. I would only use high strength concrete where necessary.

Hope this helps.

 

[RLC32681]

I agree that cost of column is negligible as compared to the total cost but my concern why asking how to design the most economical RC column and beam is that I am comparing what kind of structural frame (whether reinforced concrete or steel) to used in multistorey frame. I am given a task to make a study what kind of framing system (either steel and reinforced concrete) to use in multi storey building. I have in mind 3 criterion; 1. cost 2. construction time frame and 3. story drift. I am planning to compare these by considering a model structural frame and design it using reinforced concrete and steel and compare base on the criterion above. any comment is highly appreciated.

 

[JAE]

Can you describe the project in more detail? How many stories, is it in a high seismic area? What location is it in as this might affect the relative economies of concrete vs. steel (some areas have more concrete frames than steel due to lower labor costs, etc.).

 

[rowingengineer]

worth a read:

http://www.wpi.edu/Images/CMS/CEE/tallbuidings3.pdf

When in doubt, just take the next small step.

 

[rowingengineer]

I am currently doing some preliminary design research this just happens to show up, not perfect some good points

http://users.tce.rmit.edu.au/stuart.harrison/tech/lectures/span-table.pdf

When in doubt, just take the next small step.

 

[dik]

Like hokie, but in addition to varying the concrete strength, vary the reinforcing from about 1% to about 3%, keeping the column size as uniform as possible.

Dik

 

[RLC32681]

I have three structural frame, 4 storey, 6 storey and 10 storey. it is located in high seismic and wind risk area. concrete and steel are locally available.

by the way, thanks for your smart opinion guys. i appreaciate if you can add more information specially on how to undertake this problem. please share with me if there are some factors that i forget to consider.