quick and dirty beam design

[uglukinpjs]

i would like other engineers' thoughts on doing a quick beam calculation in this manner

if i design a beam with similar top and bottom steel, i want to just calculate 0.9*As*Fy*d as a max beam moment.
of course i'll check for min and max steel in the concrete section. On every section i checked, the concrete compressive capacity has been higher than the steel.
I want to do this without having to rely on software or complicated hand calcs.

I think this would be a very conservative design, unless i am missing something here.

 

[SlideRuleEra]

Once-upon-a-time, long, long, ago (pre-1963)... a method similar to your description called Working Stress Design was THE way reinforced concrete was designed. WSD remained allowable for use until the early 21st century, as I recall.

 

[lexpatrie]

Here:

https://www.eng-tips.com/faqs.cfm?fid=2793

 

[uglukinpjs]

Once-upon-a-time, long, long, ago (pre-1963)... a method similar to your description called Working Stress Design was THE way reinforced concrete was designed. WSD remained allowable for use until the early 21st century, as I recall.

What are your thoughts on this?

Did they stop because its too conservative or not accurate?

 

[Tomfh]

A lot of engineers use similar equations as basic design tool. Most of the time people use a proportion of effective depth as their lever arm (eg 0.85d), as opposed to the distance between top and bottom steel.

 

[lexpatrie]

The change to Ultimate design in concrete wasn't a life-safety motivated change. It limped along in the appendix for a fair while as well. The two approaches (if you ask me) don't produce dramatically different results, similar to ASD and LRFD, but for whatever reason, limit states design in concrete went though more smoothly. I'd say the LRFD version of concrete is around 1956 version (as an Appendix) when it first shows up, but I may be wrong. So they did it about 20-30 years before Steel.

As a side note, the 1971 is where it's in the main code, and that's also when static moments for slab design actually matches reality.

 

[BAretired]

ACI 318-63 contains both Working Stress Design (WSD) and Ultimate Strength Design (USD). Either method was permitted by code, but USD was preferred by most engineers.

 

[DaveAtkins]

Did they stop because its too conservative or not accurate?

If I remember correctly from my college concrete classes (40 years ago!), the switch to ultimate strength design was made because the approach is more consistent. You factor up loads, and you reduce strength. Using working stresses for all situations does not yield consistent results.

It is advantageous once in awhile, such as when your load is mostly dead load and you get to use a smaller load factor.

DaveAtkins

 

[SlideRuleEra]

uglukinpjs - WSD could result in designs without enough shear reinforcement... leading to problems that were difficult to repair. Many vintage highway bridges had shear cracking of their cast-in-place beams (deck girders) as they aged and had to be replaced.

 

[dik]

We always did limit states design, starting in '65... and other than for historic reasons never really did any working stress design. For small percentages of reinforcing we used to use As = Mu 12 /(phi .9*d fy) as a quick estimate.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[civeng80]

When I was doing my course in the late 1970s, concrete design was by USD and steel was by WSD (in Australia). My professor was the eminent Len Stevens and I recall him telling us that all material design would eventually be by Limit state design for consistency and because it was a better, safer, logical way of design (or something along those terms, this is 45 years ago).
In the early 1980s steel design went to Limit state and there were a lot of unhappy engineers including me. Now I would never dream of designing by WSD even preliminary design.
This is just a thought bubble from me but I would be interested on other comments particu;larly in Australia.

 

[mtu1972]

I took my first concrete design class in 1971 and they taught both WSD and LRFD (USD) because they were not sure that LRFD would catch on. My first boss in 1973 had never heard of LRFD, so he let me use it and checked all of my calcs with his WSD method. After a while, he was convinced that my results were OK and very much in sync with his.

gjc

 

[civeng80]

Actually my mistake, I recall steel design went to limit state in the late 80s early 90s in Australia. It took about 10 years from when I graduated.

 

[DaveAtkins]

USD makes sense for concrete design, because deflection rarely (if ever) controls the design.

ASD makes sense for steel design, because deflection almost always controls the design. Although, I don't use ASD anymore for steel design, I use nominal divided by omega design.

DaveAtkins

 

[dik]

Our concrete and steel were both limit states in 1965...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[civeng80]

Still have my vintage AS 1250- 1975 from my University days. My hand just cannot put it in the bin.

 

[Retrograde]

USD makes sense for concrete design, because deflection rarely (if ever) controls the design.

For a lot of the reinforced concrete slab designs I get involved in deflection is controlling the slab thickness.

 

[IDS]

For a lot of the reinforced concrete slab designs I get involved in deflection is controlling the slab thickness.

Especially if you calculate the deflections properly, taking account of the effect of creep and shrinkage, and reduction in the cracking moment over time.

Also crack width (or crack width related stress limits) and fatigue often control the design on the concrete structures I work on.

All different limit states, included in limit state design procedures, but only the strength limit state uses steel strains past the yield strain.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[DaveAtkins]

For a lot of the reinforced concrete slab designs I get involved in deflection is controlling the slab thickness.

I always choose a minimum thickness per ACI, then deflection does not need to be checked.

DaveAtkins