LRFD Design
LRFD Design
(OP)
I have heard some DOTs push for LRFD design. I am familiar that AISC has Steel Design based on LRFD. What about concrete part? Latest I knew was ACI is based on Ultimate Strength Design, so LRFD design of a bridge means Steel with LRFD and concrete portions with Ultimate Strength???
Just I am a bit confused!!!
Thank you for helping me
Skj
RE: LRFD Design
I believe ACI has been using ultimate strength design since 1963. AASHOadopted LFD concrete design in the 1974 spec but retained the ASD specification through the last version of the Standard Specifications. LFD was USD. In college in the 70's we were taught both methods in concrete design.
RE: LRFD Design
Rod Smith, P.E., The artist formerly known as HotRod10
RE: LRFD Design
So I can use the ACI Ultimate Strength design method and claim is AASHTO LRFD?
BridgeSmith (Structural), thank you for your response too. I appreciate you opinion on the questions I asked above too.
Thanks
Skj
RE: LRFD Design
SKJ25POL - The short answer: No.
While the bridge doesn't know if you used ACI or AASHTO the owner will know. If you're legally bound to use AASHTO then use AASHTO. ACI does have a bridge design spec, 343R-95 but it's not AASHTO. If something goes wrong, even if it's not serious, the owner's lawyer will ask why you didn't follow the nationally recognized bridge code? AASHTO & ACI are both ultimate strength but they're not an apples to apples comparison in terms of design requirements.
Parts of the AASHTO concrete spec are straight out of ACI 318. A few times I've used ACI because AASHTO didn't address particular design issue.
RE: LRFD Design
bridgebuster (Civil),
Thank you very much greatly appreciate it.
Skj
RE: LRFD Design
RE: LRFD Design
RE: LRFD Design
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: LRFD Design
ASD generally uses unfactored (expected or nominal) loads and provides a factor of safety (FOS) with a single value reduction on the material strength.
LFD, which I think is similar to ULS, uses factored loading, where the expected loads are increased by load factors based on the variability of the source of the load. Material strengths are generally input at their nominal minimums (typically 5% exclusion values, i.e. 95% of specimens exceed the strength).
LRFD, as the name implies, incorporates load factors in similar fashion to LFD, but also incorporates resistance factors that reduce the material strengths, based on the variability in that strength in a particular application (resistance factor for concrete in compression is greater than in shear).
Rod Smith, P.E., The artist formerly known as HotRod10
RE: LRFD Design
In Australian codes there is a single reduction factor applied to the section capacity (which may vary depending on the type of load), in the same way as US codes, whereas the Eurocodes apply partial reduction factors to each material, with a further overall factor to cover uncertainties in the analysis, etc; but both approaches are referred to as Limit State Design.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: LRFD Design
Rod Smith, P.E., The artist formerly known as HotRod10
RE: LRFD Design
RE: LRFD Design
RE: LRFD Design
Also back then, there wasn't any consensus on loading rate a bridge using LRFD. About a year or so after we finished the project the client provided us with a methodology.
RE: LRFD Design
Well, I don't know how he got that much of steel, and I don't know the way you used to correct the problem, but to my knowledge, the intent of crack control is encouraging the use of smaller steel with a closer spacing to minimize the crack width. Yes, typically the quantity would be higher, but 4 times....! Also, the fs in the calculation is service load stress, I don't know how AASHTO address this.
RE: LRFD Design
I had a chance to compare Japanese Code versus AASHTO on the executed design, and found these to be almost identical. Same goes with the Eurocode.
On one positive side the LRFD design provide more efficient design, but complexity of it, and lack of basic understanding what all this factors means, leads to the gross errors, ending up in catastrophes.
The bizarre rebar quantities are the result of the lack of basic understanding of the difference in between crack control reinforcement, and minimum reinforcement required for the element to be considered "reinforced concrete" - typically at the substructure, and on most projects given to the compulsory subs - as these are the less risky elements of the project...
As my old Professor teach me - design a safe structure, and then check it if it complies with the code. It appears that common practice this days, is to comply with the code using computer, without even knowing what's the result.
The best example - the discrepancies in between the models for the Miami Bridge - a simple supported truss, which a student of the fourth semester should be able to solve by hand in few minutes - as it's just the total weight of the bridge and angle of the diagonal.
RE: LRFD Design
You issue an new edition of the Code, and charge $395.
Early editions of LRFD were focused on superstructure design. In the early 2000's FHWA began offering courses in substructure design.
Wiktor - Let me clarify my remark about the rating. The AASHTO Maintenance Manual addressed load ratings. There wasn't any equivalent LRFD publication.