AISC Appendix 3 Fatigue Design

[jheidt2543]

In equation A-3-1, is there a "typical" or rule of thumb regarding the value of N, the number of stress range fluctuations, for an industrial crane runway and its connections?

I have no idea how many lifts/day the Owner may make with his 5 ton crane. I searched the threads and the Internet for some guidance and should have started here first. I'm looking for a range of use for a "light" industrial operation.

 

[civilperson]

Assume 10,000/year, this gives such a low number that fatigue can be ignored, (since the deflection controls on structural members the stress is usually a small fraction of yield).

 

[nutte]

I disagree with civilperson. The limit when fatigue need not be checked is 20,000 cycles. That's over the life of the structure, not per year.

 

[jheidt2543]

Thanks for the responses guys, perhaps I should expand on by question. I am looking for a reasonable or traditional value for N as it related to a light duty overhead 5 ton crane.

First, am I correct in assuming that one cycle is a lift (up & down)?

Secondly, let’s say the crane makes 2 lifts/hr in a 10 hour day, 365 days/yr over a 30 year life span for the crane and runway. That equates to 219,000 cycles over the useful life for the value of N. Is that reasonable? For (6) lifts/hr under the same conditions gives a value for N of 657,000 cycles. When plugged into AISC Eq. A-3-1 give 39 and 57 respectively, which is greater than Fth = 7 from Table A3-1. Therefore, fatigue can be ignored – right?

Are these values of N reasonable in light of standard practice?

 

[nutte]

A cycle is the full loading and unloading of the live load, so yes, what you're talking about is a cycle.

39 and 57 are greater than Fth, therefore you MUST consider fatigue. You have it backwards.

I don't know if your N values are reasonable or not. Perhaps someone else can chime in on what values are typically used.

 

[kslee1000]

I am still holding on to AISC 9th ed. For which, please look into Chapter K4 (p. 5-84) & Appendix K (p. 5-106). You may find the answers. Also, commentary section K4 (p. 5-177) may provide great help.

 

[JAE]

This document:

http://www.mhia.org/vango/core/orders/product.aspx?prodid=72

may be helpful - have to buy it.

Also - from an AISC conference proceedings in 2000:

National Steel Construction Conference Proceedings, 2000, Las Vegas. "New Fatigue Provisions for the Design of Crane Runway Girders," Fisher, J.M. and Van de Pas, J.P.

This is available from the AISC website (free to members and non-members can purchase)

 

[spats]

The Crane Manufacturers Association of America(CMAA has established service classifications, Classes A-F. Class A is for infrequent use, and should probably not be used. Class B is for "light duty", where load can vary from no load to full rated load, with an average load of 50% of rated load, with 2-5 lifts per hour, averaging 15', not over 50% of the lifts at rated load. Design lifetime cycles is considered to be N = 50,000 full range cycles over a 40 year period. This is a guideline only.

You don't have to count every lift, because not all lifts are at full rated load/full stress range. Also, with a standard work year being 2080 hours, at 8 hours per day, works out to be the equivalent of 260 work days per year. you don't have to use 365 days, unless that is the actual case.

AISC fatigue provisions have changed since the 9th Edition. The attached reference is the latest info I have.

 

[kslee1000]

Be conservative when deal with cranes and associated structures. I am working inside of steel mill, the excessive use a crane is more common place than under use. Fatigue has been the number 1 enemy for cranes and welded girders.

 

[jheidt2543]

JAE: Thanks for the reference, I did download the AISC paper from their website.


Spats: Thanks! I used the 365 days because that is what is in the AISC Eq. A-3-1, however I like your formulation better, it seems more realistic.

Thanks to all who replied.

 

[Teguci]

Talk to your client. Give him the options with your recommendation. Also, point out annual inspection requirements. When the client has to pay for a welder to air arc gouge and replace broken welds, he will be on the hook for it and should be understanding.