Factor of Safety for Overturning - Foundation Design 1

[oengineer]

I am working on designing a foundation for a jib crane. [highlight #FCE94F]The jib crane is 16 ft tall with a 24 ft cantilever arm. The maximum lifting capacity of the crane is 3 tons.[/highlight]


From CRSI Design Handbook 2008, it mentions to use a Factor of Safety (FOS) of 2.0 for Overturning, see image:

A FOS of 2.0 is also mentioned in Principles of Foundation Engineering, 8th Edition by Braja M. Das:

However, it seems PIP mentions the use of FOS = 1.5 or 1.2 for overturning, see images:

The only explanation to these various FOS I could find is mentioned in the Foundation and Anchor Design Guide for Metal Building System by Alexander Newman:

For my particular situation, what would be the appropriate FOS for overturning to use for a jib crane foundation based on the crane description I provided above?

I am inclined to use FOS[sub]overturning[/sub]=2.0, but I would like to have justification for my design which ever route I go.

Comments/suggestions are appreciated.

 

[le99]

I think the difference in "safety factor" is mainly caused by the use of Non-factored load (as in ASD) and factored load (as in LRFD). At the time of ASD, the factor against rotation was usually 1.5 for normal cases, and 2.0 for cases with elevated uncertainty.

 

[JLNJ]

How certain are you of the loads? If I know my loads are very conservative then I'm OK with 1.5. The more uncertainty in my loads, the more I tend to bump up the safety factor.

 

[JStephen]

You're comparing jib crane recommendations to pressure vessel recommendations, and things like that vary from industry to industry and from application to application.

 

[oengineer]

I think the difference in "safety factor" is mainly caused by the use of Non-factored load (as in ASD) and factored load (as in LRFD). At the time of ASD, the factor against rotation was usually 1.5 for normal cases, and 2.0 for cases with elevated uncertainty.

Would a foundation for the jib crane I described be considered "elevated uncertainty"? If so, then would FOS[sub]overturning[/sub] = 2.0 be justified for the foundation design?

Is there any technical document that states criteria for design of "elevated uncertain" foundation conditions?

Also, the CRSI Design Handbook 2008 & Principles of Foundation Engineering, 8th Edition are not documents that were published so long ago (within at least the last 20 years - the Foundation Engineering book was published in 2016). So I would think that these issues would have been considered when these text were written.

 

[oengineer]

How certain are you of the loads? If I know my loads are very conservative then I'm OK with 1.5. The more uncertainty in my loads, the more I tend to bump up the safety factor.

I was provided the overturning moment & vertical weight of the jib crane by the vendor. I also went ahead and ran my own calculations in STAAD to come up with an overturning moment of the situation with wind loads included (the vendor did not specify if the applied moment included wind loads as well).

The overturning moment I generated from using STAAD was 3 k-ft greater than what the vendor given me.

For this situation, would you still recommend using 2.0 for FOS[sub]overturning[/sub]?

The vendor provided a drawing with a recommended jib crane foundation requirements, but it appears to be based on a FOS[sub]overturning[/sub] of 1.5.

 

[oengineer]

You're comparing jib crane recommendations to pressure vessel recommendations, and things like that vary from industry to industry and from application to application.

Fair enough about the jib crane being a different equipment for the vertical vessels.

However, the PIP Structural Design criteria mentioned 1.2 & 1.0 for the FOS[sub]overturning[/sub]. This is less than what is mentioned in IBC 2012(see image):

That seems too "care-free" of a design to me, using FOS = 1.0.

If you have some more insight on jib crane foundations, please let me know.

 

[centondollar]

Ignoring any safety factors, the principle behind the FoS is straightforward: when you idealize the foundation as a beam and calculate the contact stress below it due to eccentric load (M = P*e)(normal stress = P/A +- M/W), you will find that a factor of safety of 1.5 corresponds to a 50% compressed footing-to-ground interface, while a factor of safety of 3.0 corresponds to a 100% compressed footing-to-ground interface. A factor of safety of zero means that you have 0% compression below the footing, indicating that it is overturning.

How this principle is applied in practice e.g., through kern equations (limit eccentricity to "width/6", where "max_eccentricity = M/P <width/h" to achieve 50% compression etc.) or through direct computation of the contact stress, or through direct calculation of resisting versus overturning bending moment, is a matter of local practice. My understanding is that most codes do not specify any specific method, since at the end of the day, overturning (of the structural type, i.e., loss of contact) and also sliding are based on basic structural mechanics. Draw a picture and do a calculation - as long as your loads are estimated reasonable accurately, your design should be on the safe side.

PS. Geotechnical failure modes of soils (e.g., shearing or block sliding etc.) is another topic that might be relevant for foundation designs, but the geotechnical expert should be able to provide those calculations.

 

[le99]

I wouldn't consider the "jib crane" has elevated uncertainty, as it is usually installed indoors, under a well-controlled environment, with defined loading conditions. In comparison, in any aspect, an outdoor crawler crane would have much higher uncertainties.

 

[oengineer]

I wouldn't consider the "jib crane" has elevated uncertainty, as it is usually installed indoors, under a well-controlled environment, with defined loading conditions. In comparison, in any aspect, an outdoor crawler crane would have much higher uncertainties.

Actually, this jib crane is going to be located outdoors 100% of the time when in use & shall be exposed. It is NOT going to be contained inside of any building or shed.

 

[le99]

IMO, if the jib crane is in the middle of a dirt yard, or near a retaining wall, for which soil condition/property can easily change, then, yes, it is facing high uncertainty. Also, you shall consider other factors - worker's life/safety, cost of the replacement of the crane, severeness of the downtime if the crane has failed... Unless you can point out a specific code requirement, you need to prepare yourself to defend the use of the higher safety factor.

 

[oengineer]

IMO, if the jib crane is in the middle of a dirt yard for which soil condition/property can easily change, then, yes, it is facing high uncertainty. Also, you shall consider other factors - worker's life/safety, cost of the replacement of the crane, severeness of the downtime if the crane has failed... Unless you can point out a specific code requirement, you need to prepare yourself to defend the use of the higher safety factor.

My main purpose in creating this post is to be able to defend the use of using a FOS[sub]overturning[/sub] = 2.0 for the design of this foundation.

My initial defense would be to point out the first two images I mentioned in the original post showing where it is mentioned that a FOS[sub]overturning[/sub] of 2.0 is required for foundation designs. i originally thought a range of 1.5 to 2.0 (primarily 2.0) was industry standard for industrial facilities.

[highlight #FCE94F]My concern is that referencing the CRSI Design Handbook 2008 and the Principles of Foundation Engineering, 8th Edition will not be enough to justify the use of FOS[sub]overturning[/sub] = 2.0 for the jib crane foundation design.[/highlight]
If my concern is not valid, I am open to hear why these two documents are sufficient to proceed with using 2.0.

 

[HTURKAK]

PIP STE03350 is for Vertical Vessel Foundation Design however, PIP STC01015 Structural Design Criteria still suggests F.S.=1.5
(...The minimum overturning “stability ratio” for service loads other
than earthquake shall be 1.5 ...)

Apparently you were provided with the overturning moment & vertical weight of the jib crane by the vendor. If the OT moment provided is based on capacity , plus a hoist & trolley allowance , you may add and an impact allowance (1.25)

IMO ,Your concern is valid and if outdoor installation, add the effect wind loads ..

My personnel experience, i always provided stronger foundation than anchor and pillar capacity ..

I always keep in mind the saying ; Anything that can go wrong will go wrong ( by Edward A. Murphy, jr. ) (means, if there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong )..

My opinion only...

 

[BigH]

HTURKAAK - O'Mally's corollary is that Murphy was an optimist.

 

[HTURKAK]

Mrs. Murphy’s Corollary: Murphy was an optimist! I did not know this.. Learning Has No Age Limit! ...

 

[le99]

I don't against using a higher safety factor for this case. But beware of the question on the suitability of adopting the CRSI standard, and the criteria offered by a single textbook. If the jib crane belongs to a specific industry, you should check what is the prevalent design code/standard used in that sector.

 

[oengineer]

I don't against using a higher safety factor for this case. But beware of the question on the suitability of adopting the CRSI standard, and the criteria offered by a single textbook. If the jib crane belongs to a specific industry, you should check what is the prevalent design code/standard used in that sector.

So I found some mention from the Jib Crane vendor about their thoughts on the foundation design regarding overturning moment (see image):

It seems to me from ASCE 7 that foundations with a cantilevered column type should have special consideration for overturning (see image):

PIP STE03350 is for Vertical Vessel Foundation Design however, PIP STC01015 Structural Design Criteria still suggests F.S.=1.5
(...The minimum overturning “stability ratio” for service loads other
than earthquake shall be 1.5 ...)

Apparently you were provided with the overturning moment & vertical weight of the jib crane by the vendor. If the OT moment provided is based on capacity , plus a hoist & trolley allowance , you may add and an impact allowance (1.25)

IMO ,Your concern is valid and if outdoor installation, add the effect wind loads ..

My personnel experience, i always provided stronger foundation than anchor and pillar capacity ..

I always keep in mind the saying ; Anything that can go wrong will go wrong ( by Edward A. Murphy, jr. ) (means, if there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong )..

My opinion only...

I like how you mention that 1.5 is the MINIMUM factor of safety for the overturning moment. Due to the fact that this Jib Crane is located outdoors (i.e. exposed to wind loads - which I am not sure were considered by the manufacture) & the cantilever span is 24 ft long carrying a maximum of 3 tons (6 kips)...to me it seems a FOS[sub]overturning[/sub] = 2.0 is justified.

Question: Would you consider the vertical weight of the jib crane when designing for overturning or not include that weight as a help to resist overturning due to the capacity of the crane times the moment are of the cantilever? My initial thoughts are that the weight of the jib crane would not be effective if the crane is trying to rotate due to a maximum applied load on the cantilever arm.

 

[MotorCity]

You are legally obligated to follow the governing code (IBC for me). So regardless of what other publications may use for a safety factor, you need to follow the code. Keep in mind that design codes are minimums, you are free to exceed the requirements.
In the absence of a code requirement, the IBC allows you to refer to other publications for guidance that have been widely accepted. That, along with engineering judgement, should guide your decision.
My personal feeling is that I am comfortable with a FS =1.5 for all foundation and retaining structure failure modes unless a lower value is permitted by code.

 

[le99]

Who is the client that requests the design? (This info is relevant to choosing the code)

Self-weight should be included in all steps of the analysis - to determine soil pressure and to determine inertia force during the earthquake.

The cited ASCE7 provision is for seismic considerations.

 

[oengineer]

Self-weight should be included in all steps of the analysis - to determine soil pressure and to determine inertia force during the earthquake.

The self weight of the foundation & the equipment has been considered in verifying the soil bearing, sliding, & strength checks. I was just specifically thinking it would be more conservative to consider mainly the weight of the foundation itself to resist the applied moment caused by the cantilever & max capacity of the jib crane.

After further thought, I will continue to consider the weight of the Jib Crane as a ballast in my Overturning Check. This is since I would think that the Jib Crane manufacture would include a counter balance to keep the crane itself for rotating/overturning.