AS1418.4 Tower Cranes - use of support structure co-efficients 1

[Temporaryworks]

All,

I would like to start a discussion on the calculation of ''dynamic'' and ''stability'' loads for tower cranes using AS1418.4 in conjunction with manufacturers loads. This in the context of foundation design and checking of lateral deflection for building clearances.

I am posing this question as this is how I have been taught to do it but AS1418 gives no guidance on the use of the different Load Combination numbers and I haven’t found any useful guidance on google. I wanted a verifier that I was interpreting the code correctly.

I would love to get your opinions and suggestions. I have attached excerpt from AS1418.4 to refresh people’s memories.

KEY IS = in service case; OOS = out of service case


LOAD CALCULATION

To get my design loads, I take the manufacturer's loads and multiply by factors to obtain a dynamic moment (IS only); dynamic vertical load (IS only) and stability moments (IS & OOS).

1 – IS dynamic moment calculated by taking the proportion of moment caused by the load on the hook at maximum radius, multiplying by AS1418.4 dynamic factor = 1.3 and then adding this to the manufacturer given static moment.

2 – IS dynamic vertical load calculated by taking the part of the total vertical foundation load caused by the load on the hook, multiplying by AS1418.4 dynamic factor = 1.3 and then adding this to the manufacturers static vertical load.

3 - IS stability moment load calculated by taking all of the manufacturer’s static moment and multiplying by 1.45 (AS1418 Table 2.3.1(A) Out of operation with wind)

4 - OOS stability moment load calculated by taking all of the manufacturer’s static moment and multiplying by 1.2 (AS1418 Table 2.3.1(A) Out of operation extreme wind)


CHECKS

Then when checking the stability & geotechnical capacity of a gravity foundation: IS - I use loads 2&3 for IS loads; OOS – Use load 4 & the raw vertical load for OOS case from manufacturers manual. I check bearing pressure against allowable values.

Then when checking the geotechnical capacity of a piled foundation: IS - I use loads 2&3 for IS loads; OOS – Use load 4 & the raw vertical load for OOS case from manufacturers manual. These are compared against allowable geotech values.

I also take in to account the horizontal loads and any surcharges for all of the stability checks etc.

When checking the lateral sway/deflection of the tower I use the dynamic moment and vertical loads (1&2) for IS and the raw loads for OOS case from manufacturers manual.

For the IS structural strength check I take the dynamic moment and vertical loads and factor in accordance with AS1170. For the OOS structural strength I take the raw manufacturer’s loads and factor in accordance with AS1170.

How do people use the minimum factors of safety against overturning given in AS1418.1 6.2 in conjunction with the AS1418.4, these are given below:

The stability margin (FS) shall be not less than the following values:
(i) Crane in service................................................................................................... 1.4.
(ii) Crane out of service subject to the design wind loading ........................................ 1.2.

http://files.engineering.com/getfil...d65f58dbe&file=AS1418-4_2004_Tower_Cranes.pdf

 

[Agent666]

Because of the way manufacturers provide the total reactions for the two service cases (sometimes an envelope of worst case loads, i.e not always in equilibrium), with little information on what load cases they are actually using or what governs its can be impossible to get straight answers. They generally don't provide unfactored loads to enable you to manually review the worst case reactions in accordance with the AS1418 load cases (or to even check their final numbers).

My understanding of their loads is they include the lifted load appropriately modified, so you don't add it again as you are proposing. As noted in the footnote to the table you linked there is no requirement to add further dynamic factors when looking at the stability. The manufacturers take the maximum rated lift for the given height and jib arrangement and use this to work out their reactions.

I find this very frustrating, usually the loads you are given are by some sales people not engineers (here in NZ anyway) with little guidance on application. Often it is to an older loading standard or different wind zone, or neglects earthquake loads here in NZ (usually not governing but best to know I would have thought). I wish they would improve the way they presented their loads to increase transparency for others using this information.

The way I deal with it is to look at the load cases in AS1418 and pick the worst load factor for in service and out of service cases (and include their 0.9 factor on any restoring actions from the foundation self weight) and simply scale their total reactions by this factor as you never know what you are being given due to the lack of transparency in their supplied numbers. Sure there is some double counting, but this is at least conservative. Tower cranes falling over are a good way to make it on the 6pm news, which is not good advertising for your services! I don't you should sharpen your pencil too much on tower crane foundations.

Don't forget the slewing loads as well as adds additional shear at each bolt set. Usually I just add this directly as an additional load factored up as appropriate. I see people forget this all the time and you need a loadpath to prevent pad foundation from spinning for example.

I have talked to the sales people about getting exact numbers breaking down to constituent loads. But I'm informed usually this entails spending like 1500 euros and waiting about 2 weeks for the manufacturers engineering department to get back to you.

 

[Agent666]

Additionally I would assume that using the 1418.4 load cases with appropriate strength reduction factors on the geotechnical capacities would automatically satisfy the AS1418.1 stability margins (or are otherwise deemed satifactory as all AS1418.4 notes is your stabilising loads must be greater than the destabilising loads for achieving satisfactory stability.

 

[Temporaryworks]

Agent666,

Apologies for the late reply, I have been away on holiday for some time.

Thank you very much for the informative reply. I can see that it the load factors don't bear too much overthinking and your global safety factor approach using AS1418 factors makes sense to me.

I am also interested to hear that you do not multiply your moments by stability AND dynamic factors. Two different companies I have worked for in Aus have very different opinions on this - leading to significantly different gravity base/pile design sizes.

My current company actually also scales up the manufacturers moment loads by an additional ''P-Delta'' moment which I don't agree with as this must already be taken into account as part of the foundation load calculations undertaken by the manufacturer prior to being printed in their manual.

Perhaps though going back to your earlier point, it doesn't really matter if 20% more concrete/steel is used by taking a mega-conservative approach, as long as the foundation is robust and my company doesn't end up in court.

Many thanks for your time.