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Tower crane anchor design - pre-load query

Tower crane anchor design - pre-load query


I have a query regarding pre-load applied to tower crane anchors. I am a Temporary Works Designer and have experience with designing tower cranes with sacrificial cast in bottom tower sections but not traditional holding down bolts. I have produced a preliminary anchor bolt design including specifying pre-load to apply to the crane and also have requested confirmation of pre-load bolt testing to be supplied to me. I am not entirely 100% confident that my specified pre-load is correct and I am also not 100% sure what I want to see in the test results when they come through.

Some background to the question is bulleted below and shown in attachment which you will need to read at the same time to get a feel for this:

- Tower crane is anchored in to a deep reinforcement concrete base which is piled.
-To give you an idea of the tower crane: It is a Potain MC85B and is 36.1m height under hook (approx 45m from top to bottom for those not in the know) and has a jib length of 45m.
- Unfactored tensile load in crane foot = -716.13kN. 4no anchors per foot therefore -179.03kN tension per anchor.
- Yield load of 32mm anchor = 764kN SWL of anchor = 1/3 of yield = 229.2kN.
-Pre load = 50% of elastic limit of anchor road = 382kN as per manufacturer's recommendations which are shown attached.
- Difference between specified pre-load and working load = 203kN or +122% of working load

I have been reading eng-tips threads on pre-load, and John H.Bickford's book and have come to the conclusion that bolt pre-load determination can be very complicated! I think that my pre-load is more than enough to make up for elastic losses etc + accuracy of pre-load application etc etc and still have a decent margin of safety.

My first Qn is - is my calculation of the pre-load, as per manufacturer's instructions, sufficient/detailed enough given the risk and high forces involved in a tower crane anchor design? I am fairly sure that most TW designers do not use anchor design methods beyond what I have described here but I wonder if we should be doing this?

My section Qn is - what preparation do I need to do in readiness for the test results from the bolt pre-load contractors or do I need to give them more specific instructions regarding the sort of test that I want?


Thanks in advance.

RE: Tower crane anchor design - pre-load query

I forgot to mention that I also have a shear load of 7.5kN unfactored per anchor.

RE: Tower crane anchor design - pre-load query

If I understand your post and attachment correctly, the crane manufacturer recommends a pre-load of 50% of Fe (hence 382 kN per 32mm dia anchor). I would have thought they would recommend a value as a % of the max. service load, but be that as it may.

With a maximum service load of 179 kN to each anchor you therefore have an applied pre-load of more than 2 times the max service load. Seems substantial, but I have no experience with tower crane bases and pre-load magnitudes.

I assume the service life of this anchor/foundation will be short - less than 2 years? - so stress relaxation of the bar and creep of the concrete can be checked but probably not significant, especially since you have ample pre-load compared to service load.

For a 850 mm 'free length' and applied pre-load of 382 kN (A = 804 mm2) you will have an estimated elongation of 2.0 mm - not a whole bunch, so any unaccounted for deformations (mostly long-term effects mentioned above, as the immediate/short-term effects will be 'taken up' as the rods are stressed) over the service life will reduce the pre-load, however, the ULTIMATE strength will not be effected. Even if you 'lost' 50% of your pre-load (from 382 kN down to 191 kN) you would still be a fraction greater than your calculated max. service load, so probably no serviceability issues would result.

For testing: I am not experienced with tower crane installations, and my background is post-tensioning of concrete structures, so user-beware of the following info. For threaded high-tensile bar we would specify a temporary stressing chair/stool, a temporary threaded coupler, and a temporary pull bar, and use a 50 or 100t hydraulic center-hole ram to stress to rods. We would use a dial indicator to the top of the pull bar to measure elongation. After each of the four rods are stressed we would then do a lift-off test to each of the four rods to check if there were any immediate elastic/seating losses, and re-stress accordingly, as required.

If you go this route, then the 50 mm dimension you have above the top lock nut may need to be increased to accommodate a half-length of a temporary threaded coupler:

I assume you are also responsible for the concrete foundation design too, you will have to also check global concrete pull-out etc etc. of the anchor assembly.

The 20 mm thick x 200 x 200 embedded bottom plate seems 'thin' - stresses may check out ok given that it is embedded, but probably not the place to 'skimp' on the plate thickness.

Be careful specifying DYWIDAG EU [Link] bar if you are not in Europe (the WR designation is EU specification, I believe) and make sure you get what you specify: for example DWYIDAG AU [Link] and DYWIDAG US [Link] are both different from EU products. You may be best to specify/reference an AS or ASTM with min. mechanical properties rather than a product name.

I am not sure if you have considered temporary corrosion protection of the anchor rod - the annular space over the 850 mm length may need to be grouted or grease injected, or some other form of protection.

RE: Tower crane anchor design - pre-load query

I really appreciate your reply, thanks for taking the time.

The service life will be <1 year. However, from the reading that I have done on the things that cause short term relaxation , there are a myriad of things that can cause loss of pre-load and the forumlae that go with this require alot of experience and knowledge especially given the risk involved with a tower crane. It would seem that this is within the realms of a mechanical engineer or a specialist structural/geotech engineer (such as yourself). Also, the site will contract the pre-load technicians and therefore it would be hard for me to manage the application of the pre-load even if I were to have confidence with calculating it properly.

With this in mind, I think that perhaps Manitowoc suggest 30% of yield for SWL and 50% of yield for pre-stress as a conservative means of ensuring that the pre-load is not skimpy. So you have probably answered my first question, In Bickford's book he talks about short term pre-losses in the region of 15% and so I can see from this and your comments above, this provides an ample margin of error.

Thanks for the comment on the thread being too short, I will contact the pre-load technicians and get some advice. I will also speak with the threadbar manufacturer with advice on concussion protection, I think a grease would be a suitably cost-effective option given that these bolts will only be installed for a short period of time.

Regards the plate, it has since doubled to 40mmm thickness.

RE: Tower crane anchor design - pre-load query

Hi Temporaryworks

I have used this formula in the past for bolt tensioning

Also if you google tentec they have lots more info on bolt preload

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Tower crane anchor design - pre-load query

Quote (Temporaryworks)

I really appreciate your reply, thanks for taking the time.

No problem.

I have used TenTec info in the past (desertfox's reference) but it is more mechanical joint-type applicable with high C/D ratios (nom. rod diameter to rod clamp 'free-length'), but for your application with a very low C/D of 32/850 = 0.04 I am not sure how applicable it is for your application. It may well be, I am just not experienced with its use for such values.

Here is a engineering paper from the AISC with a more 'structural connection' approach entitled "Applications of Pretensioned Anchor Rods in Industrial Facilities" Link. It may not assist you on this project, but a good paper to have on your 'digital bookshelf'.

RE: Tower crane anchor design - pre-load query

Hi again

If it's of any help we use tensioners oh foundation bolts using this method and had no problems with our machines which are often fastened onto concrete.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Tower crane anchor design - pre-load query

You may want to read Shapiro's Cranes and Derricks. There is an extensive section on tower cranes. The section on mast anchorage seems to parallel the information provided by your reference on Manitowoc crane care sheet.

RE: Tower crane anchor design - pre-load query

desertfox thanks for that site, it has some interesting downloads and rule of thumb formulae.

Ingeuity, the AISC document is really what I was looking for and has helped me to understand pre-load applications for tower cranes so thanks for that! I may come back with a question once I've re-read it but otherwise it is a very relevant document for me.

Chicopee, I have heard so much about this Shapiro's Cranes and Derricks, unfortunately I have been unable to get hold of a pdf copy - do you have a link? Otherwise I may have to bit the bullet and buy it!

RE: Tower crane anchor design - pre-load query

Temporaryworks - you are welcome.

Also, chicopee's book reference is a very good one. I picked up a used copy several years ago from Abebook's for a few bucks. Definitely worth getting yourself a copy. Try here: Link

desertfox - those TenTec automatic hydraulic rams/load cells are very, very nice. I assume they are also are expensive.

RE: Tower crane anchor design - pre-load query

chicopee thanks for the Shapiro book recommendation, it was well worth the money!

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