Corrosion protection of bolts and structures

[xian555]

Hello All,

In reading up on the subject over the last few months, including this posting

http://www.eng-tips.com/viewthread.cfm?qid=48335#post

on this forum, it has generated a number of questions.

The application is a lattice guy wire tower of 200ft, 3 faces, faces are 8ft centres, legs are tubular 6"OD HSS, lattice work are standard angle iron. Load is a small wind turbine of 65 ft OD, aprox. Applied standard is CSA S37-01 (similar to TIA-222), adapted to our use. Tower sections are 20ft each. The turbine load dead weight is about 10 tonnes, and we've design for a wind pressure of 600Pa (12.5 lbs/sq.ft, or a wind speed of about 70mph, with a gust factor of 1.6, or peak winds of about 111mph) and 1" of ice. Having read a study on remedial action on lot of communications tower that were over 50 years old, with innumerable other structures much older, we are attempting to design for a life cycle of 50 years.

1) one approach is to assemble the 20 foot tower sections in the shop, with plain finished bolts and send these sections off for hot dipped galvanization. After reading the above post on Hydrogen Embritlement for the pickling process prior to the hot dip, it's clear that this is not advisable for the bolts. So question 1: but what of the tower legs and lattice work? Will they not be affected by HE like the bolts that are hot-dipped galvanized ?

2) From the above mentioned posting, the best galvanization process I understand to be mechanical, for structural bolts. So in the case of A325 bolts, if a mechanical galvanization is done, is it worth also going for Type 3 versions, made of atmospheric resistant steel alloy?

3) Our former structural engineer suggested we use SAE Grade 5 bolts in place of A325, as the head size for lattice tower applications is not that important, since essentially all loads are in shear, and bolt pretesioning is not a significant factor, also per my understanding in "Specification for Structural Joints using ASTM A325 or A490 Bolts - 2009 RCSC". It is also standard practice at Hydro-Quebec, currently fifth largest utility in the world, to use Grade 5 bolts in place of A325 for their power transmission towers. However just because a large company does something it's not always for the same reasons.
I've seen on this forum some postings where some senior structural engineers are strongly against such as substitution. Given this information would some people care to provide insight, comments and observation on this possibility.

4) In the case of Grade 5 bolts, it's my understanding that there is not Type 1 (plain steel) and type 3 (atmospheric resistant) versions as for A325. Rather you choose a Grade 8, which has superior corrosion resistance and greater load capacity. If one were to use Grade 8 bolts, would it be wise to still mechanically galvanize them.

5) What of cost and stocking of A325 Type 3, and SAE Grade 8. Are we talking much more expensive than Grade 5 or A325 Type 1 bolts, and special orders (or are they usually stocked) ?



Best Regards,


Christian Martel, P. Eng. PMP

 

[hokie66]

My preference would be for the 20 foot sections to be shop fabricated as all welded sections, then hot dip them. You would need to make sure of adequate drainage for the zinc in the HSS legs. I have never known of hot dipping already bolted assemblies. Then the connection of the sections would be done by bolting, using hot dipped galvanized bolts. Mechanical galvanizing does not provide the same degree of corrosion protection. I would use A325 bolts, which are readily available HDG. The connection bolts for the sections could be in shear, but could also be in tension, depending on the type connection used.

 

[xian555]

Hi Hokie66,

Yes fully welded has been discussed but we're leery of fatigue cracks around the welds (wind turbine applications have more cyclic loading than comm tower applications). We would need to do some thermal stress relieving, or we're told, heat the zone to be welded prior to welding, but we do not know to what temperature, nor what cooling profile or if quenching would be required. Any thoughts?

Another possibility would be to preassemble the sections with mechanically galvanized bolts, and hot-dip the assembled sections. This way the bolts would be protected from the HE no? However gusset plates to connect the angle iron lattice work to the tubular legs is still required in this approach, and so the above question about stress relieving the weld connections for the gusset plates remains.

Best Regards,


Christian Martel, P. Eng. PMP

 

[metengr]

Most large wind turbine towers are welded construction, using rolled and seam welded ASTM A 572 plate with bolted flanges. The tower material may not require PWHT, it is a high strength, low alloy steel that is either welded using SAW or FCAW processes with preheat.

 

[xian555]

Hi Metengr,

As indicated in my initial description of the project, about the dead load at the top of the tower, we're talking small wind turbines, in this case of 65kW. Large wind turbines are now on 80m and 100m towers and are usually in the 2MW power range. The turbine in this project is 65kW on a 60m (200 ft) tower. In this size towers are not tubular. Tubular towers are the worse choice from a financial point of view as their wind drag is much higher and require much more steel and concrete for a given power level. Aesthetics is the driving force for a tubular choice. In the case of small wind, customers are paying out of pocket, and financial efficiency is the driving factor. Lattice guy wire towers with 3 faces are the most efficient deisign from a materials point of view.

In our case we're using HSS round tubes for legs and 50ksi carbon steel angle irons for lattice work. With this in mind, any other thoughts ?



Best Regards,


Christian Martel, P. Eng. PMP

 

[metengr]

A welded lattice would suffice with fatigue taken into account by design. Most likely, it can be done with preheat only based on the size you mentioned, and anticipated materials of construction. You can qualify welding procedures and welders using a structural welding code.

 

[CoryPad]

1) Hydrogen embrittlement only affects high strength steel. SAE/USCAR-5 Avoidance of Hydrogen Embrittlement of Steel lists a hardness of 353 HV and above where precautions are necessary. The tubes, sections, etc. commonly used for structures are well below this strength/hardness level.

2) The best coating process for high strength steels are the zinc-rich styles specified in ASTM F1136 and F2833. These are the only ones allowed for use on ASTM A490 high strength bolts. Type 3 atmospheric resistant alloys are meant to be used uncoated, you would never use them with any of the zinc coatings.

3) There are multiple reasons to avoid using SAE J429, please stick with ASTM A325, A490 and RCSC Specification for Structural Joints Using High-Strength Bolts.

4) Grade 8 in SAE J429 is only higher strength, not more corrosion resistant. It is highly susceptible to hydrogen embrittlement.

 

[racookpe1978]

OK. So if it is a solid steel (rolled tubular) tower, why not put the flanges inside out of the worst of the exposed weather. Disadvantages of course are: harder access for welding the inside flange, harder bolting and torquing access (but easier if platforms made inside at the joints) and a smaller bolt-circle diameter = more stress per bolt under the same external wind loads.