Very High Strength Bolts 1

[wahmit]

We want to strengthen the leg connections of tubular towers with flanged joints by replacing the bolts, the rest of the connection can carry the additional load. The use of grades 12.9 or 14.9 is discouraged due to the relatively low ductility and the increased risk of hydrogen embrittlement.

Can anybody suggest a suitable materials for the bolts, typically M16 (5/8&quot bolts with required working loads of 85kN (19kips) and 100kN (22kips) in a galvanised steel structure exposed to the weather.

Thanks

 

[CoryPad]

If you need strength equal to or greater than 12.9, and you want resistance to environment assisted cracking, then some alloys to consider include 431 stainless steel, alloy A-286, and MP-35N. The cost for these alloys is much higher than that for low alloy steels used for 12.9 fasteners.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Metalguy]

Can't you simply use slightly larger bolts?

 

[wahmit]

Thanks CoryPad we are checking these materials.

No can do Metalguy, the structures are existing and there is insufficient edge distance to increase the hole size. It is always worth suggesting the obvious; it's amazing how often the obvious is overlooked.

 

[ccw]

Hi Wahmit,
Here is another obvious: go to fine(r) threads?? For same tension and or shear load, root stress is smaller.

Use stainless bolt material to maintain ductility and reduce embrittlement risk. Be sure to use nickel-eze or other thread galling preventative with the stainless. Tighten all bolts to 80% proof minimum.

Here is another obvious. Drill smaller bolt holes on the same bolt circle between the larger bolt holes.

(Ed)I expect when the owner gets the bill he will not think the $value of the rating boost is worth it, unless you use Tiawann commercial grade S/S bolts which may be prohibited by your tower design/quality criteria.

Curious: Are these free standing towers?? I assume so.
In that case, the bolts are always in tension but the adjacent flange to tube weld joint may be in 100% reversal in bad weather loads. Have you taken this into consideration? What element in the flange joint takes the horizontal shear?

 

[wahmit]

Thanks ccw,
The use of fine threaded bolts is a last resort, we are talking expensive bolts being installed by guys hanging on short ropes off the side of the tower high above the ground, the risk of cross threading is greatly increased.
Unfortunately there is in insufficient room to install additional bolts between the existing, but we are looking at clamping the flanges, but that has another set of problems, most critically matching the stiffness of the bolts and clamps.
For the curious,
These are free standing towers designed in the late ‘60s, the flanges and welds were designed to match the capacity of the leg and welds have been checked for fatigue. Grade 5.7 bolts were originally used, which were adequate for the original load and much cheaper than higher grade bolts at that time. Since then further loads have been added, upgrading the bolts as required using grade 12.9 and 14.9 to match the leg capacity. To reduce the risk of failure we want to replace these bolts; we will also want to strengthen towers in the future in order to carry additional load
We own and maintain the structures so we are paying for the bolts, however the cost and consequences of a tower failure justify some reasonably expensive bolts
In these towers the shear is carried by the bracing, which bridges the leg connections. For interest, where the flange carries the shear load the friction between the flanges of the compression leg is usually sufficient to carry the shear.

 

[unclesyd]

Here are some good sources for high strength fasteners:

http://www.spstech.com/aero/products/bolts/hex.html

http://www.arp-bolts.com/pages/tech/matspec.html

http://www.nova-nsa.com/html/novalloy.html

 

[IFRs]

Exploring further into wild and crazy space:

Idea #1: Run guy wires from top to bottom, with a spreader bar bolted to the flanges to move the wires outward to a larger circle at the flanges? The connections at the top and bottom of the towers can be bolt-on half flanges. The cables will add significant strength and stiffness to the tower.

Idea #2: use a lockbolt like "Huck" - they do not use threads, therefore do not have
reduced diameter issues.

Idea #3: weld an extension on to the flange so you get more leverage for the bolts.

Idea #4: add guy wires that extend from the flange to the ground, and/or guy wires that extend fromthe tower to to the ground.

Idea #5: bolt a few vertical I-beams to the flange and bolt the top and bottom of the I-beams to the tower with bolt-on clamps.

 

[diamondjim]

I assume if you are using higher strength
bolts you are using hardened heavy washers.
If you have room you might want to spec out
thicker washers to increase the clamp length.
How close are you to your specification?

 

[ccw]

Hi wahmit,
Thanks for the additional info. To reduce the risk of cross-threading (whilst hanging from a rope!), here is a trick from the hot cell manipulator operators.

Procure the bolt, extra long. Machine (grind) off the first few threads back to the root diameter of the bolt, for a length approximately equivalent to the height of the nut. This will provide a starting and alignment post to square up the nut to the bolt.

Machine (grind) the imperfect part of the start thread back until a full thread cross section is obtained, and smooth/blunt the resulting start thread face for both the nut and bolt.

Of course, inspect all threads for imperfections caused by mishandling, etc. One might consider a fresh set of tap and die for go gages. And, train/qualify the installers to do the task safely. Certainly, one does not want a $400 bolt and nut partially installed, and cross-threaded or galled, through a tower leg flange!

Good luck with the project.

--CCW

 

[CoryPad]

A better solution to cross-threading issues is the MAThread design. This design uses rounded starting threads on the fastener that prevent cross-threading. It is produced during standard fastener manufacturing, so you don't need to do any of the extra operations (grinding, etc.) You can learn more at:

http://www.mathread.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.