PURLIN CLEAT HOLE DIM AS/NZS

[Veer007]

Hey Guys, I have the case that I'm using 12mmØ bolt grade 4.6s/pb1230 for purlin connection cleats, what my concern is I have to use 6mm tolerance in steel connection plate, which is fine?

Also, the below hole values seems so high like 22mm dia hole for 12Ø bolt.. Please let me know your thoughts on this

Also the bolts should be galv or can be plain?

Thanks in advance!!

 

[Tomfh]

They are the standard hole sizes.

The 22mm holes are for m16 bolts.

Some engineers aren’t happy with +6mm oversized holes, and demand +2mm, however that’s fairly uncommon.

 

[human909]

Huh! I learn something new every day. I've always used and seen 14mm holes in cleats. Given the purlins have slotted holes you'd have to have things pretty out of wack for you to need 6mm clearance for erection tolerances! I figure it is an allowance to ensure that the purlin slots wont experience bearing failure and plastic deformation during excessive deflection. (Ensuring the two bolt connection actually functions as a true pin connection.)

A quick check of other jobs that were done "by others" and yep 6mm clearance is the norm. I guess next time I specify such things I'll follow the supplier guidelines and keep the 6mm clearance!

 

[Veer007]

Oh, then 6mm tolerence is fine for steel plates? I didn't encounter this before

Thanks in advance!!

 

[Veer007]

Can Connection bolts be plain? Without galvanizing?

Thanks in advance!!

 

[Agent666]

Well I've never seen or heard of 6mm+ sized holes being acceptable in purlins in NZ, it's always 2mm+.

AS/NZ steel standards only allow 6mm+ holes in baseplates I thought, even then its termed you may, meaning up to the descretion of the engineer. Also some of the assumptions around member restraint (providing twist or lateral restraint) specifically requires 2mm+ standard sized holes (at least in NZS3404).

Nor have I ever seen or heard of slotted holes being fsbricated in purlins.

 

[Agent666]

Can Connection bolts be plain? Without galvanizing?

No. All structural bolts should be galvanised.

 

[hokie66]

The Australian practice is to provide 18 high x 22 long punched holes for up to 250 deep purlins, and 22 round holes for the deeper sizes.

M12 and M16 bolts, respectively. The purlin manufacturers furnish zinc coated bolts to go with the zinc coated purlins.

https://cdn.dcs.lysaght.com/download/lysaght-zeds-cees-purlins-and-girts-user-guide

 

[Veer007]

Generally, as per AS/NZS, all structural bolts are galvanized? or only purlin bolts?

Thanks in advance!!

 

[hokie66]

No, in enclosed structures, the bolts for structural steel connections are often black bolts.

 

[r13]

In the US, edge distance will determine the member size that having oversized hole.

 

[Agent666]

Never heard/seen black bolts used, primarily because the nuts/bolts/washers require the same level of protection against corrosion and durability as the main structure. Can you be certain your bolts will last 50 years with no protection or whatever time period the Australian building code puts for durability of items that cannot be easilly accessed for replacement or inspection.

You'd just have to come along and paint them if they were not galvanised (many times they are painted anyway for structural steel to match steel as well as being galv, but never seen purlins get painted which are galvanised as standard in NZ), and even then you're only able to paint what you can see after erection, the grip length remains uncoated and hence unprotected. Similarly your Australian oversized holes seem like a good addition to provide a potential path for water into the unprotected grip length. Typically in a roof they might be subject to condensation for example.

Basically, I think you'd be mad not to .... but if its the norm, go for it. No idea on the cost difference, but the risk of some issue due to fastener durability just doesn't seem worth the risk.

 

[Agent666]

I'd point out as well that in AS/NZS1252.1:2016 it states something along the lines of unless specified otherwise bolts/nuts/washers shall be supplied as galvanised.

This is the default approach, you have to make a conscious decision to do something different and depart from the standards default position/intent.

 

[hokie66]

You are correct about most buildings in Australia. Australia is not really a steel country, so we use steel primarily in roof structures, and HDG 8.8 bolts are the norm. I was thinking more about countries, and cities, where steel is the primary construction material, rather than concrete. For example, New York City has been traditionally a steel town, while Chicago is a concrete town. Many buildings in NYC and other places all across the US have been constructed over the last century using unprotected steel and black bolts. If the steel had a coating, it was just "red lead" paint for some protection during construction, then black bolts were used. Or rivets, again unprotected, until the latter part of the century. As I noted, black bolts are acceptable only in enclosed or protected environments.

 

[human909]

I'd point out as well that in AS/NZS1252.1:2016 it states something along the lines of unless specified otherwise bolts/nuts/washers shall be supplied as galvanised.

Purlin bolts are typically zinc plated not galvanised. Typically 4.6, M12s.

Though as mentioned in the large sizes you use 8.8 M16 which typically are galvanised.

Regarding black bolts. I would have thought sourcing these in the required quantities would generally be MORE expensive due to lack of availability of appropriate bolts.

 

[hokie66]

If that is true, I would be surprised, and would be a market issue rather than based on performance. I have seen plenty of black/plain structural bolts. But with almost all bolts now imported into Australia, perhaps you are correct.

 

[Agent666]

M12's in Australia then. But in NZ M16-G8.8/S galv is standard for purlins unless I have not been paying attention. M12's only typically specified for use for timber.

Here are the requirements you are missing in AS4100 regarding the purlin connections to achieve 'F' or 'P' restraint. Curious at braces/flybraces/other points of restraint if you don't put the slotted holes or accept you have to tension the bolts to /TB condition to prevent slip (and obviously check load is less than slip limit)? I would imagine it would be hard to achieve the required number of threads in the grip length from AS/NZS5131 I would have thought without packers or additional hardened washers as discussed in 5131.... All adds up to extra stuff to get forgotten on site. Different folks, different strokes, I guess.

 

[Tomfh]

We generally use M12 4.6/S with 6mm oversize holes on purlins, and +2mm on bracing elements.

S vs TB is a bit irrelevant since they all get tightened to 3 ugga duggas regardless.

 

[human909]

Yep. As you point out we don't have that in our code, nor in practice. But nor would I even consider the connection between a purlin and a rafter to be a moment connection! So the point seems moot. Though it does make sense to avoid any oversize holes for fly braces.

Slotted holes, "elongated holes" are the standard punched hole size and are nominated in the literature. I'm not even sure if you can get regular circular holes!

 

[Tomfh]

Yep. As you point out we don't have that in our code, nor in practice.

I've heard Australian Engineers make Agent666's argument that our industry standard purlin connections are fundamentally non compliant with the code. E.g. AS4100 provisions for hole size which limit to +2mm oversize, and the bracing rules which prohibit anything which allows slip to be a bracing restraint.