Bolted Connections

[haggis]

Hi all,

In a plastics manufacturing extrusion tower,built of HSS,would a bolted connection designed in such a way that a 3/4" plate is welded to the section and then drilled and tapped through the plate and the HSS to accept an A-325 bolt be as acceptable as a bolted connection using a bolt and a nut.

Thanks for any help in advance.

 

[ishvaaag]

It is always better to follow standard conection details. The problem with those that are not is that one may leaving some significant check behind, and hence diminished functionality or safety -or even true risk- may appear.

For what I read the plate seems to be added to reinforce the thinner HSS shape, ensuring solidary behaviour through the welds, and then bolts holes are drilled and then bolts put to work there. This of course may work to transfer forces to the extent the detail permit, and even less than the total width of the HSS section plates are used welded to reinforce the section at a connection. I have even some mathcad sheet for such things but do not involve bolts.

Using such complementary plates moment, shear, and axial force can be passed to the HSS member. The size of the plate and the welds will limit to what extent, and need be checked at least for the forces being passed to the member.

 

[unclesyd]

Will the A325 bolt see any temperature?

 

[haggis]

Unclesyd,

No, the bolts won't be subject to any temperatures.

 

[stel8]

How long is the tapped hole (3/4 + ?)and what size is the bolt?

 

[unclesyd]

Just checking as we have a lot of supports bolted to tool steel components that see the operating temperature of 600°F. If we think or know a bolt might see this temperature we will use a bolt rated at the maximum temperature seen by the part.

 

[wiff]

Getting away from a standard structural steel connection is risky. The A325 bolt capacities listed in the codes presume matching nuts of A563 and washers ASTM F438 unless specially designed fasteners are provided. There are considerations for faying surfaces, too. Bolting to a steel plate and using it as a nut is way outside the parameters on which load capacities of bolts are based. It should be no surprise if the threads tear out of the steel, or otherwise fail, when the connection bolts are tested with a Skidmore test apparatus as is required for A325 bolts.

 

[unclesyd]

wiff,
We have hundreds of connections where we have A325 bolts (black) in connections with machine components. We have had no problems with any of these connections over the years. An A325 cap screw is about the same strength as a grade 5 cap screw and if you treat it the same you will have no problems of thread stripping. We only change to a B7 or better if the part supported is heated. If the supported part is heated above 400°F we will use an intermediate material in the connection.
You would be surprised on how many times a grade 5 is substituted for A325 in the packaged building construction.

 

[haggis]

First of all, let me thank all concerned for the responses. I can see you all have valid points.

Unclesyd, you see that there would be no problems with thread stripping and I tend to agree as the bolts at these connections would not be in tension apart from being tightened. I also see the point wiff is making as a mild steel plate threaded, would strip before a hardened A563 nut would. Maybe Stel8 was thinking along these lines when he asks how thick the HSS is. The HSS would have a 3/8" wall thickness givng a total thread length of 1 1/8". Question now is, would this thread length in mild steel be as effective as say 5/8" thread length in the nut in terms of risk against stripping. The bolt size would be 5/8" dia.

A question I am surprised nobody has asked is why build this tower from HSS members and have the bother of this type of connection. Believe it or not, the client thinks it looks nicer and isn't worried about the higher fabrication costs. Go figure!

Thanks again to all,

Haggis

 

[unclesyd]

haggis,
Threading both parts can be troublesome. Watch the length of the A325 fastener as you might have to use a Grade 5. What hardness is the H.S.S.?


It is not unusual to see H.S.S. construction of components used in the polymer/plastics industry.
We used A2, D2, T15, H11, extensively for components used in polymer service. Where we don't use these materials we use 4340, 4140, and the PH alloys.
We have a particular process where multiple tool steel components are bolted to a large A283 block with H11 bolting. The only thread stripping noted was when some idiot puts a short screw and gets the top threads.
We also have tool steel dies threaded and bolted into 4340 flanges.

I may stand corrected but I think Dupont was one the first to use tool steels as pressure containing parts in a production process.

 

[haggis]

Unclesyd,

From your last reply, I am under the imperession that you think I'm dealing with machinery components. This is purely a four story tower on which to place machinery.

The HSS material would be G.40/21 grade 350W

Sorry if I've misled you.

Haggis.

 

[unclesyd]

No problem.
As I mentioned we have High Speed Steel supported by High Strength Steel. Like I stated watch the stacked threads.

 

[haggis]

Unclesyd

Do you see a problem with the HSS and plate being welded first and drilled thru' then tapped in one shot. I could see a problem if the plate and HSS were drilled and tapped separately.

Haggis.

 

[unclesyd]

There could and would be a problem if the plates are not in intimate contact and all holes drilled and tapped at the same time. They will have to be clamped while installing the fasteners.
I would stay away from this condition if possible.

I might think of going to smaller fasteners only more of them. A325 is limited to 1/2".

 

[haggis]

Unclesyd,

Thanks for your insight into the problems that I could encounter with this connection. I will have to work on convincing the client that it would be better to use other AISC sections and simplify things all round.

To you and the other people who had the kindness to respond, a very merry Xmas and a prosperous new year.

Haggis.