Specification for screws in steel? 1

[ikerdanolcam]

Hi there. I usually only use screws for wood, wood/metal and steel/metal applications for situations I know how to solve vis-a-vis NDS and ICBO reports. I have a situation now for a steel-to-steel connection and I don't even know where to find resources.
My particular situation is a client who wants to fabricate custom tee-sections from plates by screwing through the plate forming the horizontal leg of the T into the butt end of the plate forming the vertical leg of the T. My concerns are many but AISC doesn't provide design guidelines for screws (at least not that I've found by searching their site) and I don't know who does.
Can anyone offer any guidance?

 

[desertfox]

Hi

If you want strength grades of steel fasteners try this link:-

http://www.roymech.co.uk/Useful_Tables/Screws/Strength.htm

 

[ikerdanolcam]

Thanks desertfox.
The strength of the screws themselves isn't the primary concern.
How do you calculate the withdrawl value of a 3/8" screw into the edge of a 1/2" plate? I'm all but certain this isn't permissible, but then what is the edge distance requirement? My understanding is that screws aren't considered engaged until 4 threads are through the material, thus is it even possible to connect into but not through steel?
Then there's the fact they want to use 3/8" diam flathead screws through 3/8" plate (then into the edge of the 1/2" plate.) This leaves less than 1/8" thickness of plate that would be the "shank" diameter.
As is usually the case with bolts, the strength of the connection would be controlled by the steel plate members, not the screws. I'm looking for the code/reference for screwed steel connections.

 

[hydtools]

I'm not a structural guy, but based on my machine designing and building experience flathead screws in a structural joint do not sound like a good idea. Only one screw in the pattern will have a fully engaged head. All the rest will have eccentricly loaded heads. Any shear load will translate into a bending load of the screw head before loading the screw shank. You may even have screws fail in the assembly process.
I'd turn this request down, if I would ultimately be responsible for the integrity of the joints.

Ted

 

[dinjin]

Are you responsible for the design or for the machining
and assembling?

 

[ikerdanolcam]

hydtools: It would be a single row of fasteners, so I don't think eccentricity is an issue, but I don't see how fastening into the end of a 1/2" plate with a 3/8" fastener can work.
dinjin: I'm responsible for the design.

Is the AISI S100 the standard for screws? I've never worked much with metal studs, so I don't have this code. If it doesn't apply to structural steel and/or connections where the screw doesn't penetrate through one of the connected members aren't addressed, it's not useful for me. All connection data I've seen (eg ICBO reports) are "through connections." Screws that "dead end" into concrete and wood are used all the time, so I have to imagine the same can be done with steel.

 

[connectegr]

If this is an AISC structural application for the WT, there is not a "structural" screw for this assembly. I assume the intent is to drill and tap the top face of the WT web and screw through the flange into the web. AISC provides design information for welding (AWS D1.1) and structural bolting (RCSC). Other connection choices would be non-structural.

If the intent is not structural, there as several mechanical screws that can be used to connect the section for a cosmetic "T". Drilling and tapping the web with counter sunk screws through the flange. But, again AISC would not recognize this as a structural connection, intended to transfer shear and tension to the elements.

http://www.FerrellEngineering.com

 

[ikerdanolcam]

Thanks, connectegr,
That's what I suspected, but to satisfy my curiosity, do you know why the AISI specification couldn't be used? Many (most?) cold formed shapes are made out of A36 and it seems like it should be applicable. Also, I was thinking, if this particular connection (your understanding of it is correct) were fabricated such that the web of the T is 9/16" and tapped well beyond 1/4", thus matching the dimensions of a nut, why could it not be considered equivalent to a bolted connection since A307 specs match A36 specs?

 

[dhengr]

This is probably a problem which is much more amenable to testing and good sound engineering experience and judgement, than to a specific find of a code paragraph covering it. I doubt that any code would condone this. Although, there will be plenty of evidence, text and research on why this is a difficult problem and thus generally not allowed by the codes. However, most codes do allow you to do things, or use methods and materials, which are not specifically covered by the code, with sufficient engineering proof of the scheme.

3/8" screws into the edge of a ½" web pl. doesn’t allow for much edge distance, and your predrilling, countersinking and tapping operations had better be almost perfect. Even if the drill and tap is perfectly centered on the web, you might get some deformation (bulging) on the side faces of the web. Twice as many 3/16" or 1/4" bolts might be better. I would make sure that the screw or probably machine bolt was the weaker element, the failure mechanism, because of the almost nonexistent edge distance on two sides. Then design the screw spacing, length, etc. accordingly, with an appropriate FoS. You should be able to develop pull out and shear flow type strengths assuming that the loads aren’t too outlandish. You may actually want to, have to, machine the edge of the web pl. for good fit-up and bearing on the flg. pl. before drilling, etc.

What are the loads and how are they applied? Pull-out will be a function of machine bolt length and some torque limit to assure you have good bolt head bearing on the flange, to resist the applied loads. And, prying on the bolts could be an issue. The shear or shear flow problem involves a machining and assembly method which assures, with some confidence, that you pretty much bring all the bolts into play at the same time. The tap for the thread in the web pl., threads not in the faying plane, a counter bore which exactly fits the bolt shoulder and is concentric with the tapped hole, and finally some bolt torque for clamping. This leads to a multi-axial state of stress in the bolts and in the steel immediately around them. The tolerances in typical bolted joints generally causes a few on the bolts to come into play, in bearing on steel pl. holes, and actually causing yielding in bearing, before more of the bolts are brought into play. Thus, 10 bolts in a joint doesn’t mean each bolt is loaded to (the load on the joint/10). Some of these problems tend to go away when using self tapping screw on light gage materials.

The work and expense of what you are trying to do will be a real budget buster if you are going to do it right and with some confidence, and will probably have to be done in a machine shop setting. It is probably not advisable in a field connection situation, where tolerances and control will be lost. I’d work with the client to devise another connection solution, unless his reasons truly justified the costs. You don’t give us a hint as to why your client is making things so difficult, and I wouldn’t stick my neck out for him without some real justification. Whatever your 3/8" bolts at 2" o/c will do, two 1/8 or 3/16" fillet welds would do with far less cost and pain and suffering.

 

[ikerdanolcam]

It would be shop fabricated. The client does a lot of machined non-structural work, but in this case, they need to carry structural loads. I'm not planning to stick my neck out, but before I tell them no, I felt like I needed to be sure I couldn't justify this connection via readily available methods. Testing is definitely out, but I wondered if thickening the web plate couldn't justify their preferred connection.
Everyone's input is much appreciated. I'm still unsure why AISI's screw standards (which I don't have, but seem potentially relevant) couldn't be used for structural steel.

 

[hydtools]

ikerdonalcam,
the point I was trying to make is that center-to-center distances will not exactly match, tapped hole to countersunk hole, and all screws except the first one installed will not exactly match their resprective countersunk holes therefore eccentrically loading each head.

For edge distance, take a cue from the hex nut size for the screw.

Ted

 

[dinjin]

I would not use the distance between flats of the nut
as they certainly are much harder than A36 Steel.
I think if I were to tap these, I would put them in
a vise to ensure that bulging does not take place
in the tapping operation. In the bearing industry,
we often used 1.5 times the bolt diameter to determine
the od of the rings. This equates to .75 x bolt diameter
from bolt or screw centerline to the edge of the rings.
We used slightly less for years until stronger bolts
ie grade 8 came into specs and more common in applications.
Not certain if it applies to your design or not.
Often rather than thru holes, our rings were tapped
approximately 1.5 x diameter deep allowing a thru bolt thru the mating part and into the tapped rings. I think the general rule was that the bolt must penetrate into the tapped part 1.2 x diameter to ensure that they not pull out.
Using the 1.5 ratio for a .50 inch plate would indicate a 5/16
dia. max bolt or 8mm dia. bolt. I am amazed that testing is not allowed.

 

[ikerdanolcam]

Much appreciated insight, thanks. Often us structural guys don't get much into the machining and fabrication process.
It's not that testing isn't allowed per se, it's just that if testing is required to justify the assembly, it makes more sense instead to go welded, as much as the fabricator would like to avoid that.
Just looking through my email correspondence, I realize the fabricator wants to spec stainless for screws and plates. Not sure how or if that changes the equation for fabrication of this would-be assembly.

 

[dinjin]

If stainless steel is used, the ratio might be ok for the width vs bolt diameter. Is the 1/2 inch the stock size or are you machining it after tapping the holes?