Stiffener Spacing for a rectangular conduit 1

[flamby]

One steel conduit for carrying water under pressure needs to be designed. It is a box section of say 4m X 6m. The box will be made by say 20mm thick steel plates and the box will be stiffened by regularly placed stiffners on outer surface (circumferencial). Stiffeners will be flats of about 16mm thick plates. My confusion is about the spacing of the stiffeners. What is the width of 20 thk plate which I can take as coacting width to make a T section with the 16 thk stiffener?
Please advise.

 

[austim]

Flame,

I would have expected your local steel structures code to tell you that.

In Australia there are two codes.

Our Allowable Stress code (AS 3990) allows you to consider up to 560/sqr(Fy) * thickness to be the effective width of a plate supported on both edges by stiffeners. (Fy in MPa), so you get about 35 * thickness for basic mild steel with Fy=250 MPa, less with higher quality steel.

Our Limit State code (AS 4100) is a little more stringent, and specifies thickness*30*sqr(250/Fy).

If your local codes do not have any relevant provision, you should be OK with stiffener spacing up to 600mm (and your nominated 20mm plate), and taking the total section as fully effective.

 

[flamby]

Austims, Thanks for your comments.

Our code (Indian IS:800) in same and specifies 256T/sqrt(Fy) to a maximum of 16T as width to be considered on either side of the web.

The client's remark suggest us to take 0.11*(span of the tee section, 4000mm here) to be used as co-acting width based on another code that deals with hydraulic gate (stiffened plate for holding water behind it). While this 0.11*span figure is indeed there in that code, I have no clue to its basis or its applicability on other similar structures. It feels like I will have to be content with the value as suggested by you and fight it out.
Regards,
flame

 

[austim]

flame.

That criterion of 0.11*span sounds rather like a provision to account for 'shear lag'. There could be a justification for listening to your client.

I would be quite surprised if it makes any practical difference to your design whether you take an effective width of plate of 440 or 600. Either way, the maximum stress will be in the stiffener outstand, and it will probably not be affected greatly by reducing the effective width to that extent.