Railings: IPE Top rail Connection to Post

[AK4S]

I am working on the connection detail of a Guardrail system and not able to figure out how a commonly seen system works!

Railing system: Aluminium post with IPE top rail (no other sub rail).
This appears to be a common system as I could find several fabricators online (See attachment for photos)

Looking at the Connection between the IPE rail and the post:
The connecting screws would see: (1)Shear force at the interface (lateral load at the top rail)
(2) Tension/uplift when the Top rail sees a vertical downward load in the span and the ends want to rotate about the support at the cap plate.

(Note: sketch above shows a T-shaped post. I am using a tube)

Question:
[ol 1]
[li]Will the connection screws actually see a tensile force?[/li]
[/ol]
There are two scenarios: (A) the IPE top rail is continuous over the support (B) When two adjacent rail sections would terminate at a post as shown in the plan sketch above (either 2 or 4 screws on each side).
Logically I feel if the IPE rail rotates about the post cap plate, then the screws should see a tensile force, since they are resisting this rotation. But if I use this logic, then the tension on inner set of screws = moment/(distance from edge of cap plate to c/l of inner set of screws). This will be huge.

2. Can I detail it in some fashion which will make it behave purely as a pin at the support and the screws are resisting only shear forces from the lateral load. The photos from the attachment show only small screws, but I could not find any reference for their design.

 

[JAE]

I think the screws closest to the post will see some uplift tension if someone leans downward on the IPE board within its span somewhere.

The magnitude of that prying uplift would depend on the relative stiffness of the IPE and the cap plate.

Can you put the required screws all in one line (vs. a 2x2 pattern as you've shown?)

 

[r13]

What is the force you are designing for? The most critical cases will be:

1) Horizontal push against the edge.
2) Vertical downward push/lift at the inner edge (causing rotation).
3) Inclined force applied at the inner edge.

 

[AK4S]

@JAE: I will have to check the screws to limit them for 2 on each side as shown below:

The cap plate will primarily transfer horizontal shear to the Post. I can make it thin (less stiff, so will attract less moment from the IPE rail at the continuous rail section?).

What happens at the location where two adjacent rail sections terminate at a post. With one screw line, it behaves more like a pin, while still allowing for rotation of the top rail, without any significant tension on the screws. Am I understanding this correctly?

 

[AK4S]

@retired13: Yes, primarily 1 & 2 per your list.

 

[r13]

Maybe you can use some idea from detail below.

 

[JAE]

Technically per ASCE 7 the guardrail force is in ANY direction (50 plf or 200 lb. concentrated load).

[blue]What happens at the location where two adjacent rail sections terminate at a post.
With one screw line, it behaves more like a pin, while still allowing for rotation of the top rail, without any significant tension on the screws. Am I understanding this correctly?[/blue]
Even with one screw line there is a small (very small) moment arm between the screw line and the edge of your steel plate.
I'm not sure the prying action would be all that great if the screws were set off the plate edge by only 1/2" or so.
Also - you aren't necessarily limited to 2 screws in a line. Depending on your rail width it could be more.

 

[AK4S]

@retired13: Thanks for the ideas.
@JAE:Yes, you are correct. I could add another screw to the line if I am approaching the max. capacity of the screw in shear. will give me some buffer for any tension it may see.