Design of Sloped Steel Joists

[DHKpeWI]

I have a few questions about the design of steel joists on a 3/ 12 sloped roof.

1) Do you use the horizontal projection for the span and loads?

2) Isn't there a component of force parallel to the top chord? Does the roof diaphragm resist this load?

Thanks

 

[frv]

1. For Lr, Yes; For DL, No.

2a. Yes. 2.Yes

 

[larsacious]

For 1) you could break the dead load into components. Note that this will have a negligible effect (i.e. 12/12.37 = .97 of the dead load is normal to the joist span).

 

[JAE]

Any vertical gravity load will be a vertically applied load.

The joist will bend and shear based upon the normal force (the component force) and the longitudinal component will be taken up by the diaphragm as frv states.

The live load is an amount required by the code and is applied to the horizontal projection of the joist. The bending moments and shears in the joist will be based upon the horizontal span, not the diagonal span.

To "see" this in action - realize that as the joist slope gets steeper, a vertical load will create smaller shears and moments. As you approach a vertical joist condition, the joist will approach 0 shear and moment.

 

[DHKpeWI]

Thanks guys!

 

[BAretired]

1) Do you use the horizontal projection for the span and loads?

I use the horizontal projection for span when considering all gravity loads. Dead load must be increased by a slope factor.

2) Isn't there a component of force parallel to the top chord? Does the roof diaphragm resist this load?

A gravity load may be resolved into normal and parallel components but the roof diaphragm does not resist the parallel component.

BA

 

[JAE]

BA I think the diaphragm question does depend a lot on the actual framing configuration (monoslope vs. gable, etc.).

But if there is a parallel force, and the diaphragm deflects with the structure in that direciton (which it will), then there is load applied to the diaphragm (Hooke's Law) and thus the diaphragm takes at least SOME of the parallel load.

 

[BAretired]

If a sloping joist or beam carries only gravity load and bears on a horizontal surface with a horizontal shoe at each end, the member is stable and requires no assistance from any other part of the structure.

The deck will become slightly stressed by virtue of its attachment to the top chord which is in compression but the original premise suggested by the OP that the deck resists the parallel component of the load is wrong. It does not. That component is taken by the joist itself.

For loads other than gravity loads, the deck will participate.

BA

 

[JAE]

BA - I think you are correct - gravity is down only.

 

[BAretired]

JAE - I agree.

BA

 

[WillisV]

A sloped joist does resist axial load, but it is resolved internally to the joist and does not rely on the deck as stated by others. However the joist mnfr needs the parallel component to properly check the joist for this internal axial load, and SJI requests that it be put on the drawings and labeled as "parallel load component - XXX plf"

 

[AVak]

WillisV,

Can you point me to the section of the SJI code that requires the parallel to joist load to be shown on the drawings? I was unable to find it in my copy of the 2005 SJI code. Thank you.

Adam Vakiener, P.E.

 

[WillisV]

Not a requirement - a request - see pages 101-103 of the attached for Vulcraft's take on sloped joist which discusses the inclusion of this component (and would also help the O.P.)

http://www.vulcraft.com/downlds/catalogs/DesigningWithVulcraft.pdf

 

[ash060]

If the joist is sloped on both the top and bottom chord (parallel chord joists) the span used for design shall be the length along the slope as per SJI.

Also, remember that if you have a 3/12 slope that the joist seat may have a larger depth than the typical 2-1/2" there is a chart in the Vulcraft guide for this, but you may want to contact a manufacturer to get the real depth.

 

[AVak]

WillisV,

Thank you for the information. I have a copy of the book that you posted the link to.


Adam Vakiener, P.E.

 

[JAE]

ash060 - that section of the SJI specs (5.13) does state that the span used is the sloped span. However, it also states that you should use the component of the load perpendicular to the joist.

For shears and moment calculations, that simply equates to using the vertical load and the horizontally projected span - same results.

The sloped span usage is for the tables which are for plf perpendicular to the span.

We above (at least I was) were getting into a discussion about the "pure" loading on a sloped member and forgot that the OP was asking about joists in particular.

 

[ash060]

Right - I never said not to use the load perpendicular on the joist, I was implying to use the right span when checking