effective length of Cantilever 1

[nicam]

Hi I'm currently designing a steel awning structure and I'm trying to determine the effective bending length, AS4100 has 3 k factors kt, kl & kr for segment lengths. I know that with cantilever beams kt = 1.1, kl = 2 & kr = 1 for unrestrained members.

my awning is attached.

the outrigger members(cantilever) are laterally restrainted at both flanges (top & btm) at 1.2m c/c the total length of the outriggers are 4.9m & 3.7m, the end of the cantilever is fixed to the sourrounding truss. The supporting column is 5.3m high and has no intermediate restraints. so my effective bending length is 11.66m

should the ouriggers Le be 10.78 & 8.14m?

 

[BAretired]

The structure you show in your diagram is unstable, so your question cannot be answered until you find a way to make it stable.

BA

 

[msquared48]

Unless the columns are totally fixed at the bases, I have to agree with BA here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[nicam]

The structure is stable the columns are fully fixed, that's how space gass shows full fixity on the 3D render output.

attached is the normal screen view showing bending moments.

 

[hokie66]

With restraint at 1200 centres, why would you use the full cantilever length?

 

[nicam]

only because I wasn't sure if you need to consider the whole cantilever length or just the segment lengths.

When designing simple beams I would use the segment length between the restraints but thought the rules are different for cantilevers.

 

[rscassar]

The T frame is stable, I designed something very similar the other day.

When the load is a gravity load and it is acting through the shear centre to cause a destabilizing action then yes the effective lengths will be as you mentioned.

The size of the column with be stiffness-governed for an unbalanced live load acting on one T-frame only. As for the rafters, they would either be stiffness or strength controlled.

 

[FinnB]

I haven't got a steel code in my hand but I would do the following

1. Look at the end restraints for the outriggers. If you make the connection between the outrigger and column as providing lateral and torsional restraints and restarint against rotation on plan and then provide lateral restaint only at the tip of the outrigger you can take an effective length (Le) for the cantilever of between 0.7 and 1.0 times the span as the loads you have do not appear to be destabilizing..

2. For the column I would take an effective length of 2 times the height of the column.

3. The deflection check is very important and I would use the computer model to find the deflection. I would probably set the deflection limit around height/200 depending on how sensitive your roof cladding is to deflections. If it metal cladding you could go to 1/100 for deflection.

 

[ToadJones]

I'd use K=2.1 for the columns in the strong axis direction. In the weak direction I'd use 1.2.

 

[nicam]

Hi just an update. Attached photo of my awning! See BAretired the structure is stable!

 

[BAretired]

Yes, indeed! Nice piece of work.

BA

 

[nicam]

Thanks BAretired!

 

[StructuralEd]

Looks like so many gas station canopies that I worried about this past winter.
Don't forget to investigate unbalanced loading.