CISC handbook omiga 2 in figure 2-15

[burtonSTR]

case 1 for L1, it shall be 1.75 per CAN S16-01 for k=0. Why omiga 2 is 1.0?

Anyon could give me an answer?
Thank you.

 

[BAretired]

I have started to plow through the section of the code to which you refer. So far, I cannot make any sense out of it. If it does make sense to someone, please let us know.

BA

 

[calculor1]

omega 2 = 1 when the bending moment at any point within the unbraced length is larger than the larger end moment or when there is no effective lateral support for the compression flange at one of the ends of the unsupported length.

 

[msquared48]

Huh?

Mike McCann
MMC Engineering

 

[dik]

For beam columns, the main 'design' is based on a member that has a uniform moment for the length, ie. the BMD is a uniform value for the length of the column. This is overly conservative for most conditions and omega2 is used to increase the member resistance by reflecting the actural bending moment over the length.

On a positive note, our snow has just about melted!

Dik

 

[burtonSTR]

To calculor1?

Yes. But I can't find L2 in fig 2-15 case 1 match this note.
Same as L2 in case4.

To dik ,

We can use omiga2=1 conservative in design, but it's no the reason in this case.

 

[BAretired]

burtonSTR,

Figure 2-15 is part of the Commentary to CSA S16-01. It deals with bending of laterally unsupported members. Four cases are illustrated.

Omega2 is a pain in the neck to write out each time, so I will use w2 instead. You will know I mean omega2.

The factor w2 is 1.0 when the moment is constant over the unbraced length. That is conservative when a moment gradient exists.

For Case 2 in the figure, for each span L1, moment is 0 at one end and M at the other. For the middle span, L2 the moment is constant. Thus w2 is 1.75 for L1 and 1.0 for L2. It is simply saying what the code says.

For Case 1 which you queried, there are two adjacent spans, L1 and L2. In L1, the moment is maximum in the span, so w2 is conservatively taken as 1.0 just as if the moment were constant throughout. For span 2, the maximum moment occurs at the left and curves down to 0 at the right end. No part of the span has a larger moment than the end, so w2 is taken as 1.75.

For case 4, span L2, the moment is constant for the left half of the span, then drops to 0 at the right. The value of w2 is conservatively taken as 1.0.

All four of these cases are simply illustrating how the code defines the factor w2.

BA