unbraced length question 1

[delagina]

i have a steel platform with grating with no horizontal brace. see attached for partial plan.

unbraced length of this i set as total L for new structure or i add horizontal brace.

but this is existing and i dont want to retrofit this if not necessary. any chance this can work out as L/8. i've read about 2% flange force and also grating might help.

looking for some clarification.

my biggest worry is original engineer (this is a very old structure) might have considered L/8 as unbraced length for the beam which will cause them to fail in my new analysis if i consider total L.

thanks,

 

[delagina]

a2mfk,

yes, point taken. i'm probably on the "weak" side technically compared to other posters here. i need more reading and practice more hand calculations. maybe because i've been spoiled with good analysis and connection softwares eversince. or maybe because i'm naturally not gifted technically. haha.

anyway, all i was asking was can i use the grating or 2% of flange force to assume unbraced length as L/8.

i would normally use full L here if it's a new structure since it doesnt have horizontal brace.

p.s. as far i am concerned this thread is close and my question has been answered.

thanks,

although i dont agree i dont need staad for this structure. i just posted "partial" plan. i actually have the whole equipment struture to design with different levels.

 

[a2mfk]

Don't be too hard on yourself. Treat software like a tool and not like a crutch and you will be OK. Sometimes you have to dive head first into AISC to really understand the intricacies of bracing and lateral buckling (and any of the other finer points of steel design)... And also take a step back from your design and look at how your beam is braced, if at all, including reversed loading. Its easy to miss this in a roof beam for example that has uplift loads that may control the design if not braced.

Also ask yourself on any project- can I design this by hand, but software will make it a lot easier to reach a final solution? Always make sure you understand all the boxes you are checking and unchecking, and all of the variables and other inputs come from you, and that all of your actual design assumptions are reflected in the analysis. Its when you become too removed from all of these processes that software can become dangerous.

But to conclude on your problem I think unless the grates are welded or otherwise attached in a manner where they act like a diaphragm, they will not properly transfer the axial buckling force into the side beams (assuming they are properly braced)...

You can always add lateral bracing, but often its much cheaper in the long run to avoid bracing and the labor involved and just upsize the beams...