AISC Chpater F

[SmithJ]

Hi Everyone,

Chapter F of the 9th Ed. AISC ASD Manual states that the equations in that section apply to ..singly or doubly symmetric beams including hybrid beams and girders ..".My understanding of this means that the equations would be applicable to built-up beams where the flanges are made of sections other than flat bar (e.g. Channels, Angles, or Tubes) as long as they are I-sections.

I am currently in a discussion with another Engineer who says that these Equations are "only applicable to standard shapes and standard built-up girders" meaning flat plated sections only. I do not see this limitation in the code and was wondering if anyone out there could shed some insight into this matter.

As long as the needs for unbraced length and compactness of the section are met as required by the code, I see no reason why the equations can not be used to calculate bending strength in a hybrid beam.

Your thoughts on this would be highly appreciated.

Thanks,
JS

 

[ajh1]

We have certainly used the AISC provisions to do things like crane beams where you have a combined section of a W shape with a top cap channel. Not aware that anyone has ever questioned the use of AISC. We also do stepped crane columns where the outer flange is a plate and the inner flange is a wide flange shape with strong axis perpendicular to the web of the column.

 

[VoyageofDiscovery]

Hi smithj,

The commentary on page 5-146 half way down even makes reference to crane girders and page 5-50 F-6 makes reference to channels.

Regards

VOD

 

[pmkPE]

Page 5-46 under section 2. second paragraph states "For built-up members meeting the requirements of Sect. F1.1....." I have always used setion F for evaluating built-up sections. I agree that you can use a variety of members for your built-up sections including angles and channels (not just for crane rail beams).. Check out some of the old style designs from the 30's through the 70's and you will find such configurations using a variety of plates, angles and channels to create and even build-up existing beams to increase their capacities.

If your section is deep then Chapter G applys for Plate Girders.

 

[SmithJ]

Thanks for your replies.

I seem to find it easy to make the connection between Chapter F and built up beams made with with Angles and Channels and I am wondering if the same would apply if the flanges were made from HSS Tubes. I think the equations would still apply as long as local buckling is adequately addressed but would like to know your thoughts on this issue as well.

Thanks.

 

[pmkPE]

My first thought is in regards to punching shear of the tube. They are usually very thin and the resultant forces from a web at their mid section could easily "cut" through. I think AWS D1.1 has guidlelines for punching shear on tube to tobe connections. I have not seen HSS members used to beef up other members as they are relatively thin and do not necessarily provide a cost effective solution as their weight to cross section area is very high.

What exactly is your application: span, loads, sizes etc? Why not use standard hot rolled sections such as plate, angles and/or channels?

 

[UcfSE]

The AISC publishes a manual for HSS connections that includes limit states for the punching shear of a connecting plate. I don't know if that can be extended to your application but you may be able to come up with something. It's only in LRFD though. I have used HSS tubes to retrofit in place wide flanges but they were welded to the bottom flange only so punching wasn't a concern. If punching shear is a problem you could always make the web plate a through plate but now you're talking about a very expensive section. Why do you want HSS flanges anyway? pmk is right, that seems to be the more expensive way to go. In my application it was better to use a tube but not in general.

 

[phdunn]

My understanding of this is as follows:
The formulas are based on the Iy of each flange assuming a rectangular section, Iy=bf^3*tf/12. If the shape is going to give you a stiffness greater than this, such as a channel cap your even better. If you reinforced with a small bar at the center of the flange, then you might be in trouble. In a graduate class I had, we showed a flat plate stood on top of a wide flange could make it buckle at a lower load!
Another example of this is in coldformed channel with a lip where the lip will buckle if made too long, which will buckle the flange.