Moment of Inertia for built-up Beams
Moment of Inertia for built-up Beams
(OP)
Question:
If I have an I-Beam and a C-channel stacked and welded as one, to find the "I" of the "built-up" member can I just add the two "I" values together, or is there some other method of finding the combined "I."
Thanks
If I have an I-Beam and a C-channel stacked and welded as one, to find the "I" of the "built-up" member can I just add the two "I" values together, or is there some other method of finding the combined "I."
Thanks
Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario






RE: Moment of Inertia for built-up Beams
RE: Moment of Inertia for built-up Beams
As a quick summary for obtaining I for "two members"
1. get neutral axis of combined section.
y_bar = sum(Ay)/A = (A1y1 + A2y2)/(A1+A2)
2. Iy = I1 + A1 d1^2 + I2 + A2 d2^2
where d1 = distance to center of A1 from y_bar
and d2 = distance to center of A2 from y_bar
This is as simple as I can explain it. Good luck.
RE: Moment of Inertia for built-up Beams
also, isnt there a table of section properties for I beam and a challel in the AISC steel manual? Unless it is a bizarre combination, you should be able to find the values you need from the steel manual.
RE: Moment of Inertia for built-up Beams
Thanks Again
Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario
RE: Moment of Inertia for built-up Beams
Your question about structures (acutally its about section properties - aka deformable bodies) makes me wonder what you are using this built up seciton for? Finding the capacity of an I section with an attached channel is more than just finding the I. Could you tell us what this is used for? Thanks
RE: Moment of Inertia for built-up Beams
Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario
RE: Moment of Inertia for built-up Beams
You don't mention how the channel is attached to the I beam. Horizontal shear connectors at intervals are required to utilize the composite section for bending.
Regards
VOD
RE: Moment of Inertia for built-up Beams
VOD
RE: Moment of Inertia for built-up Beams
Only connection you need is to be able to resist the amount of shear flow between the two pieces so they act compositely. Like VoyageofDiscovery sugggested, intermittent welds or even bolting at certain spacing may suffice.
RE: Moment of Inertia for built-up Beams
Whyun: No, we wouldn't do a continuous weld over the 40', probably wouldn't be much more than a 1-12 1/4" fillet.
Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario
RE: Moment of Inertia for built-up Beams
If 1" long 1/4" fillets at 12" spacing can carry the shear flow. That's all you need. If two rows of bolting is cheaper you can also go that route.
Of course, continuous weld entire length works designwise, but its a waste of money.
RE: Moment of Inertia for built-up Beams
In all of built-up sections, I prefer welding to bolting as if I go for bolting, the codal provisions eat up the effective area of the sections. I have to made deductions for holes and moment of inertia of the section decreases. This proves too be much more expensive and makes me hate bolts in such cases. Even continuous weld is not a very bad idea as compared to bolts.
RE: Moment of Inertia for built-up Beams
bolts on the other hand are cheaper and installers need not have certifications. in many jurisdictions, you may also be able to avoid inspection.
all in all, i also prefer welds over bolts (in the case of attaching channel to a WF beam)
RE: Moment of Inertia for built-up Beams
RE: Moment of Inertia for built-up Beams
Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario
RE: Moment of Inertia for built-up Beams
The reason for my quesiton was to ask if you are trying to design this as a beam, or just determine the deflection. There are a lot more things going on. The connection between members is one thing, the lateral stability of the beam, if the beam is continuous, is the support designed for the load, what about fatigue, what about vibration.
I just wanted to make sure you weren't simplifying things to much, or making assumptions.
Just trying to be careful.
-Doug