I-Beam Deflection Calculation Problem

[Motorhead]

I work at a community college and doing some bending beam experiments for student labs. When doing simply supported beam deflection calculations and measurements, I am seeing some inconsistent data that I cannot explain.

If I test a basic cross sectional shape (square, rectangle, tube), I get very good results with theory. When I try to do an I-Beam however, the data is way off from theory...on the order of 50% more measured deflection that what have been predicted. Even if the area moment of Inertia between basic shape and I-beam are about the same.

I know the general theory says that stiffness should be proportional to 1/(EI). So why should I see a difference? My gut feel says that there is something different on how the load is transferred between the web and the flange that is not present in the basic shape. In other words, the strain may not be constant laterally across the cross section as basic theory would imply.

My background is Mechanical and my basic structural books do not address anything like this. I am hoping this is something addressed and understood by structural engineers. If you could help with some insight, it would be much appreciated. It always a little frustrating when you cannot explain unknown results to students.

 

[ishvaaag]

Just elaborating on the shear thing,

http://www.bth.se/tek/amt/mtd009.ns...0CE00529A64/$FILE/przemieniecki_p_201_208.pdf

this must download, that at desertfox's post didn't (I think)

You have also a nice shear deformation section in Timoshenko and Gere's (I have a 2nd edition in spanish)

Mecánica de Materiales
Grupo Editiorial Iberoamericana where it is at section 7.12

Also, to not suffer that much and for general use, not a simple beam case, use some finite element program with elements where shear deformation can be taken unto account. This way you will get a finer appraisal (as much as your model and the inner modeilzation allows, sometimes are not coincident). In RISA 3D you have at global parameters a box to check if you want shear deformations to be included. That I remember I always left it checked, so, less surprises.

 

[MechEng2005]

Perhaps we are overlooking the obvious... Have you accounted for the weight of the member(s) in your calculations?

-- MechEng2005

 

[hemis]

Shear deflection has to be included in the design of timber joists. The formula is KM/AG where M is the max BM, A the area and G the Shear modulus (for timber it is 1/16 E). K is the shape factor which is 1.2 for a rectangular section. I think I read in Roarke the method to calculate the form factor for other shapes.

Check out the Timber Designers manual by Baird and Ozelton. It has a good section on shear deflection.

I doubt that shear deflection would be the problem. Maybe a non-linear analysis of the beam need be considered - especially if it became plastic.