Posted: 8/28/2026 2:50:53 PM EDT
I'm using the website optimabeam.com to figure out the strength of a custom made beam. Here is what is got me confused. The Moment of inertia. The higher the number the more resistant to movement I'm assuming. the moment of inertia in the Y axis (7.875) should be alot higher than the X axis (331.002) in this shape. Is the website doing something goofy or am I misunderstanding the Moment of inertia. I'm concerned about bending in the Y axis alone, no forces on the X. I'm plugging the Moment of inertia along with the Elastic Modulus along with the span of the beam to get the deflection. I'm working to get an acceptable number for a relatively low weight (225lbs) Thanks |
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Originally Posted By bsbg: Looks right to me. Ix is the moment of inertia about the x axis, so how hard it is to bend around the x axis. If this is a floor beam, gravity acts down (-y direction) and causes deflection about the horizontal x axis. So I have been reading it wrong. Typical for me I got it backward. |
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Originally Posted By bsbg: Looks right to me. Ix is the moment of inertia about the x axis, so how hard it is to bend around the x axis. If this is a floor beam, gravity acts down (-y direction) and causes deflection about the horizontal x axis. Do you mean this is computing what happens when the beam is squeezed by gravity, then spun aroune the axis? So instead of a skinny beam spinning around its center axis, you get an arch spinning around the axis that goes through the end points? Still not sure what the center point for the X axis rotation is then. |
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I'm trying to get the deflection of a beam. The beam is going to be loaded so that it goes down (gravity or a -Y movement) I'm assuming up and down are the Y axis and left and right or side to side is the X axis. I am using the website https://intelcalculator.com/construction/beam-deflection-calculator/ to get the deflection. To get the deflection I'm feeding it the following info. Center point deflection. span of beam (1.2 meters) point load (1kN) Elastic Modulus (3000) Moment of inertia (use the x axis moment of inertia?) To get the Moment of inertia I was using the website "optimabeam.com" as mentioned in original post. I just noticed that the moment of inertia is being output as a number expressed in inches to the 4th power. I need to convert the number expressed in inches to the 4th power to cm cubed as the beam deflection site is wanting that to be inputted. So in a nutshell I need to confirm that I should be using the X Moment of inertia number from the optimabeam website convert that to cm cubed and then the beam deflection calculator will give me the deflection. Deflection up and down or Y axis. How to convert inches to the 4th power to cm cubed? |
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am I right to get inches to the 4th power converted to cm cubed(or cm to the 3rd power) I do the followeing example 100 to the 4th power is equal to 100 x 100 x 100 x100 or 100,000,000 or its equal to 10,000 to the 2 power (squared) 10,000 square inches is equal to 64516 square centimeters (1 sq in = 6.4516 sq cm) find the square root of 64516 cm = 254 so in this example 254 cm cubed would be equal to 100 inches to the 4th power? |
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Originally Posted By DirtyDirk: I'm trying to get the deflection of a beam. The beam is going to be loaded so that it goes down (gravity or a -Y movement) I'm assuming up and down are the Y axis and left and right or side to side is the X axis. I am using the website https://intelcalculator.com/construction/beam-deflection-calculator/ to get the deflection. To get the deflection I'm feeding it the following info. Center point deflection. span of beam (1.2 meters) point load (1kN) Elastic Modulus (3000) Moment of inertia (use the x axis moment of inertia?) To get the Moment of inertia I was using the website "optimabeam.com" as mentioned in original post. I just noticed that the moment of inertia is being output as a number expressed in inches to the 4th power. I need to convert the number expressed in inches to the 4th power to cm cubed as the beam deflection site is wanting that to be inputted. So in a nutshell I need to confirm that I should be using the X Moment of inertia number from the optimabeam website convert that to cm cubed and then the beam deflection calculator will give me the deflection. Deflection up and down or Y axis. How to convert inches to the 4th power to cm cubed? If you need cm cubed it sounds like you need the section modulus, not the moment of inertia. Frankly, you don't seem to understand basic math. DIY structural engineering, even with websites, seems like a bad idea. (very senior mechanical engineer, who often does structural engineering) |
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Originally Posted By Lost-Drive-In: If you need cm cubed it sounds like you need the section modulus, not the moment of inertia. Frankly, you don't seem to understand basic math. DIY structural engineering, even with websites, seems like a bad idea. (very senior mechanical engineer, who often does structural engineering) If what you say in your first line is true then why is the website (beam deflection calculator) asking for Moment of inertia? I'm no math wizard I'll admit but to you this might be "basic" math but to me its not obviously. Figuring out the deflection of beam doesn't seem like it should be so hard. Size of the beam, span, strength of the material (elastic modulus) and the load in the center while supported on both ends. What am I missing here? |
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You're looking for the area moment of inertia. Units are to the 4th power. In^4, mm^4, etc. The area moment of inertia (also called the second moment of area) measures a shape's resistance to bending and deflection. A higher value means the shape is stiffer when a force is applied.Standard UnitsSI System: \(\text{m}^{4}\) (meters to the fourth power) or \(\text{mm}^{4}\) (millimeters to the fourth power)Imperial System: \(\text{in}^{4}\) (inches to the fourth power) Deflection of a simple supported beam with a centered point load is PL^3/(48EI) Units will solve out to mm, in, whatever units you use - but have to be consistent, E will be in newton's per mm^2 or pounds per in^2. |
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What is the material, and where did you get the Young's Modulus, E? The deflection of a simply supported beam with a point load at the center is: Deflection, d = ([load, P][length,L]^3)/(48[Young's Modulus, E][Moment of Inertia, I] d = PL^3/48EI Use the Ix number. For a beam fixed at the ends, d = PL^3/192EI Units: d, inches = lbs(in^3)/(lbs/in^2)(in^4) Write it down, cancel terms to arrive at inches. Repeat for SI units to come to millimeters, centimeters, meters, or whatever. This is called dimensional analysis, and is unfortunately not taught or taught poorly. It's critical that each entry in the equations have consistent units, I.e. all kilograms or grams, all mm, cm, or meters. Don't mix, or the answer will be wrong. inches^4 (2.54 cm/inch)^4 = cm^4 The elastic modulus is a measure of stiffness, not strength. Bending stress is another calculation you should make. Ask someone here to run the numbers so it's correct. |
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Originally Posted By DirtyDirk: am I right to get inches to the 4th power converted to cm cubed(or cm to the 3rd power) I do the followeing example 100 to the 4th power is equal to 100 x 100 x 100 x100 or 100,000,000 or its equal to 10,000 to the 2 power (squared) 10,000 square inches is equal to 64516 square centimeters (1 sq in = 6.4516 sq cm) find the square root of 64516 cm = 254 so in this example 254 cm cubed would be equal to 100 inches to the 4th power? No. It's not correct, and not close. Both values must be ^4. The program asking for cm^3 is after something else. What stops this beam from rolling over? |
Keep your powder dry, and watch your back trail.
@DirtyDirk The moment of inertia directly from the program in the first post cannot be used as is, it must be calculated (transformed to) at the elastic centroid of the beam. "lbf" is pounds-force. Someone will probably want to argue about pounds-force and pounds-mass and whether they exist as units of measure. I won't, and they are wrong. |
Keep your powder dry, and watch your back trail.
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Originally Posted By DirtyDirk: If what you say in your first line is true then why is the website (beam deflection calculator) asking for Moment of inertia? I'm no math wizard I'll admit but to you this might be "basic" math but to me its not obviously. Figuring out the deflection of beam doesn't seem like it should be so hard. Size of the beam, span, strength of the material (elastic modulus) and the load in the center while supported on both ends. What am I missing here? Units for section modulus are length cubed - used for stress calculations. The x-axis is shown along the bottom surface and not the neutral axis. If the number is right, the illustration is wrong. What is the span and are the flanges braced to prevent the beam from rotating and buckling? (Also an old/senior ME who did a lot of structural type engineering) 
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The material for the beam is Baltic Birch plywood. MOE =6,000–7,000 MPa or 3000-5000 depends upon where you get your info. and of course that is with the correct orientation (compression of the grain) The end of the beam is notched out in just the vertical "I" section to slip over a beam running perpendicular. The top and bottom horizontal sections overlap the other beam. so rotation of the beam should not be an issue I suspect. |