Inventor General

## Inventor General

Member
Posts: 4
Registered: ‎07-30-2007
Message 1 of 11 (939 Views)

# Simple Beam Stress Analysis

939 Views, 10 Replies
08-24-2007 06:58 AM
I am having difficulty understanding results for stress analysis of a simple beam. W14 x 398 30 ft long with two point loads of 150k symmetric 36 ksi steel. Comparing hand calcs and a beam program (with similar results) to the output from Inventor 2008 Simulation Suite, I find a big difference.

(1) The only way to get similar reactions on the constraints is to use fixed points and use the components for X and Y as 0, however this gives me the weak spring’s error. Does this affect my results?
(2) Hand calcs and beam program give me deflections of .693 and .787 inches respectively. Inventor gives me .3 inches. This appears incorrect.
(3) Both hand cals and beam program conclude that the beams allowable bending is 21.6 ksi and allowable shear 215.2 kip based on ASD 9th. The maximum moment and shear with simple diagrams equates to 1154.8 k-ft and 155 k respectively. The beam should be within allowable stress at about 21.12 ksi for bending based on a Sx of 656 cubic inches. How come my Equivalent Stress is so high at 61 ksi?

These are similar loads that I have used in the past without failure. What is it that I am doing wrong?
*Expert Elite*
Posts: 4,153
Registered: ‎04-27-2005
Message 2 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-24-2007 01:35 PM in reply to: kevinoneill
What is the distance between the two loads? I assume that the loads are parallel with the web of the beam (Ix = 6000in^4).
Member
Posts: 4
Registered: ‎07-30-2007
Message 3 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 07:12 AM in reply to: kevinoneill
10 ft. apart
Yes, in line with the web
Correct (Ix = 6000in^4).
Thanks
*Teun Ham \(IV10 SP3 / IV2008 SP1\)
Message 4 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 07:24 AM in reply to: kevinoneill
Wow, that's massive!

What does the white & red sign say? "DO NOT DROP"? ;-)

--
T. Ham
CDS Engineering BV

HP xw4300 Workstation
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Windows XP Professional SP2
Autodesk Inventor Series 10 SP3a
Autodesk Inventor Suite 2008 SP1
--
*Troy Grose
Message 5 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 07:51 AM in reply to: kevinoneill
I think its supposed to say "Do Not Bump", but everytime I read it's
coming out as "Do not Hump"

Teun Ham (IV10 SP3 / IV2008 SP1) wrote:
> Wow, that's massive!
>
> What does the white & red sign say? "DO NOT DROP"? ;-)
>
Distinguished Contributor
Posts: 1,052
Registered: ‎01-15-2006
Message 6 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 08:35 AM in reply to: kevinoneill
It is reassuring to see someone do all the planning needed to safely
do any sort of rigging or lifting . that being said .

use a bigger beam and let extra safety margin keep you warm at night

it may be a photo thing but that beam appears quite deflected .
*Richard Hinterhoeller \(AIS 2008 Sp 1\)
Message 7 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 09:09 AM in reply to: kevinoneill
I believe you are reading it correctly as "Do Not Hump".

I believe "Hump" is a railway term. I'm no expert in this but my guess is
that it refers to a humping yard where trains are sorted by pushing cars
over a hump, after which they gravity roll down a gentle incline and get
switched onto the appropriate siding so as to become part of the train

The incline has to be steep enough so that a poor running car will roll, so
the really smooth running cars can build up enough speed to cause damage
when it meets the stationary train. It may be undesirable to take this risk
with a payload so one would advise the railway to not sort using a humping
yard.

But then, I could be all wrong.

Richard
"Troy Grose" wrote in message
news:5701824@discussion.autodesk.com...
I think its supposed to say "Do Not Bump", but everytime I read it's
coming out as "Do not Hump"

Teun Ham (IV10 SP3 / IV2008 SP1) wrote:
> Wow, that's massive!
>
> What does the white & red sign say? "DO NOT DROP"? ;-)
>
*Expert Elite*
Posts: 4,153
Registered: ‎04-27-2005
Message 8 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 12:46 PM in reply to: kevinoneill
Kevin,
I had similar questions about the beam calculator. It has too many problems right now for me to use it. I don't use it because of all the problems. Try going back and editing your beam calculations. When I do it, the graphical rep of the supports are moved away from the beam and I can't get them back on it. I have to delete everything and start over each time. I am using a "Custom" beam and not one from the Content Center. That may make a difference. The link to my post is
http://discussion.autodesk.com/thread.jspa?threadID=567615. You may find something in it that may help you out. Just be aware that the deflection from beam self weight is included in the deflection calculations.

I ran the numbers for the loading with both Inventor and another beam program that I use. Inventor gave me a deflection of .769in and the other program gave a deflection of .761in.

I have attached my results here for a comparison. I added two additional supports (more like your picture), just to see what would happen. The deflection then dropped to .193in at the center.
*Expert Elite*
Posts: 4,153
Registered: ‎04-27-2005
Message 9 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-27-2007 12:57 PM in reply to: kevinoneill
Here are the results from the Inventor Beam and Column calculator.
Member
Posts: 4
Registered: ‎07-30-2007
Message 10 of 11 (939 Views)

# Re: Simple Beam Stress Analysis

08-28-2007 04:16 AM in reply to: kevinoneill
Hi Karthur
Thanks for all you help. However I am not trying to use Inventors beam program but the FEA Stress Analysis. I design rigging that I normally can not load test. I purchased the Inventor simulation to test my designs and see weak points. To get myself familiar with the product I took a simple span beam and load that was close to max.
I use a program called Enercalc for my beam checks and MathCAD spread sheets for others aspects such as web crippling and buckling as well as check bending shear and deflection.
Attached is the beam I used in Stress Analysis.
My real questions:
Are my constraints and loads not correct in FEA?
Is the FEA so precise that some judgment is needed?
Are both my MathCAD and Enercalc incorrect?
This is a very simple example with conflicting results especially with allowable stress.

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