NOT QUITE SO SIMPLE BEAM CALCULATOR

[CipesDesign]

Dear NNA, Unless I'm missing something, the "Simple Beam Calculator" (found in VWA version 11) does not actually calculate beam sizes. It does give a mathematical solution, but it does not size beams. Therefore it is nearly useless for real world design as another entire program would be needed to actually input data like 1) is the beam wood,steel,glulam,etc? and 2) at what dimensions does the chosen type of beam work/fail. Also, we generally need to be able to print the beam calc's. All of these suggestions would be welcome and useful additions!

[tom kyler_dup1]

Peter, indeed it is a "simple" beam calculator. For a simple beam to withstand a given allowable stress though, many different variables can be altered to satisfy the stress requirement numerically; however, the software doesn't know which variable you may want to alter and therefore, for most circumstances, it is not be a prudent design method to let the software size the beam, but rather you design the beam and let the beam calculator give you feedback on your design.

With a good understanding of beam loading, stress and mechanical properties, sizing beams is not to difficult or time consuming and the beam calculator eases the process greatly. As an engineer, I don't like the idea of architects or designers trying to use Vectorworks to size beams automatically...hence my two cents of input....however, I could see engineering properties working on structural shape plug-ins without having to convert them to polygons. This would make sizing structural elements a bit quicker.

Tom Kyler

Mechanical Engineer

San Antonio, TX

[ 07-13-2004, 05:46 PM: Message edited by: tom kyler ]

[CipesDesign]

Hi Tom, and thanks for your reply. I understand and agree with most of what you said, but still have a couple questions. Since I'm not an engineer, please bear with me... I assume from the instructions regarding the Simple Beam Calc'r (SBC) and from your comments, that instead of a user entering a specific size and type of beam (I'll get back to this in a minute), we are asked to enter the design (or engineering?) properties of that beam. I can find the Modulus of Elastisity, and have the equation for the Section Modulus, but I am stumped on the Moment of Inertia (I basically understand what is, but do not how to calculate it). Some help there would be appreciated.

It also seems to me that this data could be calculated by a beam calc program based on known material strengths, etc., thereby making it much more user friendly. And just so you know, almost all my work gets looked at by a structural engineer, but it would be nice to have some idea of beam sizes, etc., earlier in the design. Also, there are times when I have very small things (spans less than what is generally shown in tables, for instance) that I would like to be able to calc/verify.

Thanks Again,

[Kevin]

Peter; Unless things have changed since I was in school, the data that you are looking for is listed in tables. If you are using steel, then you can find the data in a Steel Manual.

This has always been a trial and error process. There are so many variables such as beam type: steel, sawn wood, engineered wood; depth of beam and width of beam, not to mention what the desired deflection is.

While I have not used the calculator yet, I would imagine that it would allow me to "rough size" a beam to test my design. It can give you an idea of whether a wood beam can span the distance or if you must go to steel.

I will have to take a closer look at this new feature of VW. I would say though that with beam tables you should be able to size a beam just fine. The math is still the same whether you use a slide rule, pocket calculatior, or this "new fangled" beam calculator.

[tom kyler_dup1]

Peter, I agree totally that you should be able to do quick, early calculations for sizing beams...so a quick engineering lesson. Fortunately, VW has a command called "engineering properties" in the tools menu that can give you the moment of inertia (MOI) for any shape you can draw. Since sizing a beam generally means specifying the cross sectional shape and dimensions and defining the material (modulus of elasticity value) You can alter the shape of your beams quickly and get the new MOI for the beam calculator.

The size and shape of a beam (cross-section)is described by dimensional geometry..web thickness, flange thickness, flange width, overall height, arc radius, whatever, etc....and this dimensional geometry is what is used to calculate the Moment of Intertia of a section, it's calculated with geometry only. Vectorworks can calculate the moment of inertia of any closed polygon and if you want the MOI of a I-beam or something similar, you can look in a table, or use VW structural plug-ins and convert them to groups to calculate their MOI. When you select an object and go to TOOLS>ENGINEERING PROPERTIES, you'll see several values, notably Ixx, and Iyy. Ixx and Iyy are the moments of inertia about the x and y axis that passes through the centroid of the area. Almost all the time, you'll use the Ixx value, because the load (down) is normal to this axis and the beam is bending in this plane...that's a simplification, but it works.

If you want to try different sizes, you just change the cross sectional shape (taller or wider, thicker flange or whatever) and recalculate the MOI and put that value into the beam calculator. You can then get the max stress and compare it with allowable values. This is the iterative process for Vectorworks.

You can work backwards by specifying the maximum deflection or maximum stress, and figuring out the beam dimensions, but this involves rearranging the equations manually and is probably longer than using the iterative method which is what I do now.

Hopefully, you're not thoroughly confused and you can make some sense of it, because a thinking designer is always a pleasure to work with.

[ 07-13-2004, 10:55 PM: Message edited by: tom kyler ]

[CipesDesign]

Thanks Tom! No, you've explained it pretty clearly. I assume that I draw the beam profile to calculate the MOI (eg: a rectangle 3.5 x 7.25 for a 4 x 8 beam)? One more question? What is considered an acceptable number for "yMax" for a solid wood beam? Is 1/2 inch excessive in an 8 ft span?

[tom kyler_dup1]

That's exactly right, the beam profile is where the MOI is calculated from. With regards to Ymax, that depends....every Ymax will have an associated maximum stress associated with it. Assuming that the stress at a given Ymax is well within acceptable levels, then the issue isn't the stress anymore, but rather what kind of problems might occur from that kind of deflection. Cracks, or visual deflection might be unsightly and would be undesireable in such cases. As far as 1/2" being excessive, it would depend on the beam, if it was a 2 x 4 beam laid on the flat, then 1/2" ain't bad, but for a 4 x 4 x 8' beam, 1/2" would be pretty bad. In general, I think 1/2" deflection for an 8' span in construction is not very good, and a stiffer beam is called for, either by changing material or changing the configuration or dimensions. What's tricky here is using a proper numerical value for the modulus of elasticity. Solid wood would be different from a gluelam of course, as would a truss. The published numbers are averages and are only a general guide, but are the values commonly used.

Common sense goes a long way here, and if a beam supports the load OK, then go with what you feel. If you're the designer and you don't like 1/2" deflection for whatever reason, even gut feelings you can't explain, then go ahead and make a change..you might piss off a few engineering types, but you can't please everybody. As engineers, we're responsbile for making sure designs work and are safe. Anything above and beyond that is up to you, I'm a big fan of maintaining aesthetics and don't mind over-engineer stuff if it looks good.

[ 07-14-2004, 02:21 AM: Message edited by: tom kyler ]

[CipesDesign]

I knew the answer would be not-so-simple. But nothing that's any good ever is... Thanks for the explanation!