Engineering FAQ's

Modified on Mon, 29 Mar, 2021 at 12:33 PM

Engineering FAQ's

TABLE OF CONTENTS

Difference between ASD and LRFD?
VMD Deflection in Diagrams vs Adequacy Cards
Set Default Beam Size and Material
Floor Joist/Roof Rafter/Collar Tie design not adjusting when spacing is updated
Bearing Length used as part of a beam span in the calculations?
Why isn't Total Load listed in the Print Preview document?
How do I change the Duration Factor?
Why does 1 ply of a certain width beam work, but multiple plies with the same total beam width may not work?
What support types should be used? (ex. Fixed, Pin, Roller)
Where is the Live Load Deflection? Where is the Total Load Deflection?
What are "Strength" and "Magnitude" in the adequacy cards and the printout?

Difference between ASD and LRFD?

They are two different ways of combining the loads on members. Typically ASD is considered to be simpler. It compares your actual loads with allowable loads, hence Allowable Stress Design (ASD). LRFD is more complex, and applies factors to the loads and also to the "resistance" (similar to capacity), hence Load and Resistance Factor Design (LRFD).

They are basically just 2 different ways of looking at the same thing, and typically give very similar results, within a few percent.

Wood has historically been done mainly with ASD, but some designers have been switching to LRFD in recent years.

Steel used to be done mainly ASD, but many designers switched to LRFD (probably because that's what they started teaching in school), but recently ASD for steel seems to have gained more popularity.

Concrete is required to be done with LRFD, but footings are a bit mixed - the concrete capacity is ALWAYS basically done LRFD, but the bearing capacity of the soil beneath is ALWAYS done ASD. But that is done automatically by the program, so you don't need to worry about it for footings.

For residential structures with mainly wood, ASD is probably the most common. Also, ASD is the only option that Legacy StruCalc has.

VMD Deflection in Diagrams vs Adequacy Cards

The VMD diagrams typically show the worst case load combination for a failure mode like bending, which may be a load combination that is NOT required by the building code to be checked for deflection. For example, the VMD diagram may be showing a combination like 1.2D+1.6L (if in LRFD mode), but code only required deflection to be checked for D+L. So the VMD diagram will still show what the deflection WOULD be for 1.2D+1.6L, but it isn't actually being checked for that case, it is a fictitious load case as far as deflection is concerned. So it may show a very large deflection for that case, but it is ignored by the deflection check because it is not actually required to be checked by the building code. The adequacy card will list the worst-case deflection for only the load combinations that are required to be checked for deflection.

Set Default Beam Size and Material

We plan on adding the ability to save custom templates in the future. As a workaround for now, you can save beams with the typical values you use most often, and then use the SaveAs feature to create copies of them to use. You could even create a Level in the Project Explorer called Templates to save your custom beams in. And you can save a project file with these typical members, and then create a copy of the project file for each new project.

Floor Joist/Roof Rafter/Collar Tie design not adjusting when spacing is updated

You probably need to change your loads from Linear to Distributed. You can do this either by clicking on the load and changing it in the popup, or in the Load Card at the bottom.

The difference is that Linear loads are plf loads applied directly on top of the single rafter in question, so the spacing doesn't affect it. A Distributed load, however, is a psf load being applied across an area, so it is automatically adjusted as you change the spacing.

Bearing Length used as part of a beam span in the calculations?

No, the bearing length is not used in the calculations for bending, shear, deflection, etc. The span is considered as the distance between the 2 supports. This is necessary to model a beam for computer analysis. Where required, the span should be increased to include any bearing length that should be part of the span.

The bearing calculation takes the loads at the support and checks bearing by applying them over the bearing length and the beam width.

Why isn't Total Load listed in the Print Preview document?

This is because the program is running through all the different load combinations, and each load combination would have a different result for total load (especially if you're running a calc in LRFD with the LRFD factors, like 1.2 D and 1.6 L, etc.). And each load combination would also have a different duration factor applied if you're talking about wood design. So you could actually have one combination with a higher "total" load, but another combination could actually control because it uses a different duration factor.

You can see more about how this works by using Connectors. If you click on the hanger, and enter info on the first screen, then at the top of the 2nd screen you'll see several "total" loads for different load combinations grouped based on duration factor.

Also note that if you're trying to come up with a total load to transfer to another beam or footing, you can use Links to do that automatically.

How do I change the Duration Factor?

You don’t. You don’t need to.

The program will run through every combination of loads and apply the highest load duration factor that applies to each combination (as required per code), based on the loads in that particular combination.

So for example, if you input loads for Dead, Live, Roof Live, and Snow all on the same beam, it will run through the following combinations (and others), and I'll list the duration factor, and the IBC load combo equation it comes from (see attached).

D (Eqn 16-8) [with Cd = 0.9]

D + L (Eqn 16-9) [with Cd = 1.0]

D + Lr (Eqn 16-10) [with Cd = 1.25]

D + S (also Eqn 16-10) [with Cd = 1.15]

D + 0.75 L + 0.75 Lr (Eqn 16-11) [with Cd = 1.25]

D + 0.75 L + 0.75 S (also Eqn 16-11) [with Cd = 1.15]

etc, etc.

This is the correct way to do it per the IBC and NDS. The Legacy StruCalc program allows the duration factor to be checked because it limits the load types, and therefore limits the load combinations being considered. It only allows a SINGLE type of "live load" (live, roof live, snow), and you in essence control whether that load is live, roof live or snow by changing the duration factor. So in essence it is only running the first 4 combination equations above, and you have to have all of your "live" loads be a single type of load. That means Legacy StruCalc cannot automatically combine different load types into the combinations for all of the other IBC load combinations.

Vitruvius runs ALL of the combination equations, allowing you to input ALL different load types, and applies the correct duration factors for each combination.

If you want to use your engineering judgement to, in essence, hard-code a specific duration factor to a calc, you can basically do that by changing all of your "live" loads (live, roof live, snow) to a single type, and it will calc those first 4 equations the same as Legacy StruCalc, and will use the duration factor that applies to that equation.

This may be conservative in some situations, but may also be NON-conservative in others (same as Legacy StruCalc).

If you go through all of the IBC load combinations and figure out how many separate combinations those turn into, and combine them with the LRFD equations (and the equivalent time effect factor), and then consider that Vitruvius allows wind and seismic loads to be entered separately in two separate cases, it ends up being between 90-100 load combinations that are figured out automatically by the program, with the correct load duration/time effect factor applied to each case.

So there is no way that a duration factor override could be used properly for that many equations - that would most likely result in a lot of users applying NON-conservative duration factors to load cases where they shouldn't be applied, resulting in non-conservative calcs.

So you can still "hard-code" the duration factor by changing the load type, as explained above. But we've tried to make everything much simpler so that that isn't necessary when dealing with the potential of so much complexity in the equations.

Why does 1 ply of a certain width beam work, but multiple plies with the same total beam width may not work?

The difference is in the Stability Factor, CL, which is calculated based on the width of the member in question. For the Stability Factor to include the width of all plies acting together, it would technically require full composite action between the plies (like being glued together like a glulam).

So the program calculates the CL factor based on a single ply. This is slightly conservative, since nailing plies together obviously does provide SOME composite action. But it doesn't provide complete composite action, so to use the full width of multiple plies together for the Stability Factor calculation would actually be non-conservative. So the program calculates CL based on a single ply. Other design software uses this same slightly-conservative approach.

What support types should be used? (ex. Fixed, Pin, Roller)

A typical simple span beam would normally be modeled with a pin and a roller in any computer software.

The "pin" is not a true pin in the real world, it is a way of modeling it in computer analysis to show that the beam can rotate slightly at the end. And the "roller" is not a true roller - it models that the beam can both rotate and the supports can move closer together if the beam flexes.

Most typical wood construction would be modeled with one pin and one roller to represent the real-world situation as closely as possible. Real-world connections are typically not rigid enough that they would truly prevent rotation or flexure in the beam. Even heavy duty column caps will still have some “give” in the bolts or screws that will allow some slight rotation of wood beams. And if a beam were sitting on two walls or columns, and the beam flexed downward under load, the walls or columns would flex slightly closer together.

Typically a “fixed” support would only be used for a connection that truly allows no rotation or movement (like a beam embedded in concrete).

Where is the Live Load Deflection? Where is the Total Load Deflection?

Currently the program shows the worst-case of live load deflection and total load deflection. Both are being checked, but only the worst-case is being shown.

What are "Strength" and "Magnitude" in the adequacy cards and the printout?

“Strength” is used to indicate the capacity of a member.

“Magnitude” is used to indicate the actual load or demand on a member.

If you have any questions or suggestions, please don't hesitate to reach out to our customer service at support@thevitruviusproject.com.

Thanks for reading!