Worked Example: US Wood Beam to NDS 2018

Background

In this worked example, we’ll go through the design process of a multi-span ridge beam. Note that this references our one-page worked example for a US Wood Beam - Ridge Beam. You can download the one-page PDF to follow along. If you’d prefer to follow along with the video tutorial, check out the video below.

Given Information

You’ll see screenshots of a simple CAD Elevation and Roof Plan below. By the time you’re coming into ClearCalcs to design your ridge beam, you’ll likely have these plans drawn up (or at least the preliminary plans done) to know the geometry of your roof. You’ll also see your “Design Criteria” (determined by your roof geometry, your client, the code, and/or your business standards of practice) and “Loading Data” (determined by the local jurisdiction in which you’re designing this ridge beam). Note that you can disregard the letter callouts (A, B, C, etc.) in red circles as these are specifically relevant in our one-page PDF example.

Opening the US Wood Beam - Ridge Beam calculator preset

To speed up the design process, ClearCalcs offers common calculator presets where information is pre-filled based on your Project Defaults. In this case, you can select the Ridge Beam preset under “Wood Beam (ASD)” (note that the workflow will be the exact same if you use LRFD). Learn more about calculator presets in ClearCalcs here.

Entering your Design Criteria

First, it has been decided that this ridge beam will be a Microllam LVL (see “Design Criteria” in our Given section above). You can assume that you have a good relationship with the local supplier and can get Microllam LVLs at a good price, plus you’ve had positive experience in the past using this type of LVL for ridge beams. You can use the Member Selector and filter down the list to only see what you’re interested in. We’ll come back to this later once we’ve set up the rest of our Design Criteria and Loading Data.

You can go ahead and exit out of that Member Selector for now - we’ll come back to it once we have the rest of our Design Criteria entered into our calculator. Next, we’ll enter our beam plan length which is given in our Design Criteria as 40’-0”.

Next, we’ll input our support locations and bearing lengths. Our Design Criteria tells us that we have a total of (3) supports, each of which are 20’ o.c. (or, on-center). In the screenshots below, notice that you’re able to add as many support locations as you’d like in ClearCalcs. Pro Tip: Place your supports as relative locations instead of hard-coding them. For example, notice below how you can input the supports at 0, L/2, and L. ClearCalcs knows that L is the full length of your beam. This way, if you ever change the length of your beam, the supports will stay at the same relative locations along the beam.

The bearing length input in the one-page worked example shows 3in. However, if we have a 40ft long, 3-ply ridge beam for our roof, it’s more likely we’d be bearing onto a few 6x6 posts. Because of this, let’s assume that my bearing length input is 5.5in (because us Americans like to be confusing and a 6x6 post is actually 5.5in x 5.5in). Pro Tip: Notice how the Bearing utilization percentage dropped from 99% to 57% after we changed the bearing length from 3in to 5.5in. This is expected because we’ve more than doubled the bearing length.

Next, let’s verify that our Short-Term and Long-Term Deflection ratios are correctly set per our Design Criteria. We’ll scroll down on our ClearCalcs calculator to the “Design Conditions” sections on the left hand side of our screen. Pro Tip: More often than not, you won’t need to change any of the inputs in this “Design Conditions” section. The values in this section default based on the code you’ve selected in Project Defaults, and the preset you selected when you added a new calculation. Of course, it’s always good to confirm these values are what you need to design to!

Last but not least, let’s insert our Loading Data into the “Loads” section of our calculator. Because we opened up our US Wood Beam - Ridge Beam preset, ClearCalcs already pre-filled common ridge beam loads into our calculator. While this saves us time, it’s important to double check that these loads are correct for our specific design case.

Let’s check our Roof Load, which according to our Design Conditions in our Given section tells us that we need to design for 10psf Dead Load and 20psf Roof Life Load. We’ll need to modify our Dead Load preset from 15psf to 10psf. Click that “Edit” button on the right hand side of the Rood Load row. Since we don’t have a Live Load here, and just a Roof Live Load (the code differentiates between the two), we can remove the Live Load row (last screenshot).

Next, let’s check that our Snow Load is correct to our specific design case. In this case, we’re all set because the ClearCalcs preset is 30psf, and our Loading Data in Given is also 30psf.

Next, we can remove that Wind Load preset since our ridge beam example doesn’t say we need to design for any wind loads. Of course, in the real world, there’s a high probability you’ll be designing with wind loads in mind. This is where your engineering judgment and knowing the local wind speeds/loads come into play.

Now that we have our load magnitudes set for our Distributed Loads, one very important input we can’t forget to check is our Tributary Width. If you’re unfamiliar with tributary width, check out this article here, and check out the visual below (a picture is worth a thousand words here).

In our case, we can determine our tributary width from our CAD Elevation and Plan View. To determine the tributary width (see screenshot below), we know that we have 12’ from our ridge beam to the next support on both sides. We take half the distance from each side (6’ to the left and 6’ to the right) to get our total tributary width of 12’. Note: The Elevation view is a bit misleading as you might think the distance to the next support is 14’. However, note that there is a 2’ overhang.

Now that we have confirmed our tributary width, we need to update this for our Roof Load and Snow Load to ensure our ridge beam can withstand the correct tributary load. Note: ClearCalcs has two tributary width input columns - Tributary Width Start and Tributary Width End. More often than not, these two fields will be equal to one another. They will only differ when you have a triangular or trapezoidal loading pattern (such as a hip/valley rafter, or a wonky deck you’re designing.)

The last load we’ll need to confirm is the Alternative Minimum Live Load. In our Given section, we know that we need to apply an Alternative Minimum Live Load of 300lb. Check out the blurb below about what this load is, and why ClearCalcs applies it as a preset load default in our ridge beam calculator. Usually, an ‘Alternative Minimum Live Load’ will appear here by default. This Alternative Minimum Live Load, with load type ‘L2’, is NOT applied at the same time as the normal live load. For some types of surfaces, the building codes require that beams be able to support at least a minimum concentrated live load, regardless of the normal live load, and that is this value. If it is blank (zero), then the default surface type you have selected in your Project Defaults does not require an alternative minimum live load.

We’ve now given ClearCalcs all our design conditions! It’s time to head back to that Member Selector and select the most optimal member for our ridge beam. Since we already set our filters at the beginning, the only members we’re seeing within the Member Selector are the ones we’re interested in (Microllam LVL).

We can now select the member we are most comfortable with. In this case, the most optimal member (theoretically) is the 1-3/4x11-1/4 Microllam LVL 2.0E-2600Fb (remember we have 3-plies of this selected). Once we select this, our ClearCalcs calculator will update.

We’re now ready to print and submit to our permit reviewer for a quick and easy permit review process! Click that printer icon on the top right of your screen, and select either the One Page, Standard, or Detailed Summary printout option.

Want to learn more about our Wood Beam presets? Check out the following help articles and tutorials:

How to Design a Wood Beam, including Joists, Rafters, and Ridge Beams to NDS 2018

​Background

​Given Information

​Opening the US Wood Beam - Ridge Beam calculator preset

​Entering your Design Criteria

Background

Given Information

Opening the US Wood Beam - Ridge Beam calculator preset

Entering your Design Criteria