Loads

Background Information

This example will look at the snow load calculations for a building located in Chicago, IL. Our given information is that the ground snow load is 25 psf. We are designing the snow loads for the roof of a single storey commercial building. You can follow along with the video of this example, and you can check out our other article for further design of these steel joists here.

Entering your Properties

Site and Building Properties

The site and building properties can be set in Project Defaults, or can be overridden in the snow loads calculator. To navigate to the Project Defaults, we can select the tab on the left hand side of the screen.

From here, the important values for snow loads calculations will be entering the exposure category and ground snow load. For our example, we will assume our building is located in the city and use exposure category B, so that we are using the most conservative roughness category. This determines the fully exposed, partially exposed and sheltered factors. We can enter in our ground snow load here, as 25 psf.

Still in Project Defaults, the building risk category can be set as II - Regular Building.

Moving back to the snow loads calculator, our roof exposure can be set to sheltered, since we are designing to be located in the city with other buildings as nearby obstructions. Last, we will assume our thermal condition of the roof is warm.

If you want to override the project default properties in the snow load calculator, you can input these values as shown in the image below. You may want to do this if you wanted to calculate a snow load with different conditions than the rest of your design. The values for the rest of the load calculations will be taken from Project Defaults unless the override option is set to “Yes”.

Roof Properties

The horizontal distance from eave to ridge is equal to half of the building length, which we can input as ‘(21ft+4in)/2’, and ClearCalcs will calculate that to be 10.7 ft. Our roof pitch was indicated on the drawings as 2:12, which calculates the roof angle as 9.46 degrees.We can leave our roof surface type as “obstructed or non-slippery surfaces” and we can remove any additional properties for snow drifts. We’ll come back to those later.

Results

Our balanced snow load is found as 21 psf. ClearCalcs also calculates unbalanced snow load for a rafter system, which is 25 psf. For this example, we would use the balanced snow load of 21 psf. In other scenarios you might instead need to use the unbalanced snow surcharge for general cases, where we would add a rectangular surcharge load (9.43 psf) to a width of 8 ft, 9 in. Unbalanced snow loads do not need to be considered for slopes > 30.2° (7:12 roof pitch), but since our roof pitch is 2:12, we do need to consider it. These loads can now be used in any of your other designs.

Consider Snow Drift

For the second part of this example, let’s consider the same scenario but with an additional rooftop unit. The roof joist design would require a triangular load added to our existing snow load. To add this, we can go to our inputs for snow drift next to projections and parapets. Our length of roof is from the edge of the building to the rooftop unit, and our roof step height is indicated on the drawings as 84.7 in

In our results, we now have a section for snow drift loads. The important values that we may want to consider in our design would be the snow drift pressure, which is the additional pressure caused by snow drift, to be added on top of the existing balanced snow load (8.74 psf), as well as the width of snow drift. For a joist design for example, we would apply an additional 8.74 psf for a distance of 2.03 ft from the edge of the rooftop units, on both sides.

Switching to the joist calculator, you can input the triangular loading based on the tributary width. Adding to our balanced snow load on the roof, we can set the load from 6.97 ft (9-2.03 ft) to 9 ft. To replicate the triangular load, we can use a starting tributary width of 0 ft and an ending tributary width of 4 ft, equivalent to the joist spacing. Our snow load magnitude will be 8.74 psf, as defined by the snow loads calculator. Our final loading on the roof joist will be as follows: