Background Information

In this example, we’ll go through the design process of open web steel joists on the roof of a single storey commercial building. Our building will be located in Chicago, IL and will be designed with a K-series joist.

Scenario 1: Roof Steel Joist for Single Storey Commercial Building

The steel joist (ASD) calculator uses SJI 100-2020. As shown in the image below, we will focus on the design of joist J1.

If you prefer to follow along with a video, check out a clip from our webinar!

Given Information

 
• Length = 21’-4”
 
• Spacing = 4’

We will consider uniform loads as follows:

 
• Dead Load = 10 psf
 
• Roof Live Load = 20 psf
 
• Ground Snow Load = 25 psf
 
• Flat Roof Snow Load = 21 psf (calculated from the above value, using the Snow Loads Calculator)

Entering your Properties

Key Properties

The joist span length of 21’-4” can be inputted into the Key Properties as “21ft+4in”.

For now, we’ll leave the designation as a 16K5 joist, and can determine the most efficient section after we input our loading scenario and design criteria.

Loads

Our center-to-center spacing (or tributary width) is 4 ft and all of our uniform loads can be defined within the distributed loads section.

Given that each load is a uniform load, the load end location will be at the end of the beam (L, or 21.3 ft) and our total tributary width at the start and end is both 4 ft.

Design Criteria

As per the IBC 2018, we can keep our deflection limits as L/360 for the span criteria.

Here we can see that our design is pretty low in utilization (governed by 35% utilization in bending and 36% in shear), so let’s find a lighter joist section in order to optimize cost savings and member depth.

To do so, we can enter our member selector next to “Designation” in the key properties. We can see that a 10K1 section is not passing, but a 12K1 is governed by 75% utilization in short term

Results

For our steel joist, the final passing design will be with a 12K1 section. This section has a lighter weight and a shorter depth than the 16K5 section we originally designed our joist with.

The utilization is checked at various locations along the beam's length. For this example, we can see the results shown below. Each envelope is determined and plotted with the graphed load combination.

Scenario 2: Roof Steel Joist with Additional Point Load and Snow Drifts

Background Information:

Taking the same example as above, let’s try to design our steel joists with an additional rooftop unit. We’ll need to consider the point load for our unit weight, as well as snow drift from piling next to the rooftop unit.

Key Properties

Loads

Since we’re designing the same single story commercial building, we can keep the same design as before.

We will add the weight of the rooftop unit by including a new point load. Our rooftop unit weighs 2000 lbs, however since it spans over two joists, we can divide this weight by two and use a load of 1000 lbs.

Other things to consider is the location, we’ll assume it’s 9 ft from the end of the joist.

However, now our results show that we’re well over capacity (179% utilization governed by shear). We can see on the graphed load combination where exactly the demand is over the envelope.

We can go back to our member selector and see that we could use a 12K5, or even a 14K4 for a lighter weight of 6.7 plf, or even a 16K3 that is 6.3 plf.

Let’s say we want to keep our 12” depth, a 12K5 section will be sufficient. The summary is as follows.

Snow Drift

If we wanted to get more accurate with our design, we should consider the snow drift next to our rooftop unit.

This calculation is quickly covered in the webinar, however we can expect the snow drift to add a snow load of 8.74 psf at a width of 2.03 ft.

For our tributary width, we have a triangular load as the snow piles, so we could use 0 ft at the start and 4 ft at the end to replicate that. It is best practice to consider snow drift on all sides of the rooftop unit.

Our snow drift under the distributed loads are as follows.

We can see that our utilization is still governed by shear capacity and the design still passes! Our new design will use a 12K5 joist section.

Again, for a video walkthrough, check out the video from our webinar that covers both of these examples.