Designing Joists

Lets get designing. To start agree to Briwood’s Terms of Use and Privacy Policy by clicking on the radio button as can be seen in the screen shot below:

Next enter your email address. The completed designs will be emailed to you when finish designing. Please put the correct email address – an incorrect email address will cause the designs to be sent to the ether and there is no way to recover them.

Then press the Start Designing button. You will be transferred to the joist entry screen, seen below:

Start by entering a Name for the joist. This is also known as the joist’s tag. Typically it will look something like ‘J1’.

Note that in this version of the code you can’t enter joist counts. This will be added to the web site at a later date.

The program, created for the Canadian market, uses only metric units.

For the possible number range for each of these input fields, check out the Design Characteristics link on the Website’s main page. The ranges are subject to change as the software advances.

Dimensions

On the line below enter the joist’s design length, from working point to working point in millimetres (mm). This length should not include the top chord extensions, shoes etc. Then enter joist’s overall depth, again in mm. The default top and bottom chord extensions will be applied to the design. Later versions of he software will allow user control over the extensions.

Other Joist Characteristics

Below and to the left of the dimensions one can enter the type of joist. The options are Roof, Floor, Classroom and Other. All of these types will cause the correct Minimum Concentrated Load to be applied to the joist, and set the basic Live Load Deflection and, if required, change the live load to a snow load.

When Other is clicked the following dialog appears:

Here you enter a joist type, as a string (for instance Floor for Passenger Cars) and the load, which in this case (Article 4.1.6.4), would be 9.00 kN.

Following that, the live load deflection can be entered as a fraction of design length.

Lastly in the left column one can choice the web pattern for the joist. It can be one of Variable (where the panel offset can be set to maximize the use of the web compression material). Warren, where the offset is set to ½ the panel length, or Pratt which has 0 length offsets. Variable suits most short span joists, Warren for joists with high uplift loads or larger pipes going through the web, Pratt typically for long span joists.

Loads

All joists have a basic uniform load, even if the joist supports nothing it has to support its self.

The web of the joist, being symmetrical around the centre line, forces the shear design forces to be symmetrical around the centre line. Hence the raw shear can not be used to design the web. A envelope function must be used to ensure that the the worst shear from each end of the joist gets used. Also some forces on the joist can reduce the normal shear. These reductions aren’t always present on the joist, the simplest case are point loads.

To make it obvious what these envelope functions look like graphs are presented below so that you can see what the envelope function actually looks like.

Uniform Loads

On the left there is a column of load entry fields.

The loads start with the uniform dead load, which is entered in kN/m. Typically the value is entered to two decimal places.

Below that comes the uniform live load, or snow load if the joist is being used for a roof. Again kN/m are used.

Following that is the uniform total load which is the total of the uniform live and dead loads. This value is for display and not entry.

Lastly comes the uniform uplift. For most roof joists a value needs to be entered, for floor joists there will be no uplift. Use kN/m for the entry.

Uniform Load Shear Graphs

Below are two graphs presenting a typical uniform load case. The first graph shows the normal unmodified shear and the second the design shear with the unbalanced load and imaginary panel point loads.

Uniform Reversal Loads

Roof joists experience one force completely foreign to floor joists, which is wind, a reversal load. Wind can rip the roofs off buildings and do extensive structural damage.

Briwood handles the Net Factorized Uplift differently. It takes the input and reverses it back to a gross load then refactorizes it with the existing loads.

The reversal wind load is entered in kN/m.

Uniform Reversal Load Graphs

For uniform reversal loads only the uniform dead load is considered resisting the load. There can be a point during construction when the joist is up and the uniform dead load is applied but not any roof units etc. Hence with a heavy enough uniform dead load one might not get any uplift reversal in the joist.

The chart below shows what happens when the dead load gets doubled. You’ll notice that the red line for the final combined design force never crosses the zero kN line. No uplift at all from wind on this joist.

Panel Lengths

Along with the panel pattern type the joist designer has some options on selecting the length of the joist’s panels. Many Canadian designs use variable panel lengths; the end panel often being different. The remaining panels are designed in pairs with the exception of the center panels, which depending on if the joist has Variable, Pratt, or Warren panels can have one to four centre panels.

If you select Equal Panels a modal dialog box appears:

The Equal Panels dialog box presents a number of options:

You can enter the number of panels (Panel Count) or a panel length. The panel length must be divisible into the length of the joist. When a special end panel is present the panel count will be divided into the total length of the joist minus the end panels.

To enter a special end panel length click the check box and enter the panel length in mm.

When you’ve done all the steps click the Check Equal Panels button. It will check the panels and give an O.K. if everything fits. Otherwise it will give a basic error about the panel set up.

Remember that even having selected the panel lengths and the test approving them does not mean the joist can be designed adequately and/or manufactured. Material selection might not work.

When your finished click Done or Cancel.

Special Loads

At the time of writing there are three special loads available. Point loads were added first to test the finding of zero shear, reactions, moments and the moment distribution. Additionally we now have additional uniform loads and moving point loads.

To enter any special load start by clicking on the clear blue button named for the load type you want to enter and a table with instructions appears. Above the table will be an Add button. Click this and a dialog will appear to collect the information on the load. When you have finished filling in the dialog and closed it the load will be added to the table. This whole process is described in detail for adding point loads.

Point Loads

To add point loads click on the Point Load button and a table for the point loads will appear. It will be empty when you start. You may notice that all the inputs you made above will freeze and lock. This ensures that all the checks on the point loads you’re about to enter can be run.

Above the table is some basic information about using the point load table. There are two buttons: Add to add new point loads and Close to close the table.

Pressing Add will open a modal dialog allowing the entry of the point loads:

The load is entered in un-factored kN, the location is entered in mm from the left end of the joist. The location has to be on the joist.

The load can either be a dead or live load. A dead point load is one that is constant and will never change, like a roof unit¹. A live point load is one that will either be removed and added repeatedly or a load that varies over time, like a support strut for a water tank.

One must indicate if load is going to be on a panel point. This could be an actual panel point or a reinforced part of the joist, usually by extending a member from a bottom chord panel to the location of the point load on the top chord. In the output these are noted as Local Bending and No Local Bending. For loads hanging off the bottom chord, they should be on bottom chord panel points or be reinforced to a top chord panel point.

If the load isn’t going to have reinforcement one can lock it in the centre of the panel for the moment distribution and top chord analysis by selecting mid-panel lock.

When all the information has been added close the dialog box by either clicking Add / Change or Cancel the addition or changes to the load by clicking Cancel.

Here is a filled example of a Point Load dialog box:

Here we have a 5 kN point load dead load located 2000 mm from the left end of the joist that sits on either a panel point or reinforcement. Note that the Load and Location have a green surround indicating that they have passed their respective range tests and can be used in the design. When a field does not pass a test will receive a red surround and a error message in red.

Once you’ve created your loads they will look like this the Point Loads table:

Each row of the table starts with a Modify button that allows the designer to change the load. Then the load in kN is presented, followed by the location in mm. Note that the loads are typically entered from the left most first. Then the type, Live or Dead followed by if the load is on or off a panel point. Then it is indicated if the load is considered to be locked in the centre of the panel for top chord analysis.Lastly comes a red ‘X’ button that can be used to delete the load.

Below all of this is the Close button which will close the table.

Point Load Shear Graphs

End Moments

To add end moments click on the end moment button. The list holder for them will open and then click add. You’ll see this dialog:

End moments are forces that run through the end of the joist. A pull or a push through the chords with the opposite force occurring each chord. They are created from dead loads, live loads, wind loads and earthquake loads. With this program you can have multiple end moments, in other words you can combine both wind and seismic end moments.

For the end moment to be applied to the joist the end experiencing the end moment must have a bottom chord extension that joins the wall or column that supports the joist. The program can handle single end and double end moments.

When testing the program a number of beam analysis programs are used. No standard convention exists for end moment forces. Hence in this program a positive moment puts and downward force (positive moment) on the joist with a resulting negative shear.

Note that a negative end moment will not reduce the normal forces on a joist nor will a positive end movement reduce the reversal forces on the joist.

Reversible End Moments

When the program has both end reversible end moments the end moments should be entered with opposite signs. This tips off the program to use the correct shear function. If you click on the ‘Yes’ reversible radio button the loads will be forced into this configuration.

The graph shows that the end moments force from both ends start at 6.25e7 kN and drop to zero. Then the end moment drops off the graph and becomes a reversal moment. Hence it isn’t considered a normal force pressing down on the joist.

This pattern is repeated for the reversal loads on the joist.

Wind and seismic loads are automatically reversible.

Additional Uniform Loads

Some joists have more then the standard uniform dead load, live load or snow load. The latest building code allows for uniform loads created by dead, live, snow, wind, storage, liquid and soil loads. These loads can be combined and even added in multiple times.

To add and additional uniform load click on the blue button marked Additional Uniform Load. A empty table will open. The table will show the additional uniform loads entered; the table below shows two additional uniform loads:

Screenshot from 2022-06-19 10.34.54 Too add a additional uniform load click the Add button above the table. The dialog below will appear:

Enter the unfactorized load in the Load input box and then select the load type from the radio buttons below.

Click on the Add/Change button to save the load or cancel not to make the change.

Moving Point Load

Some times when the joist is being designed or estimated the locations of the point loads from roof units and other items to be supported isn’t known. In this case using a moving point load will help. In this version of the code all moving point loads must be reinforced or be on panel points. One can enter multiple moving point loads but the code only allows for dead and live, so there isn’t a reason to have more than two. When you add moving point loads take the total of the loads of a given type (dead or live) and multiply by 1.5 to ensure that the centre of gravity of the object is accounted for¹.

To add a moving point load click on the Moving Point Load button bringing up the moving point load table. An example of a filled table is given below:

At this point click on the Add button and the Moving Point Load dialog will be shown:

Enter the unfactored moving point in the text box in kN and select if the load is live or dead.

When ready click on the Add / Change button to add or change the load, or click Cancel to not add or change the load.

Moving Point Load Shear Graphs

Snow Pileups

Snow pileups occur where snow collects behind an obstacle on the joist. Often these obstacles are walls or roof units.

To add a snow pileup click on the snow pileup button after the main dimensions and loads have been entered. This dialog box should open:

Snow pileups can face left or right, but the face must be on the joist. The end of the snow triangle can be off the joist.

Strip Loads

Strip loads are a variant of uniform loads, but the do not cover the entire joist.

To enter a strip load click the strip load button and add a load. You will enter the loads starting location and ending location, the load in kN/m and then select the load type, which can be Dead or Live.

Design & Done

Before starting the design process your screen should look something like this:

The green outline indicates everything is satisfactory in each field. If some thing isn’t right it will be outlined in red with an error message. Passing these tests only indicates that the data entry meets requirements; it does not indicate that the joist can actually be designed or manufactured.

Below all the special loads is the Design button, large, blue and screen width. When pressed the information entered is sent to the server and the joist designed. The results of the design will be emailed to the email address provided at the beginning of the process.

After you’ve created your last design a new design will appear. To finish and not design this joist click the wide Done button at the bottom of the window. This design will not be designed and the joists you have designed will be emailed to you.

You will know when this has happened when you see this screen: