Joists – The Basics

If you’ve stumbled across this site and are wondering what this is, here are the basics.

In a nutshell this site allows the design of open web steel joists…

What is a Joist?

Here is a side on view of a open web steel joist, with a graphic image of a man to give scale:

A closer view of the end panel:

And a close up of the shoe detail:

Finally here is a image of how a joist looks to the typical person (however the shoes are normally hidden). This view is from below:

A joist provides support for a floor or roof of a structure. They come in two major forms: wood joists and steel joists.

Wood joists are common in residential construction. You’ll typically find them supporting the roofs of buildings, these are often A frame like structures. Occasionally you’ll find wood joists supporting floors but usually this is done with a wooden beam, either a sold piece of wood (a 2 inch by 6 inch mounted vertically) or compound beam made of plywood.

Steel joists are found in commercial buildings or large structures. They take on certain roles were beams can be used. Joists are lighter then beams and have to be manufactured, using them plays on the trade off of labour vs materials. Joists can range in size from 2 metres long to 60 metres long. They can vary from 200 mm deep to about 3000 mm deep. They can be designed or selected for a wide variety of loads.

This program designs steel joists.

You’ll see steel joists in large box warehouses, hockey arenas, schools and shopping malls. Next time your in Home Depot, Costco, Canada Tire, or your local hockey arena look up. Typically there will be joists supporting the roof. In schools you’ll often find them exposed supporting gym roofs.

Formal Definition

The definition below is taken from the Canadian Institute of Steel Construction booklet Steel Joist Facts… Recommended Practice, 2nd Ed.

Open-web joists or joists means simply supported steel trusses of relatively low mass with parallel or slightly pitched chords and triangulate web systems proportioned to span between masonry walls or structural supporting members, or both, and provide direct support for floor or roof deck.

Joists vs Trusses

What is the difference between a joist and a truss?

In a truss the moment distribution forces are carried through out the entire truss. Hence the web, and the bottom chord all have to be designed with respect to the moment distribution forces throughout the truss. The web members of the truss are considered to be fixed to the cords and able to transfer moment to the rest of the truss. A sophisticated analysis must be done to fully comprehend the forces in a truss.

A joist only considers the moment distribution through the top chord. The moment forces are not analyzed in the web or the bottom chord. Essentially the web are considered to be ‘pinned’ to the top and bottom chords, and can rotate around their pinned location and thus are unable to transport moment force into the web a bottom chord. This greatly simplifies analysis. It also simplifies optimization, allowing a software package like this one to optimize and select material for maximum utilization.

The most optimized design is the variable panel, variable offset joist. In this design we can use all the material to the fullest extent, sometime achieving optimizations of 100, (to a stated overage limit) while still meeting code.

Design & Engineering

A typical house or small walk up apartment building only requires an architect to create the plans; the building code (at least in Canada) covers all the details to the point were the architect and builder just has to follow the rules to build a suitable structures for the local environment. For any structure larger then a shed or typical house you’ll need a certified structural engineer. They will design the overall structure and do the basic material selection (beams, columns etc.). The actual detailing might be handled by the shop that is manufacturing the structure. Amongst this detailing would be the joists.

Typically the main structural engineer would not be designing the joists; just specifying the loads for them.

In the United States the joists would be selected by calculating the equivalent tabular maximum factored moment and shear on the joist and selecting the appropriate design from a catalog. This is not how this program works.

In Canada joist selection can be done; there are a number of large companies that support this.

But in Canada a second alternative has been created and used for many years, at least since the availability of the mini-computer (the Wang computer was popular), which is design build¹. In this scenario the structural steel manufacturer takes on the role of designing and manufacturing the joists.

Most structural steel shops will have an engineer on staff or as a consultant that will verify the loads and either assist in or approve the design of the joists. For the actual input of the loads in to the program an engineer or senior draftsman will do the work.

The takeoff from the design drawings etc. is beyond the scope of this document. I give minor advice in this blog about designing joists, basically its information that I’ve picked up over the years. I’m not an engineer, though I understand the basics having studied a bit of physics and math in university.

A Certain Ratio

When designing joists one should stay within certain input ranges and ratios. In general:

For short span joists the ratio of length to depth should be approximately 1/20th to 1/12, with 1/15th or so being optimal.

For long spans the ratio can range from 1/30th to about 1/10th. Note that the program’s maximum permitted depth is 3000 mm (when long spans are implemented), which shouldn’t be used for joists shorter that about 35000 mm long.

The heaver the loads usually the deeper the joist.

The deeper the joist the better it handles deflection. Also the lighter the chords.

At some point some research and numbers on this subject of joist depth/deflection will be done.

For the exact limits on maximum loads and dimensions of the program currently please see the Design and Materials page. These ranges will change as features get implemented and/or turned on.