Definitions
These are the definitions of some terms used in this program and blog. These definitions are here to explain Briwood’s nomenclature. Different companies sometimes use different terms and phrases for the same thing.
For really accurate definitive definitions see the definitions in your region’s building code and handbook to steel construction.
To help orient the user the diagram below gives a basic idea of the parts of a joist. The image is taken from a drawing of a typical 10 m variable panel, variable offset joist with some annotation:
And the same, with a more 3D view:
Design Length
The design length of the joist is the distance between the central points of support for the joist. The actual joist will have top chord extensions that go beyond this length, typically 100 mm. For beams and walls this ‘working point’ should be in the centre third of the width of the beam or wall. A structural engineer should specify this.
Overall Length
This length includes the top chord extensions that are added to support the joist’s shoes. It does not include any cantilever lengths.
Top Chord
The top chord is formed by two hot rolled angles usually separated by the web material or spacers in Briwood’s designs. This program currently assumes that at the mid-point of every panel there will be a spacer.
Top chords can be made from other materials (HSS, etc.) Briwood does not support this at the present time.
Top Chord Extension
This is the bit of the top chord that goes beyond the working point to help support the shoe. Typically this is 100 mm each end.
Bottom Chord
Two hot rolled angles form the bottom chord of Briwood designed joists. Usually there are no separators in the bottom chord or reversal members except where reversal forces require them.
Bottom chords can be made from a great variety of structural materials as the bottom chord often works only in tension. Occasionally joists will be used in locations with reversal point loads, end moments, axial loads, or uplift that cause separators or even top chord panel point to bottom chord reinforcement.
I’ve seen rods used as a bottom chord! Briwood currently only supports hot rolled angles.
Bottom Chord Extension
This is the additional bit of the bottom chord that goes beyond the bottom chord panel point, typically 150 mm on each end. This material is used as space to weld the first panel’s web to the bottom chord. It is also used as a useful handle when moving the joist around in the shop, transport and erection. Some joists will have one angle or both extended close to the wall or beam. These can be used to hang things, like false ceilings, banners or lights from the joist near the beam or wall. Sometimes the chord will be extended to the beam or wall to transfer end moment forces into the joist.
Shoe
The joist has shoes at each end of the joist. They are designed to take all the loads from the joist and transfer the forces to the beam or all the joist sits on.
Typically a shoe will be 150 mm long and made from a pair of angles as in the image above. However, some shoes might be a simple plate welded to the chords or other design.
Joist Depth
The depth of the joist is the distance from the top of the top chord to the bottom of the bottom chord.
Effective Depth
The Joist Depth minus the st and sb of the chords. The st and sb heights are the imaginary lines in the chords in which the summary of the forces act.
Panel Parts
In the annotated image above the panel points are labeled. At Briwood we refer to the first panel as the end panel, or panel one. Often the end panel is different from the remaining panels in the joist, even in equal panel joists.
Panel Length
The length between the points where the web meets the top chord forms the panel length. It is the point where the compression and tension members meet in the web.
Offset
This is point where the compression web and tension web member meet at the bottom chord, subtracted from the panel length for that panel. This can be 0 to typically ½ the panel length.
Tension Member
The tension member takes the sheer tension in the web. Under normal conditions this will be the outside material in a panel, the first bit of material to connect the top chord to the bottom chord from the end of the joist in the first panel. The member can also be subjected to compression under reversal. This member can be made of rods, HSS, angles or other materials. If made of rod one or more sections of the panel can be made from one rod, in some cases with proper equipment three whole panels can be made from one rod.
Compression Member
The first compression member usually goes from the bottom chord to the top chord completing the first or end panel of the joist. This member will be in compression all the time. It can be made of the same materials as the tension member.
Loads
Only loads that are currently supported by this program are discussed here.
Uniform Load
This is a load that is constant across the joist. Measured in kN/m, a linear load. An example would be the concrete floor supported by the joist. Uniform loads come in three main types: dead loads, like concrete, or roof materials; live loads, which are furniture and occupants that move around or can be removed and snow loads which are the loads created by snow pileup on the joist.
Uniform Uplift Load
Wind can cause the joist, and the building to act like a wing. This causes reversal loads as the roof of the structure gets lifted. Measured in KN/m, a linear load.
Total Load
This it the total of the Uniform Loads (Dead, Live, Snow) that press down on the joist.
Point Load
Point loads are concentrated loads that occupy a single point on the joist. They can come from mechanical devices, like air conditioning units, decorative fences, and supporting struts for things like tanks, etc. Unlike uniform loads, a joist can have many point loads.
The load can be live, or dead. Roof units are dead loads. A point load created by a hanging conveyor belt support would be composed of both a live load part and a dead load part. Similarly so for a strut supporting a water tank.
Point loads are entered in KN. They have to be located on the joist in mm from the left end. Reversal point loads, though rare, do exist.
Point loads can be located on (no local bending allowance, NLBA) or off panel points (local bending allowance, LBA). This means that they can sit on the actual panel points of the joist, or sit on a reinforced part of the top chord, where a rod or some other material is extended from the bottom chord panel point to the top chord. When the joist sits on a panel point or reinforcement they are not considered in the moment distribution.
When the point load isn’t supported it has to be considered in the moment distribution. This can create a much heavier joist, because the point load (unless really light) will cause a bump in the moment forces in the panel which has to be distributed out. It also makes the top chord heavier, supporting this one heaver loaded panel, and messes with the panel optimization algorithm as the moment distribution pattern isn’t even throughout the panels.
Moment Distribution
Assume your standing beside a long handrail supported at 1 metre intervals. Between two of the supports you press down on the rail with all the force you can muster. A light, decorative handrail might show the impact of the moment you create (as a point load) in the next segments (panels) of the rail. Your segment will be pushed down, the segments on either side of you will bow up as the rail distributes the force you’ve created through the rail and supports to the ground. This is the essence of moment distribution.
The forces on the top chord create the requirement for a moment distribution. Briwood uses the distribution method to solve the equations.
The results of the moment distribution are presented in the top chord analysis on the Mf line in kN•mm. The results of the moment distribution are used in the analysis of the top chord.
Note that in the Canadian building code joist panels 610 mm or less in length do not have to consider the results of the moment distribution in the top chord analysis.
Joist Types by Purpose
Joists are designed for certain purposes. There are three main ones, being roof, floor and classroom.
Roof
These joists typically have a live load deflection of 1/240th of span. The low deflections mean lighter chords and a bouncier joist that is unsuitable for occupational use and the floor will be bouncy and feel unsafe to the occupants.
In Canada a roof joist will typically have a snow load instead of a live load. Snow loads have a different factorization then live loads.
Floor
These joists usually have a much stiffer deflection requirement, normally 1/360th of span. This will make the floor feel nice and solid under the occupants and have very little bounce. Some floors will have stiffer deflection requirements due to the activity of the occupants or the floor’s use. This higher deflection often governs the chord selection, making the joists substantially heavier then a similar floor joist; on the other hand there will be a reduction in the number of panels in the joist, lowering the amount of labour to manufacture it.
Classroom
Typically classroom joists have a deflection of 1/320th of span, in between the floor joists and roof joists. Since classroom loads are usually lighter and office equipment isn’t present the bounce of the joist can be a bit higher.
Panel Patterns
There are four panel type patterns described below.
Variable Panel
Here is a typical 10 m long variable panel joist. The person graphic gives a sense of scale.
A closer view of the end panel:
And lastly a close up of the shoe detail:
This joist is a variable-panel joist made with rods in the web. Some approximations have been done: all the rods are 24 mm diameter instead of the sizes specified by the design and the sizes of the angles have been rounded to the nearest whole millimetre.
The variable-web panel joist uses material most effectively for short span joists.
Pratt
When designing large joists the Pratt pattern is popular. It simplifies the cutting of material as all the web compression members have identical length. As can be seen from the image above the offsets have been set to zero.
The draw-backs with this panel pattern are that web material is not used ideally, the joist must always have a even number of panels and the design isn’t ideal when experiencing high reversals. Pratt pattern is often used in equal panel joists; when Pratt Equal panels are used the web compression and the tension members each have the same length. Sometimes the end panel will be an exception, having a longer tension member. On some jobs saving labour has more value then saving materials.
Warren
Warren panel joists have their offset set to half the width of the panel length.
The Warren pattern is often used when pipes, air conditioning and heating ducts go through the joists. The pattern gives useful wide gaps where the duct work can be threaded through the web easily. This pattern is also used when the joist experiences large reversals caused by wind, earthquakes or other forces. The draw-back with the Warren pattern is that the compression material has to be heavier than it would be in a Pratt or variable panel joist. You might notice that many joists have a Warren pattern in their centre panels as it is good for uplift and handling reversals. If the joist has top and bottom chords with the same characteristics the joist could be installed upside down! Apart from the end panels the joist can have the same strength and characteristics either way up, which is not the case for Pratt or variable panel joists.
The equal panel Warren pattern is also popular because, if bending rods, the bending machine only needs to be set to one angle for all the web members, with possibly, the exception of the end panels. If cutting material, all the cuts are the same for the entire web.
The image above shows the centre panels of the 10 m joist where a Warren Pattern has been used.
Modified Warren
The Modified Warren pattern adds supporting material in the middle of each Warren pattern V. This supporting material allows the panels to be longer, thus reducing the number of panels in the joist. Usually Modified Warren joists employ equal panel lengths to simplify the cutting of the material in the joist. The main drawback with this design is that the extra compression members do not do much work. The compression side the V, also, can not be optimized.
Modified Warren pattern joists are popular for ‘Joist Girders’, which are large long-span joists that support smaller joists at right angles to the main joist. This system is popular in the U.S.
Joist Type by Material
Steel joists are classified into two distinct groups; Open Web Steel Joists (OWSJ) and Long Span Steel Joists (LSSJ). The differences between the two are discussed below.
Open Web
Open web joists typically use only solid rod material in the web. They are typically less then 20 m in length and less then 1 m in depth. The joist examples presented above for the different panel patterns are all open web joists. Presently this joist program can only design open web joists.
Long Span
Long span joists are deeper and longer than open-web steel joists. Typically, long span joists are over 1 m deep and have a length that varies between 20 and 60 m. Their additional loading and size pushes the stresses so high in the web that rods can not handle the forces. Other material has to be substituted to handle the forces. In the typical Canadian joist angles and HSS sections are used.
The picture below shows a short shallow long-span joist, with HSS tension members and angles in compression. Note that no shoe has been added to this joist.
At the present time long span joist design isn’t enabled in this design program.
Materials
Various structural steel sections are used to manufacture joists. Some are described below.
Angles
Angles are used in the chords and in the web. Typically they are manufactured through a hot rolled process. Cold rolled angles can be used too, but are not set up in this program. Usually they are used in pairs except when use for special reinforcement. Below is a typical hot rolled angle:
Angles come is a large variety of sizes from 25 mm on each side to over 200 mm on each side. The sample presented in the image is a 50 x 50 x 5 mm angle that is 150 mm long.
Angles are not enabled in the web at the present time.
Rods
Rods are usually used in the web of the joist. They vary in size from 13 mm to 32 mm in diameter. Here is a typical rod:
Hollow Structural Steel Section (HSS)
HSS are usually used for the web of a joist, however, sometimes they are used in the chords. HSS come in a huge variety of sizes. Typically in joists the sizes vary from 40 x 40 mm to 100 x 100 mm. Here is a typical HSS section:
HSS sections are not enabled in the web at the present time.
Other Structural Shapes
In the production of Joists many other structural shapes have been used. Rods have been used in bottom chords, a number of companies have created their own custom chord shapes. Briwood’s software will be able to accommodate them all eventually if there is sufficient demand.