Reading the Joist Design Output

For most engineers familiar with designing joists the design output should be straight forward. Just to make everything clear this document covers what exacty each part of the joist design output covers. We’ll start at the top of the page.

The very top of the page gives the joist design number or tag, so it can be identified. In later versions of this project will add more information here to uniquely identify the joist. Right below that appears the Dimensions heading and the beginning of the joist’s information.

Dimensions

Below is the first part of the design showing the dimensions of joist:

After the title is the indicator of what kind of joist we have, in this case we have roof joist, which influences the use of building code clause 16, next to that we have a indicator of the importance factor, which in this case is normal. Presently in the joist program the importance factor can not be controlled by the user.

On the line below we have the design length, which is the distance from one design working point to the other, next comes the total depth of the joist. Both these figures are given in mm.

Lastly the user input deflection requirements as ratio of span. Usually the live load deflection gets specified, and it gets presented as a fraction of span, then the total load deflection gets presented. In this case no total load deflection was given. At the present time the program does not allow the entry of a total load deflection.

The specified loads show the loads before factorization. Always present will be a uniform dead load and a live load or snow load. Uplift will be presented if it exists. Along with the dead and live/snow load there will be a total uniform load presented. Uniform loads are presented in kN/m. Special loads will also be presented in this block, as discussed below.

Special Loads

Loads that are not uniform are considered as special loads. The will be presented immediately below the uniform loads.

Point Loads

In the example above two point loads are presented. One dead and one live. The first dead load is 5 kN at 2000 mm away from the left end of the joist. The load sits on a panel point and does not influence the moment distribution as it has No Local Bending Allowance (NLBA).

The second point load is of 10.5 kN at 3500 mm from the left end of the joist, 1500 mm further in from the first load. It isn’t on a panel point so there is a Local Bending Allowance ((LBA).

Note the point loads with a Local Bending Allowance will be treated as though they sit in the middle of whichever panel they reside in. This gives the maximum impact in the moment distribution. This feature may change in future versions of the program.

Maximum Moment & End Reactions

Here we start to get an outline of what the actual factored and un-factored forces on the joist are. Starting with the factored forces, showing the left and right factored reactions in kN. This number includes the importance factor and takes the highest of all the factorizations present at the left and right ends of the joist. The maximum moment is taken at the point of factorized zero shear, which is given in mm from the left end of the joist.

Zero shear is found through a zero finding algorithm; all factorizations are tried and the maximum moment found for each one, then these are compared and the highest moment is selected and its zero shear is output. This zero shear and maximum moment are used to find the chords etc. The moment is presented in kN metres.

Below the Un-factored forces are displayed. Again the zero shear and maximum moment are found.

Due to factorization the maximum moment and point of zero shear will often be different from the un-factored when more than uniform loads are present. When designing the joist the shear and moment at each point on the joist will be influenced by the factorization; the factorization in panel one for shear and moment will be different from the factorization used in the centre panel. In some places the factorization for moment will change from mid-panel analysis to panel point analysis.

Chords

Below the Moments & Shears comes the chord selection, which is influenced by both the moment and the deflection requirements for the joist.

Presented first is the top chord. At this time the program can only use angles for chords. They are presented as T. 2 L’s, meaning Top 2 Angles of size in mm.

After the size we present a string that indicates if the long leg of the joist is vertical or horizonatally orientated. This orientation is presented as:

    LLV - Long Leg Vertical
    LLH - Long Leg Horizontal

For most joists with unequal leg lengths the top chord will have the long leg vertical and the bottom chord long leg horizontal. For the top chord this helps the effective depth and for the bottom chord having the long leg horizontal will reduce bridging requirements. Next comes the area of the angle in mm squared, flowed by the effective area of the angle in mm². The effective area is determined by the Fy of the angle, which is presented next in MPa (megapascals). Note that all the values for the angles are calculated and not book values.

Below this line the bottom chord get presented in the same manor.

Then comes some really useful information starting with the effective depth of the joist which is the total depth (mm) of the joist minus the st and sb of the angles forming the chords. Next comes the joist’s inertia in mm⁴ followed by the calculated camber in mm.

On the line below the actual deflections are presented, with the deflection in mm and as a fraction of span. Both the live/snow load and total load deflection are presented.

Panels

Only the left side of the joist is shown.

Beside the heading for panels the total number of panels across the joist and the number of centre panels is presented. Below this is a heading line for the Panels, which indicates the panel numbers, the end panel being panel one.

The next line shows the length of the panels in mm.

Next comes the offsets in mm. The offset is measured from the end of the panel outwards to the end of the joist. In Pratt joists the offsets will all be zero, in Warren they will be half the panel length. Presently the program doesn’t do Modified Warren designs.

Last line of this section covers the web material. When the web material is just a rod for the panel the rod thickness is given in mm.

For non rod material in the web a first letter code will be used to indicate the material type:

    H - HSS
    A - Angle
    R - Rod

At the present time no other material types are supported, and HSS isn’t enabled in the online program. The details about the material selected are presented near the bottom of the design page.

Web Shear Analysis

Following the material the web gets analyzed for shear. The analysis is in a horizontal format that matches the panels. Below is an example of the typical analysis:

Right beside the web analysis title is an indication of the type of joist we have, which can be one of:

    Variable Panel
    Pratt
    Warren

Following that is a statement about the strength of the web rod material, in MPa again.

The first line after the title line indicates the shear to the normal of the panel (Vn) in kN.

Below that is the Compression Capacity (Cmp. Cap) in kN. In the sample above you’ll notice that the compression material is slightly lower strength then the Vn force. This is because the program has an over design limit, which in this case is set to 1.005 or ½ a percent. If we take the capacity of the compression material and multiply it by the over design limit we will find that the material is acceptable.

    154.39 (kN) x 1.005 = 155.16 kN

Later, when there is time the over design limit will be able to be set by the program user. It will likely have a range of 0, to 1.5,, with a recommended default of 1.25,.

Below the compression is the tension site’s tension capacity in kN.

Following along is the Vr from any reversals, uplift and unbalanced loads, again in kN. The line below gives the reversal capacity in kN. The reversal capacity of the compression side isn’t given; it is guaranteed to be greater then the compression force.

A few things to note about the shear design in a Briwood joist:

The highest shear from all forces is taken to design the web, including unbalanced loads.

If a positive shear from any load goes negative on the left side of a joist, it is treated as zero and is never subtracted off the positive shear. Thus, if you have a point load that causes a reversal in shear near the centre of the joist the negative shear isn’t considered, hence the design acts like the load doesn’t exist. The classic case of this is a building under construction when the roof units haven’t been installed yet and a snow storm loads the joist; the point loads from the roof units would reduce the shear experienced near the centre of the joist, but the design doesn’t consider that in this case.

In reversal no shears will be considered that reduce the total normal shear on the joist. A for instance of this would be a reversal point load on the joist that reduces the normal shear; the joist will be designed to resist the reversal loads in reversal but for normal shears the program assumes the negative point load doesn’t exit. The reversal might be a live load, meaning it isn’t always present or it may only placed on the joist long after the joist is installed.

For the net factored uplift in shear in this design assumes that the dead load is countering the uplift.

Top Chord Analysis

The top chord analysis shows how well the top chord is being utilized. It starts with showing the moment force in each panel from the overall moment, this force is presented in kN. Below that is the Mf line that shows the result of the moment distribution in kN•mm. In Briwood’s analysis we show the results even for the panels that are less than 610 mm long.

Below these two lines is the actual analysis for both the Mid Panels and Panel Points. The panel point line is offset by half a panel to make it clearer where the analysis point is. The first one, with the 0.629 total result is the mid panel analysis of the first panel. Note that these lines have no units, as the result of the analysis is an indicator of the fraction of available resistance under the applied force. Below and slightly to the right is the Panel Point analysis.

The formula used for this analysis is:

The individual Cf, Cr, Mf and Mr values are not presented only the resulting totals. All these numbers should be under the over design limit or unity.

Note that this design output, at the present time does not indicate if any spacers are needed in the top chord. Thus spacers should be placed at the mid point of every panel.

Bottom Chord Analysis

Beside the heading for the bottom chord analysis is the tension force in the bottom chord in kN, the tension resistance follows. Note that the joist over design limit might be imposed here but usually the discrete nature of the strength of angles in tension and deflection requirements means the angle selected usually has more strength then required.

The line below, starting with Cf, indicates the compression force in the bottom chord from uplift. When compression force in the bottom chord arise spacers and reinforcement may be required. This isn’t presented in the current design output. If you have a joist that experiences serious uplift forces a Warren pattern joist might be suitable for the job.

Below this is the Bridging (Brdg.) based on the compression in the bottom chord.

At the end of the block the maximum bridging spacing based on the Ryy values of the top and bottom chord. For the top chord this is 170 x the top chord Ryy, and for the bottom chord the bottom chord’s Ryy x 240.

Often, if the joist only has rods for web material this section will be missing, however, in long span joists it will always be present.

Below is a typical example from a long span joist:

The first section, used as the tension web member, is a hollow structural section (HSS) of 88.9 mm walls and 6.25 mm think. It has an area of 1877.00 mm2 and a Fy of 345 MPa.

Next, in the compression side of the web is a pair of angles, denoted as A1. The angle is a 44.45 mm equal angle that is 4.7625 mm thick. Due to common code for printing angles it indicates the the long leg is horizontal, not that it matters in this case. Following the angle orientation the area and effective area and the Fy are presented, in the same manner as the chords.

In the web, for both HSS and Angle members, only equal length walled HSS and leg angles are used to simplify analysis.

Below is an image of when rods appear in the web.

In this case we have a panel that has an angle in the compression side, due to the offset not being too large, and then another panel that has a rod in the tension side as the panel is a Warren and the rod doesn’t go over the kl/r of 200. Manufacturing this joist might be a bit complicated with the rods; in a later version of the program there will be a button to force the whole web in being a long span with no rods in it.

General Info

Lastly comes the general info which has some general information about the design.

The first line indicates that the design complies with the Handbook of Steel Construction, 11th Edition by the Canadian Institute of Steel Construction, and that the minimum factored point load has been added to the design, which also includes the handling of unbalanced loads as required by the National Building Code of Canada.

Below is the total weight of the joist, with the top and bottom chord plus the end panel rod extension, if present. These extensions are:

    Top Chord 100 mm each end
    Bottom Chord 150 mm each end
    End Panel rod extension 70 mm at on each end panel

The weight of the web isn’t calculated too accurately in this version of the code: the member lengths are calculated from the design length and not the cut length suitable for manufacture and welding. This will be tightened up in a future version of the program. The weight per metre result is calculated from the design length of the joist, but the weight includes the top & bottom chord extensions. Tightening this code up will likely modify the weights by less than 1,. Next there is a factorization factor which is the average of the factored moment over the un-factored moment times two plus the factored left and right reactions over their respective un-factored values all divided by 4 to give an average. Following that is the top chord and bottom chord lengths including extensions.

Lastly are the counts of the joist; right now the line is empty since there is no way to currently enter the values. This will change in a future version of the program. Following at the very end is the design date in the format of YYYY-MM-DD.