50 Pound Point Load

A number of engineers in the past have told me, and one document from an American joist manufacture stated, that you don’t have to worry about a single point load below 50 pounds (22.68 kg, 0.2224 kN) in the analysis of a joist. My take on the information was; show me the academic peer reviewed paper that backs the statement up. Never having found it, and doing my own research would easier then finding it. Here is Briwood’s research.

I suspect that this 50 pound idea was started when most joists were ordered from catalogs. When ordering from a catalog one often had to take all the loads and convert them into an equivalent tabular load to select the required joist. This could have been a bit of work, subject to error, and it was simply easier to drop a fifty pound point load from the analysis then do the full work up. Also, most catalog joist are modified Warrens with short egual panels, hence when placing a point load on them there often is a panel point nearby.

To analyze this two joists were designed, one 10 m and one 40 m. The joist are both equal panel Warrens with the 10 m having 1 m panels and the 40 m having 2 m panels. Chords were selected such that none of the panels were over stressed. Both the mid panel and panel point forces will be examined. Different dead load point loads at different locations will be applied to the joist at the middle of the panel.

The graphs below show the analysis changes by percent. This was done to normalize everything. Here is typical example of how the percentage is arrived at:

Base Joist      Panel 1   Panel 2
Mid Panel       0.256     0.472
Panel Point     0.667     0.757
  
Mid End Panel PL of 0.125 kN Unfactored (0.15625 kN factored)
Mid Panel       0.260     0.472
Diff            1.56%     0.00%
Panel Point     0.680     0.756
Diff            1.95%    -0.13%

For the first panel the analysis gave the result of 0.260 while in the base joist it was 0.256. Turning the delta into a percentage:

(0.260 – 0.256) / 0.256 * 100 = 1.5625%

We get a 1.56%, increase in the analysis result in the 1st (end panel) at the first mid point.

To keep this analysis simple only load case 3, which is the Dead load (1.25 times factorization) and Wind Load (1.5 times factorization) load case.

For this analysis we’ll consider the loading to be unsafe when it pushes any point of the analysis more than 1.0% over the original analysis.

10m Joist Tests

Taking the 10m joist first.

The load in the first case is in the first panel. You can see the variants of the load on the right side of the graph. Loads start at 0.125 kN and double to 1.0 kN.

One can see even with the 0.125 kN load the analysis totals rise by close to 2%, the 0.25 kN load causes an approximately 3.13% rise. Typically in a Briwood variable panel joist the end panel is stressed up to 95 percent plus. Adding 4 percent to that would often push the mid panel analysis to over 100%.

From the graph it would not be recommend putting a 50 pound (22.7 kg) load into the end panel. At the very least design for it. One must qualify that advice by adding that usually the full uniform loads are rarely present on a joist, and there is a safety factor of approximately 40%. There is no risk of collapse but over stressing the joist with an un-analysed point load isn’t ideal.

When we move the load into the first interior panel the following graph gets created:

Now the load is on Mid Panel 2 (MP 2) on the graph. You can see that the analysis numbers jump in that panel and at the surrounding panel points. You’ll also notice that the end panel’s mid point analysis is reduced and the panel point analysis (PP 3) also gets reduced. This indicates the moment distribution is taking the load out. However around the panel that has the load the analysis give a minimal jump of 0.5% at PP 2 and up to 2% at PP 1 for the lightest 0.125 kN load. In Briwood’s variable panel joists the two interior panels typically have the most slack, as it reduces the length of the web, reducing the weight of the joist.

Might you wish to put an unplanned 0.25 kN load on to the first interior panel, on an equal panel joist, with panels that are not stressed up? It wont be ideal. On a variable panel joist, where the panels are better utilized, it wouldn’t be recommended as the first panel point would likely be over stressed.

For the last test the panel point has been moved into the middle of the 4th panel. Below is the resulting graph:

As can be seen here the 0.25 kN load doesn’t push anything above 1% immediately around the point where the load is applied. One can also see additional work being done by the end panel to take the load to the support. In this case the panel point seems to take brunt of the work. Again for a load of 0.25 kN this presents a problem with the analysis values running over 1.5% increase.

From these graphs one can conclude on a joist that is 10 m long or similar, and is low stressed at both mid panels and panel points, a 50 pound (22.68 kg, 0.2224 kN) load can not be placed on the top chord without design or analysis on the interior panels. The load should be considered in analysis.

If the joist has really heavy dead and live loads there may not be a problem. This case is looked at below.

40 Metre Joists Tests

After the results from doing a 10 m joist were gathered it was decided to test out a 40 m joist. Again starting at the end panel we get the following load analysis pattern:

As expected with the point load of 0.25 kN at MP1 the analysis rises by 1.0% in the end panel and rapidly declines as you get into the joist. Any load above 0.25 kN causes an unacceptable rise in the results.

Again, no unaccounted for point loads in the end panel.

Moving into the first interior panel, we see the following pattern:

For the 0.125 kN and the 0.25 kN the change in analysis is at most close to 0.5%. This appears to be quite acceptable, except maybe for joists that have really stressed up panels.

Moving the load into the middle of the joist generates the following graph

Again, here you can see the moment distribution at work. At no point in the analysis does the 0.25 and 0.125 kN loads increase the analysis values by more than 0.25%. Hence one can conclude that for a 40 m joist the addition of the 0.25 kN load in any interior panel will not do the joist any harm.

Looking at the spreadsheet for the graph above most of the changes in analysis are in the relm of a 1/1000th. Hence the results for the 0.125, 0.25 kN lines are near the end panel are actually mostly noise due to rounding errors. Some of the methods used in creating the analysis results use floating point numbers where they should be changed over doubles for better precision. Also the analysis output is in the form a single digit with three figures after the decimal point, restricting the result to 1/1000 in accuracy.

The increase in maximum moment for the 40 m joist with the addition of even the one kN load is 0.72 kN or 0.26% which is negligible. Hence no change really in the design. The end panel is the excpetion where the change is most notable.

One should also consider that the other loads, uniform snow and dead are rarely at their maximums so the joist should have not have a problem handling the additional dead point load of 0.25 kN.

Doubling the Uniform Dead Load: Any Changes?

Out of interest the uniform dead load was doubled to see what impact it would have on the 0.25 kN load. The load was place at the mid point of the first panel, second panel and thrid panel to see what the anaylsis would look like. See the resulting graph below:

As can be seen having the load in the first panel causes issues, with the analysis values rising by 1.6%. Again the prohibition results in the rule of not putting such a load in the end panel. However the in the other locations we do not experiance any increase in the analysis over 1.0%. Thus in a joist with panels that aren’t too stressed up things should be fine.

Conclusions

Generally it is safe to put a 50 lbs (approximately 0.25 kN) load on a large joist without doing a full analysis as long as it isn’t in the end panel and the panels are not really stressed up. Finding the point were this transition to a ‘large’ joist occurs wasn’t part of this exercise. In general we can say not to put a unanalysised 50 pound load on a 10 m joist, but it is acceptable on 40 m joist’s interior panels.

In the best interests of safety, when designing it is a good practice to account for every load, no matter how small.

Downloadable Test Designs

These are the joists used to create the graphs. Note that the design output is a full analysis with one page per load case. This feature isn’t available on the online program… yet.

10 m Joists | 40 m Joists | 10 m Joists - Doubled Uniform Dead Load