A benefit of using Simpson Strong-Tie’s CS Truss Studio™ is managing the relationship between the truss self-weight and the dead load. Truss self-weight is checked against all applied dead loads to verify that there is enough assumed “extra” dead load to account for the dead load of the truss itself. [For all images, See PDF or View in Full Issue.]
Steps for managing the truss self-weight and the dead load:
- Open a truss and analyze.
If the dead load for the truss is not adequate, a message displays. The message is either an error message or a warning message. In the example shown below, the following error message displays:
- Dead Load does not include enough to cover self-weight of truss! Dead Load = ####, Weight = ####, Deficit = #### (##%)
- #### indicates the actual values used
The error message indicates that the total dead load on the truss is 672 lbs. Because of the heavier lumber used and the gable truss design, the weight of the truss itself is almost 650 pounds. This leaves approximately 22 pounds of dead load to carry anything above the truss weight, which is not adequate.
- To resolve the error, you must calculate what is required for the truss dead load and how much is extra and can be used to counteract the self-weight of the truss.
In the following example, the applied dead load is 672, as indicated.
- When designing trusses for Wind Loads, you must run a factored dead load case (typically 60% of the dead loads used in combination with wind loads); this ensures that the weight of materials is not overstated to reduce uplifts.
- In the example below, the factored dead load (402#) is subtracted from the dead load (672) = 270. This amount represents the “extra” amount, above the truss weight. Subtract the extra amount from the weight of the truss (650#). The result is the deficit indicated in the error message. In the example below, the deficit for this truss is 380#.
Solutions for resolving the self-weight error:
- You can adjust the deficit threshold in Env Data > Analysis > Overall Controls > Report Self-Weight deficit larger than % as Error. This value can be set from 0% to 50% (default is 10%). This could be expressed as a decimal instead of a percent, with values Min = 0.00, Max = 0.50, Default = 0.10. If the deficit divided by the weight (times 100, if using %) is more than this setting allows, the truss fails and an error displays. Adjusting this setting may result in a warning as opposed to an error that prevents the truss from passing analysis.
- Adjust standard loads on the truss. Go to Load Template Manager > Standard Load and adjust dead loads for the truss.
- If you know the actual weight of the materials, you can override the dead loads. Go to Load Template Manager > Wind Loads and enter the actual material weights. Be aware that if you enter amounts that total more than 60% of the dead load, you will actually be reducing the amount of available dead load for the weight of the truss.
- Add additional load to the truss. Remember that when you add load, Truss Studio uses only 40% of that load as part of the truss self-weight. Go to Component > Component Loading > Distributed Load and adjust the Dead Load values. Adding a Description of Self Weight may be helpful to anyone reviewing the truss design.
If this added weight is the only additional load on a single-ply or two-ply truss, check the User Load is applied over 3 or more trusses checkbox to maintain repetitive lumber values. If there are other loads that disqualify the use of repetitive lumber values, do not check this box.
Note: Verify that the repetitive member factor is used properly. See Repetitive Members.
Note: If a warning displays regarding the truss self-weight, this is informational only. You can decide whether it is necessary to adjust the dead load for the truss.
Additional benefits of truss self-weight include:
- Verify the truss can support itself, in addition to all other material weights
- Warnings or errors are presented to the user when the truss self-weight exceeds the design dead loads
Simpson Strong-Tie’s Component Solutions® CS Truss Studio™ software has the ability to use truss self-weight. Contact your Simpson Strong-Tie Representative or email CSHelp@strongtie.com for more information.
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