When designing a truss to be used as a free-spanning structural gable or party wall application, additional dead loads may need to be considered. The MiTek engineering software does not account for self-weight of the truss or for any additional weight of sheathing or drywall being applied to one or both faces of a truss. In the case of a continuously supported truss, the vertical studs will support the sheathing and transfer the load to the bearing below, therefore this is normally not a concern. On a free-spanning truss, such as a structural gable, this additional weight must be carried by the webs and chords of the truss and needs to be accounted for in the design. [For all images, See PDF or View in Full Issue.]
Let’s run an example to see how to determine the weight of the materials being added to the truss on a 24 feet long and 4 feet tall flat truss. We first need to consider the weight of the additional studs that the sheathing is getting connected to. This can be easily done by comparing the weight of the truss without studs to the weight of the truss with studs.
The difference between the weights will be the amount of additional dead load needed to account for the weight of the studs: 141 lbs – 97 lbs = 44 lbs. This will be added to the total weight of the additional sheathing to be added to the truss.
Next, we need to determine the weight of the sheathing material being used. This can be found in several places. Most manufactures provide the weight, along with other specifications, of their products on their websites or other product literature. Or you can find a list of weights of materials in SBCA’s Load Guide.
Let’s assume we are sheathing one face of the truss and our sheathing material weighs 2 psf (lbs/sq-ft). This will be the weight we’ll use to determine how much additional loading to apply to the truss. Next, we’ll need to calculate the area of the face of the truss that will be sheathed.
Multiply the Span by the Height to calculate the area: 24 ft x 4 ft = 96 sq-ft. area. Multiply that by the weight of our material (2 psf), and we get a total weight of 192 lbs. This is the total weight of all the material to be added to the face of the truss. Add this to the weight of the additional studs to come up with a total weight to be added to the truss: 192 lbs + 44 lbs = 236 lbs.
Now we need to add that load to the truss. The load will be applied to the top and bottom chords. The first thing to do is to determine the uniform load from the total load (total weight): 236 lbs divided by 24 feet span = 9.8 plf of uniform load. Half the uniform load is applied to top chord and half the uniform load to bottom chord to account for the additional dead load. 9.8 plf / 2 chords = 4.9 plf dead load applied to top and bottom chord members using Manual Loading in engineering. Please note, upward Wind load cases require a 60% reduction to dead load.
If you determine the weight of the materials being added to the 24 feet long and 4 feet tall common truss, follow the same steps, except to calculate the area multiply the Span by the Height and divide it by 2: 24 ft x 4 ft / 2 = 48 sq-ft. If weight of sheathing material is 2 psf, then total weight is 96 lbs. Let’s assume the weight of the studs is the same as in the example above – 44 lbs. Total weight to be added to the truss: 96 lbs + 44 lbs = 140 lbs or 5.8 plf. Trapezoidal load from 0 at the left end of the truss to 5.8 at the peak and from 5.8 at the peak to 0 at the right end of the truss must be applied to top and bottom chords using Manual loading in engineering.
When you are done applying the additional loading, review the loads and confirm that they are correct in magnitude direction and location.
For additional information, or if you have questions, please contact the MiTek Engineering department.