The Top Chord Live Load (TCLL) that can be used in the design of a roof truss can depend on a number of factors. There are three ways that the TCLL can be specified – Construction, Snow or combination of Construction and Snow. Each of these affects the design in different ways.
The type of TCLL you use will be dictated by the part of the country you are designed for and what is specified in the plans.
To back up a bit – a Live load is a temporary or transient load that acts on a building or structural element. For this discussion we are looking at loads that would be created by people and materials on the roof during construction or snow.
We will look at wind load in another article.
Construction Live Load
You will typically use this for non-snow areas.
When you select the Live load as construction – the load duration factors will be set to 1.25 typically. Some users may elect to make this 1.15.
The duration factor is determined by the length of time that the load will be acting on the structure. In this case it is figured to be 7 days.
Your loading will show the TCLL as the 20 psf. on the truss drawing.
Snow Live Load
When you are required to design for snow loads there are a number of different factors that can be used in your design. Again, this information should be indicated on the plans or governed by the building code in your area.
When you select Snow as the Live Load and go to the Snow Tab you have a number of options.
What are these options and how will they affect the load applied to trusses?
Terrain exposure: This deals with the obstructions to wind within 2600 ft. of the structure or 1500 ft. for structures with a mean roof height of 30 ft. or less.
Exposure B – typically suburban areas where buildings of 30 ft. in height surround the building.
Exposure C – Open grassland with scattered obstructions having heights generally less than 30 ft.
Exposure D – Building along the shoreline of water at least one mile across excluding hurricane prone areas.
Roof exposure (Ce): This deals with obstructions right next to the structure. The factor used will depend on the terrain exposure that is used.
Sheltered – Structures that are tight in among conifers that qualify as obstructions.
Fully Exposed – Roofs exposed on all sides with no shelter afforded by terrain, high structures or trees.
Partially Exposed – All other roofs that do not meet the previous two categories.
Chart below from ASCE 7-10
Thermal Factor (Ct): – This deals with how much heat escapes the space below the trusses and up to the roof to melt the snow.
Ct=1.1 – Structures kept just above freezing. This will be used for the majority of residential structures. ASCE states if an R value greater than 25 is used between heated and ventilated space, then a thermal factor of 1.1 should be used.
Ct = 1.2 – Structures kept just below freezing. This would encompass structures such as barns that are open and unheated.
Ct = 1.3 – Structures intentionally kept below freezing.
Ct = 0.85 – Continuously heated greenhouses to a minimum temperature 50 degrees F.
Ct = 1.0 – All other structures not defined by above factors.
Surface condition: This will determine how easily the snow can slide off the roof and will adjust the loading accordingly. Your two options are Unobstructed Slippery and All Others. Slippery roofs would be metal, slate, glass type roofs.
Occupancy Category: These describe the type of building that is being designed and will impact the loading based on the buildings “Importance”. This will determine the Importance Factor (Is) that is used to determine the loads trusses are designed for.
Category 1: Low hazard to human life such as agricultural buildings or temporary structures
Category 2: Those not listed in category I, III, or IV.
Category 3: Substantial hazard to human life in event of failure such as schools, jails, buildings with public assembly.
Category 4: Essential facilities such as hospitals, police and fire stations, national defense facilities.
Building Location: Most always be left as Other location.
Building Lu: Distance from the truss to the furthest eave from the truss being designed. It accounts for snow blowing parallel to the ridge of a hip roof. It will not have an effect on trusses that do not have a flat top.
How do these factors affect the TCLL?
The way that the Roof Snow is figured based on the ground snow is:
Roof Snow Load = 0.7 * Ground Snow * Importance Factor * Thermal Factor * Exposure Factor * Slope Factor.
First design: 25 psf Ground Snow, Exposure B, Partially Exposed, Ct=1.1, Occupancy II.
Results in a TCLL of 19.3 psf
Second design: 25 psf Ground Snow, Exposure B, Sheltered, Ct=1.1, Occupancy II.
Changed the roof exposure to Sheltered.
Results in TCLL of 23.1 psf. due to close proximity of trees.
Third design: 25psf Ground Snow, Exposure D, Partially Exposed, Ct=1.1, Occupancy II.
Changed the Terrain exposure to D with roof exposure back to Partially Exposed.
Results in 17.3 psf TCLL due to the structure being in flat unobstructed area exposed to wind over water.
Fourth design: 20psf Ground Snow, Exposure C, Partially Exposed, Ct=1.1, Occupancy II.
Reduced the ground snow to 20.
This will result in a 16 psf TCLL.
On this last example, the actual Snow Load that would result from the formula above would be 15.4 psf.
A value of 16 psf will be used as a minimum.
This minimum value will vary depending on the code being used.
As you can see there are numerous ways that the TCLL can be entered and affect the design of the truss. It is important to pay attention to the loading criteria that is indicated on the plans to ensure accurate analysis of the truss for the location the structure is being built. This is especially true for high ground snow load areas where small changes in the factors can make a noticeable difference in the TCLL that the truss will see.
Hope this has been informative and look forward to hearing from you on this topic.
Check back for some additional posts related to TCLL. We will look at wind loading and the factors that go into determining the loads acting on trusses due to wind.
Bill Hoover – Design Manager
Gould Design, Inc.