This whole discussion is not meant to be a detailed dissertation on engineering and loading but rather a laymen’s discussion to help a user know which selections to make in some of the various screens in the MiTek 20/20 engineering software. The loading specification is the responsibility of the building designer.
For discussion there are three main ways to setup the truss top chord loading for roof trusses in MiTek 20/20 and we will look at each of these. Then we will look at snow and wind loading in some detail. After this, we will discuss the bottom chord live loads.
Top Chord Loading
The first way is the most simple and that is to use the ground snow load directly from the snow map and enter that as a Top Chord Live Load, TCLL and call it Roof Live Load (Construction) as shown here:
This method is the one used the most due to its simplicity and long standing history. It is based on the “prescriptive codes” like the International Residential Code (IRC). One disadvantage to using this as your default loading is that you then have to remember to change it to snow when running a commercial job as this option only works for residential (IRC) jobs except in the south where snow loading is not generally considered. Another possible disadvantage to this option could occur on certain truss designs that the snow load would be greatly reduced for a steep pitch and there are no hip trusses subjected to heavy unbalanced snow loads over a majority of the span.
The second option would be to run “snow” loads per ASCE 7 as shown here, which can be used for both commercial, International Building Code (IBC) and residential (IRC or IBC) jobs:
The main advantage to using this as your default setting is probably that you would not have to make any changes when running a commercial job. On certain trusses with a steep slope and no flat sections of top chord you could get a more competitive design. One possible disadvantage to this option is that hip trusses and un-triangulated trusses may be less competitive due to larger unbalanced loads. Another disadvantage would be that the engineering drawings would show the reduced snow load as the TCLL, which can confuse some building inspectors. It is important to tell the software if the snow load you have input is a roof or ground value to insure a proper design in this option.
The third option is combine both of the first two options and to run both live and snow as shown here:
This option yields the most conservative results from both of the first two options and is therefore is perhaps the most costly of the three options. This option does have its appeals though in that you can satisfy a building designer that specifies both a snow load and a live load in the specifications. Another great advantage is that the TCLL shown on the drawing will be directly from the Live Load (Construction) input and will therefore be a whole number making the building inspectors happy.
The bottom line is that no one way is the best at all times and for all fabricators in all areas of the country. Designers should look at the type of work they do (commercial or residential) and the snow load from the map for the area they do business to setup their defaults. Many fabricators today ship to a large area and they will need to make sure they design correctly for the area where the job is located.
Now let’s look at some of the settings in the snow tab as shown here:
The Terrain exposure category is dealing with the obstructions to wind within a 2600 foot distance of the structure or 1500 feet for structures with a mean roof height of 30 feet or less. Exposure B refers to most suburban areas where buildings, etc. of 30 feet in height or more surround the structure. Exposure C is for open grassland with scattered obstructions having heights generally less than 30 feet. Exposure D is for buildings along the shoreline of water at least one mile across excluding hurricane prone regions.
The Roof exposure category deals with obstructions right next to the structure. Sheltered roofs are those that are tight in among conifers and fully exposed roofs are those that have no shelter from terrain, higher structures or trees. Partially exposed roofs are all of those that do not meet one of the first two definitions.
For a more detailed description of Terrain exposure and roof exposure, see ASCE 7-05, Minimum Design Loads for Buildings and Other Structures.
The Thermal condition deals with how much heat escapes from the space below the trusses up to the roof to melt the snow off.
The IRC states that the minimum insulation to be used in the attic space in normal heated residential structures is R-30, R-38 or R-49 depending on location. ASCE says that if an R value of 25 or greater is used between the heated and ventilated space, a Ct factor of 1.1 is to be used. Therefore a Ct factor of 1.1 should be used on almost all residential structures. On structures that are unheated such as most barns, the Ct factor should be 1.2. A Ct factor of 1.0 may apply to certain commercial structures but this would be the exception rather than the norm. Please also be aware that anytime a Ct factor of 1.0 is used that a 2X overhang snow factor is then required.
The Surface condition deals with snow’s ability to slide off of the roof. A slippery roof is one of metal, slate, glass, bituminous, rubber, and plastic membranes with a smooth surface. To qualify for a smooth surface, there must be no obstructions to keep the snow from sliding off of the roof.
The Occupancy categories are all described in the in the drop down menu of the MiTek 20/20 software. For a more complete description, refer to ASCE 7-05.
The Building Locations should be left set to “Other location”. The other options require programming to use and this does not affect loading as long as it is left at “other location”.
The Building Lu is the distance from the truss in question to the furthest Eave from that truss. It is used to account for snow blowing parallel to the ridge of a hip roof. You will notice that this has no effect on trusses that do not have a flat section.
Applying the slope reduction factors can reduce the roof snow load on trusses with a pitch and is a function of several variables including the steepness or pitch of the roof.
The Unbalanced snow loads should be set to “Do for any geometry” so that a mono truss will have unbalanced snow loads applied to it. This can be very important if two mono trusses are placed back to back to form a full roof and on hip ends where the whole hip area could be subjected to drifting when the wind blows parallel to the ridge.
The Transverse Roof Pitch is there to account for “rain on snow surcharge” and is only applicable to roofs with a flat section and a ground snow load of 20 psf or less. It tells the program that the flat section is not really flat because it has a pitch perpendicular to the truss and therefore this load is not applied. The value to enter here is the pitch of the roof perpendicular to the truss.
Now let’s look at some of the settings in the wind tab as shown here:
We have already discussed the Exposure category and the Occupancy category under the snow settings. The Wind Design Method contains many options and we will focus on three of them in general terms. The first is Components and Cladding, C-C which is the most conservative and can be described simply as dealing with pockets of high wind pressure applied to one small element of a whole structure at a time. This may be the appropriate setting for small trusses such as End Jacks although some jurisdictions and building designers require this to be used for all trusses.
The next option is main Wind Force Resisting System, MWFRS. This deals more with wind being applied over a large part of the structure and it’s resistance to that wind force. The final option is what is referred to as the “hybrid” method as shown in the example above. This method applies C-C wind and then in separate load cases applies MWFRS. The individual parts of the truss (plates, lumber and bracing) are based on the most conservative of all of these load cases but the reactions are only based on the MWFRS. The hybrid method is the most commonly used on truss designs.
The MWFRS Roof Zone options include Interior, Gable End and Automatic. The Interior Zone has lower uplifts than the Gable End Zone and may be applied to trusses that fall inside a distance 2a from the gable end of the building. Okay, we weren’t supposed to get too technical but “a” is defined as: 10% of the least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of the least horizontal dimension or 3 feet. In the above, “h” is the mean roof height. If that was in Greek to you, use either Gable End Zone or Automatic. If you use Automatic, the Truss Dist to Eave near the bottom right of the wind tab will be used to determine the zone. The C-C zone can be either Interior (1), Exterior (2), Corner (3) or Automatic. The exterior is basically the edges and pitch breaks, the corners are the corners and the interior is everything else. Most trusses pass through multiple zones and therefore Automatic is probably the best setting to be used most of the time. The Automatic setting relies on the Truss Category and the Truss Dist to Eave located at the bottom of the wind tab to apply the correct zones.
The opening condition options include Enclosed and Partially Enclosed. Most buildings in the United States can be considered Enclosed which results in lower wind pressures. Buildings that need to be considered Partially Enclosed include those in the Wind Born Debris Regions or “Hurricane Zones” and those with a large amount of openings.
The description of Partially Enclosed Buildings from ASCE states that a partially enclosed building meets both of these conditions (maybe more Greek here):
The height above ground is the mean roof height in feet.
The Max Dead Loads are used to resist the uplift from wind and therefore the smaller they are, the more conservative in this case. They are restricted to being 0.6 times the dead loads used in the general load tab except on Ag trusses where the dead loads are more accurately designed for.
The building width is the width of the building perpendicular to the truss in feet.
The sections of the truss that are exposed to wind that may be selected to be on or off include Cantilevers, Porches and End Verticals. Cantilevers and End Verticals seem self explanatory. Porch loading applies uplift pressure to the underside of the truss starting at the first bearing from the end and turning off at the next bearing.
Therefore if a truss with just a bearing at each end is designed with porch “left” turned on, the whole bottom chord will be designed for uplift loads.
IBC/IRC Bottom Chord Live Loads
Now for a look at the bottom chord live load requirements in the 2006 International codes for residential trusses:
The ICC has issued a set of 2007 supplements for the IBC and IRC. A change was made to the IRC ceiling load loading that we think is good for the truss industry, but it may be a little confusing. The same change was not made in the IBC, which complicates things.
Both the IRC and IBC require a non-concurrent 10 psf bottom chord live load for attic areas without storage. Non-concurrent means that the bottom chord live load is applied in a separate load case without the top chord live load. Both of these codes also have a 20 psf live load for attic areas with limited storage, defined as any area in which a 42" high by 24" wide box would fit within the open spaces of a truss. It is the exceptions to this requirement that creates differences between the IBC and IRC. For the IRC, there are exceptions where the 20 psf live load is not required for 2:12 and greater slopes and when the required insulation depth is greater than the bottom chord member depth. The IBC does not have the insulation exception. Both codes require that you have a minimum 10 psf bottom chord dead load whenever you meet the limited storage requirement. The insulation exception may make it rare that the 42"x24" load is required on IRC designs.
When Running the IRC Code:
We recommend running both the "IBC/IRC 42"x24" BC Load" and the "IBC BC Live Load". Having both loads turned on by default prevents you from missing the cases where you may need to have the 42"x24" load. We also recommend using a minimum 10 psf bottom chord dead load which is required with the 42"x24" load. Only one of these bottom chord live load types is required on a truss, but running both will keep you from having to worry about which load cases you have to run for a specific truss or job. You will need to have one or the other of these loads shown on a truss to be code compliant and have a MiTek engineer seal the design. Because there is no way of knowing which load is required until the truss webbing and other geometry is determined, we recommend applied both load types. If you know that the insulation will cover the bottom chord and therefore the 20 psf bottom chord live load is not required, you can technically not run the "IBC/IRC 42"x24" BC Load", but we do not recommend this.
When Running the IBC Code for residential construction:
We recommend running both the "IBC/IRC 42"x24" BC Load" and the "IBC BC Live Load". The 42"x24" load is required whenever the box will fit within the truss and you don't have a sloping bottom chord. Turning this feature on in the program will automatically take care of the conditions where it may be needed. We also recommend using a minimum 10 psf bottom chord dead load which is required with the 42"x24" load. Since the 10 psf non-concurrent IBC BC Live Load is required whenever you do not have the 42"x24" loading on a truss, we recommend that you always have it turned on to catch those cases.