MiTek 20/20 software lets you design for many different wind loading conditions. It uses two general methods for wind load design: ASCE 7 and User Defined. In some rare instances, the building designer may require an analysis using specific wind pressures so you would use User Defined, see figure 1 below.
Generally, wind design is prescribed in the building code, and determined in accordance with ASCE 7. The MiTek software uses the Main Wind Force Method (hereby referred to as MWFRS), and the Components and Cladding method (hereby referred to as C&C) in ASCE 7.
The MWFRS method is defined as: An assemblage of structural elements assigned to provide support and stability for the overall structure and the system generally receives wind loading from more than one surface. MWFRS has two wind zones: the gable end and interior. The gable end zone is defined as 2 x 10% of least horizontal dimension or 0.4 x mean roof height, whichever is smaller, but not less than either 4% of least horizontal dimension or 3 ft (0.9 m). Interior zone would be for all the rest of the trusses. (See figure 2 below for simplified diagram)
You will also notice that there are two design methods within MWFRS, Low-rise and All Heights. Low-rise is defined as an enclosed or partially enclosed building with a mean roof height less than or equal to 60 ft and a mean roof height that does not exceed the least horizontal dimension. The all Heights method was recently added to the program, and can be used to design for any roof height. MiTek recommends the use of All Heights, if enough information has been provided to answer all wind loading fields with a reasonable amount of accuracy, because it will typically help you to be more competitive with reduced uplift anchorage requirements.
The C&C method is defined as: Elements of the building envelope that do not qualify as part of the MWFRS. C&C 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 C&C method breaks the roof up into 3 zones: Interior, End, and Corner. The End zone is defined as 10% of least horizontal dimension or 0.4 x mean roof height, whichever is smaller, but not less than either 4% of least horizontal dimension or 3 ft (0.9 m). The corner zones could be defined as overlapping sections of end zone. Interior zone would be the areas between these zones. (See simplified diagram below)
The MiTek software will allow you to design with either of these methods, or a combination of both, referred to as the Hybrid method. In the absence of building designer or code specification, the Hybrid method is the MiTek recommended method, as it uses the more conservative design of either method for structural design and plating, but uses only the MWFRS for uplift reaction reporting, which will generally report uplift reactions much less than the C&C method.
The first step in the wind design is to choose the correct edition of ASCE 7 Table 1 shows the corresponding ASCE 7 code to the building code.
Now we will look at the loading dialog in the MiTek 20/20 Engineering software.
At the top of the Wind load dialog, you have the Exposure category and the Occupancy Category. The 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 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.
Probably the most impactful option in wind design is the wind velocity. Generally, you should choose User define wind and enter the velocity for the area you are designing for, which can be found on the Basic Wind Speeds map found in your state code book or ASCE 7. There are also some pre-set wind velocities. Only two would apply to our US customers: High and Hurricane wind. When running the FBC code and using the Hurricane wind (High Velocity Hurricane Zone), the wind speed will automatically increase to 140. In versions prior to 7.0, this would also give you the option for setting the Directionality Factor, which can be adjusted any time you are designing with the All Heights method in 7.0. This factor should always be set at 0.85 unless otherwise specified by the jurisdiction having authority. Using the 0.85 factor is the default setting and it takes a 15% reduction in wind loading being applied to the truss.
As previously stated, Low Rise MWFRS Roof Zone breaks the roof up into 2 zones: Gable End Zone and Interior. You can choose Automatic for the MWFRS Roof Zone, and the program will calculate which zone the trusses falls in using Truss Dist to Eave near the bottom right of the wind dialog. Note: The Truss Dist to Eave (horz. distance from truss to exterior wall) would still need to be entered to have the software determine the correct zone.
The C-C Roof Zone breaks the roof up into 3 zones: Gable End Zone, Interior and Corner. Most trusses pass through multiple zones and therefore Automatic is probably the best setting to be used most of the time for the C-C Roof Zone. 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.
When you choose automatic roof zones you will notice that you are asked to provide a few other pieces of information. You will need to enter the truss category, the Truss Dist to Eave, and some building dimensions. The Truss Category and Building Dimensions which are the overall length and width of the building; the Truss Dist to Eave is the shortest distance from that truss to the exterior wall, as noted before. Note: When importing trusses from TrussFramer this information will automatically be entered into the program in a future version.
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 and those with a large amount of openings in one wall relative to the other walls in the building. The description of Partially Enclosed Buildings from ASCE states that a partially enclosed building meets both of these conditions:
1. The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10%.
2. The total area of openings in a wall that receives positive external pressure exceeds 4 square feet or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.
Note that there is not an option for open buildings. This is because the code has historically assumed that with an open building the wind blows in one side and out the other so it is the least conservative of the three. Therefore we have historically advised using enclosed or partially enclosed types for the design of trusses as these were more conservative.
Recent code changes in ASCE7-05 created open building criteria requiring separate programming efforts and these building types cannot be covered by the previous two setting as customary done.
If you are running C&C ASCE 7-05, you will notice another choice labeled # of C-C Load Cases. If you are running 1 load case, you are not using the directional approach and therefore you are subjecting the truss for the maximum loads from all directions at once. See figure 5 below.
But if you wanted to be less conservative and more accurate, you could choose to run two C&C wind load cases. This will better approximate actual wind behavior and turbulence occurrence on the truss from different directions. This will run two C&C load cases left and right. See figure 6 below.
Per TPI 18.104.22.168 when designing for wind you can have your DOL’s set for lumber and plate to a max of 1.6, unless otherwise specified by your building designer or jurisdiction having authority.
The Max Dead Loads are used to resist the uplift from wind and therefore the smaller they are, the more conservative they are. 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 sections of the truss that are exposed to wind that may be selected to be on or off include Cantilevers, Porches and End Verticals. See Below.
Cantilevers and End Verticals are 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 truss will receive loads applied in the upward direction along the entire bottom chord. So it would be very conservative to leave this on as a default.
If you have questions regarding any of this information please free to call your MiTek Engineer.