As the building and construction industry continues to evolve, we strive to enhance and improve our understanding of the loads and conditions that we need to design for, the engineering tools and methods that we use in our analyses, and the materials with which we build. This advancement in our knowledge and understanding is readily reflected in the expansion and enhancement of the building codes.
The adoption of the ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures in the 2024 International Building Code (IBC) brought important changes to the wind loading requirements. In designing roof truss components, properly defining and applying the wind loads in accordance with the project requirements is critical to achieving a successful project and can be one of the most complicated parts of the design process.
While the Engineer of Record (EOR) or the Building Designer (BD) is responsible for defining the wind load criteria needed to design the roof truss components on a project, having an understanding of the wind loading requirements will aid in interpreting and applying these loads. This article provides an overview of the MWFRS Directional & Envelope Procedure wind design methods, Components & Cladding wind loading, Combined wind load analysis, and Tornado Loads, providing insights into the use and application of these methods in designing roof truss components.
Wind Loading
Within ASCE 7, the wind loads are defined in terms of the Main Wind Force Resisting System (MWFRS) and Components and Cladding (C&C). The MWFRS is defined as the structural elements designed to provide support and stability for the overall building. Conversely, C&C is defined as elements of the building envelope or elements of building appurtenances that are not part of the MWFRS.
For the MWFRS loading, ASCE 7-22 provides two principal options for analysis, the Directional Procedure and the Envelope Procedure. Each of these procedures has specific conditional applications and/or limitations as defined with the code. Additionally, new requirements within ASCE 7-22 require building structures that are Risk Category III, representing a substantial risk to human life in the event of a failure, or Risk Category IV, determined to be essential buildings, be designed to resist Tornado Loads where applicable as part of the new wind load requirements in the code as outlined below.
MWFRS Directional Procedure
The Directional Procedure is the traditional approach for MWFRS wind loads within ASCE 7 which defines wind loading based on the wind direction, principally wind parallel to the ridge or perpendicular to the ridge as it relates to roof truss components. This wind procedure applies to the full range of enclosure classifications, ranging from enclosed, partially enclosed, partially open, and open buildings and is applicable to buildings of all heights.
This wind design procedure accounts for wind applied from a set of defined directions and is based on external pressure coefficients (Cp) multiplied by the gust effect factor (G) and the wind directionality factor (Kd). While it is more detailed, and accounts for the directional effects of the wind, it can produce higher localized pressures at corners and edges in some cases.
Overall, the Directional Procedure is applicable to most typical buildings, covering the full range of enclosure classifications and all building heights.
MWFRS Envelope Procedure
The Envelope Procedure is the former “Low-Rise Buildings” provision in Method 2 of ASCE 7-05 as noted in the commentary to the code. It is applicable to low-rise buildings with a mean roof height of 60 feet or less which are regular-shaped buildings, or buildings with non-rectangular plan shapes, such as L, T, and U shapes based on engineering judgement. It is not intended for buildings with an arched, barrel, or other unusually shaped roof. This wind procedure is limited to buildings with an enclosure classification of enclosed, partially enclosed, or partially open.
This wind design procedure is a non-directional approach, using a global external pressure coefficient (GCpf), combining the gust-effect factor and the external pressure coefficient to represent all wind directions, with the loading “enveloped” to represent the maximum possible wind pressures. It is based on wind tunnel testing research to develop the maximum induced external force to be resisted by rotating the building model through a full 360 degrees. As the procedure is simplified, it is easier to apply but may be less conservative in some cases as it envelopes maximum average pressures but may not reflect localized peak pressures.
The Envelope Procedure is a simplified procedure applicable to low-rise buildings with a mean roof height of 60 feet or less and not intended for buildings with an Open exposure classification.
Components & Cladding
In Components & Cladding, the wind loading is evaluated to reflect the localized pressures on a small effective wind area of the component being designed. For these small effective wind areas, such as that of a single fastener, the localized loads can be quite high, but vary based on the location on the building, exposure, height above ground, and other factors. The design methodology provides specific wind load factors for low-rise buildings and other buildings with a mean roof height of 60 feet or less as well as those for buildings with a mean roof height over 60 feet, reflecting the increase in wind loading for taller buildings.
The external pressure coefficients (GCp) were developed using wind tunnel testing research and enveloped with the surfaces of the building having been zoned to reflect the peak pressures for a given design application. Each C&C element should be evaluated for the maximum positive and negative wind pressures acting on it. For roof truss component design, this typically translates to load cases of C&C wind load “up” and “down” where the internal pressures are additive to the external pressures.
Combined Analysis: MWFRS Wind Loads / Components & Cladding
In engineering truss components, analysis using Main Wind Force Resisting System (MWFRS) wind loads with either the Envelope or Directional Procedure, Components & Cladding wind loads, or Combined wind loading methods incorporating analysis for both MWFRS and C&C wind loads can be used. When utilizing a Combined wind load design method, a full set of load combinations in accordance with the code requirements should be developed for wind loads incorporating the MWFRS wind loads, using either the Envelope or Directional Procedure and a full set of load combinations incorporating the C&C wind loads, with both sets of loading combinations being applied to the full truss.
By using the Combined wind load design analysis approach, the full envelope of wind loading is applied to the full truss and individual members, with the truss members designed for the worst-case loading from the MWFRS and C&C wind loads. This design approach is consistent with wood truss and cold-formed steel truss industries best practice and the ASCE 7-22 commentary on the definition of Components & Cladding (C&C) wind loads in C26.2, where it states “The designer should use appropriate loads for design of components, which may require certain components to be designed for more than one type of wind loading: for example, long span roof trusses should be designed for loads associated with MWFRS, and individual members of trusses should also be designed for C&C loads.”
While the Engineer of Record (EOR) or Building Designer (BD) is responsible for determining the MWFRS analysis procedure and providing all of the wind design criteria required for the delegated design of the roof truss components, in the absence of specific direction on the wind load design method for the roof truss components the use of the Combined MWFRS / C&C approach is recommended.
In engineering the connections, or end anchorage, of roof trusses analyzed with a Combined MWFRS / C&C wind load design method, it is typical for the connections to be based on the reactions from the MWFRS wind loading based on the code commentary noted above, however the EOR or BD should ideally specify the project specific wind load anchorage requirements.
Tornado Loads
As noted above, new requirements within ASCE 7-22 require building structures that represent a substantial risk to human life in the event of a failure (Risk Category III) or which are essential buildings (Risk Category IV) and that meet specific design parameters be designed for tornado loads. Tornado Loads on the MWFRS are based upon a modified MWFRS Directional Procedure. Similarly, Tornado Loads on components and cladding are based upon a modified procedure for C&C Wind Loads.
The design for Tornado wind loads is required for Risk Category III or IV buildings located within the tornado prone region as defined by the code, roughly the area of the conterminous United States east of the Continental Divide, where the Tornado Wind Speed (VT) is 60 miles per hour or greater and is greater than the threshold speed, calculated using a factor based on the wind exposure category multiplied by the basic wind speed. The Tornado Wind Speed of 60 miles per hour, and the threshold speed using a factor applied to the basic wind speed, reflect the approximate threshold where tornado loads begin to govern aspects of the wind load design. The Engineer of Record (EOR) or Building Designer (BD) should clearly specify where Tornado Loads are required to be considered and provide the Tornado Wind Speed (VT).
Note that design requirements for Storm Shelters require compliance with additional critical requirements provided in the applicable building code and ICC 500, the “ICC/NSSA Standard for the Design and Construction of Storm Shelters,” and is outside the scope of this design procedure.
Summary
As the building codes and design standards increase in complexity, it is important for us to maintain an understanding of the design requirements. With this overview of the wind load design procedures defined in the building code, a better understanding of the wind load requirements for roof truss components can be achieved. With this knowledge, identifying and applying the project specific wind load design requirements to truss component design can be done with a greater level of confidence and accuracy. If you have any questions, or would like any additional information, please feel free to contact the MiTek engineering team.