ASCE/SEI 7-10, Minimum Design Loads of Buildings and Other Structures, lists two methods for calculating wind pressures: Main Wind Force Resisting System (MWFRS) and Components & Cladding (C&C). This guide will provide information to assist the building designer in deciding upon the appropriate analysis method for uplift due to wind loading.

Step-by-Step: 

Intro

Introduction

  • ASCE/SEI 7-10, Minimum Design Loads of Buildings and Other Structures, lists two methods for calculating wind pressures:
    • MWFRS (Main Wind Force Resisting System)
    • C&C (Components and Cladding)
  • Building designers, code officials and truss designers may question which method to use when designing uplift connections for trusses.
  • This presentation will provide a step by step approach to assist the building designer in deciding upon the appropriate analysis method for uplift due to wind loading.

  • SBCA recommends a hybrid (combined) analysis, using both the MWFRS and C&C method
    • MWFRS applies to the assembly as a whole, while C&C covers individual parts.
  • While this is the most common approach in the truss industry, ultimately it is the responsibility of the building designer to determine the method(s) to be used.

Step 1

Understand Responsibilities

  • The truss designer needs as much loading information as possible from the building designer in order to design the trusses.
  • The building designer is responsible for providing the structural design documents and all of the load and dimension information necessary to design the trusses.
  • If a project does not require a licensed professional building designer, the owner or the owner’s agent is responsible for providing this information.

Step 2

Check Building Code Applicability

  • The IRC design scope for wind is limited in Section R301.2.1.1 as follows:
    • IRC 2015 – where design is not required in accordance with Figure R301.2(4)B.
  • This is generally all areas with an ultimate wind speed of 130 mph(Vult) or less except in the New England states where wind speeds up to 140 mph are allowed.
    • IRC 2012 – less than 110 mph (Basic wind speed, Vasd) or where design is not required in accordance with Figure R301.2(4)B
    • IRC 2006 and 2009 – less than 100 mph in hurricane prone regions and less than 110 mph elsewhere.

Step 3

Check Site-Specific Information

  • The truss designer must rely on the building designer to provide accurate site-specific wind information per Table R301.2(1):
    • Basic (Vasd) or Ultimate (Vult) Wind speed (3 second gust) and whether or not the structure is in a hurricane‑prone region
    • Exposure Category
    • Plus: mean roof height (if not given, 15 feet would be typical for a one-story, 25 feet would be typical for a two-story)
  • The default Exposure Category in the IRC is “B”, but adjustments for Exposure Categories C & D as well as mean roof heights up to 60 feet are provided.

Step 4

Check Structure-Specific Information

  • Generally, the following design criteria for structures within the scope of the IRC can be used.
    • Importance Factor (I) = 1.0
    • Enclosure Category = Enclosed
    • Topographic Factor (KZT) = 1.0
    • Directionality Factor (KD) = 0.85
  • Always verify any assumptions made with the building design where they are not shown on the construction documents.

Step 5

Use Hybrid Approach Unless Otherwise Directed by the Building Designer

  • Most two-dimensional software analysis programs offer a choice of wind analysis methods when applying wind loads including a hybrid approach.
  • The truss designer should use a combined analysis as follows:
    • Truss, rafter, or gable frame uplift connections should be designed for wind load using the MWFRS analysis method
    • Individual truss, rafter, or gable frame members should be designed using the C&C analysis method.