MWFRS Directional Procedure¶

The MWFRS Directional section calculates the design pressures on the main wind force resisting system by the directional procedure of ASCE 7 Chapter 27 for buildings of all heights. In ASCE 7-10 and 7-16 that is Part 1 of Chapter 27; Part 2 there is a simplified method for enclosed simple diaphragm buildings up to 160 ft. ASCE 7-22 has no Part 2. You describe each building region: its plan dimensions, its levels and parapets, and its roof. Prose finds the velocity pressure at every level, the external pressure coefficients for each wind direction, and the design pressures on the windward, leeward, and side walls, the parapets, and the roof, for both internal pressure cases and the minimum load case.

Section, figure, and equation numbers in the text are the ASCE 7-16 and 7-22 numbers unless another edition is named. ASCE 7-10 numbers Chapter 27 differently; the Code Reference table gives the numbers for each edition.

Key Features¶

• Any number of wind regions, each a building or part of a building with its own geometry
• Levels and parapets at the elevations you need pressures for
• Flat, gable, hip, and monoslope roofs, with the mean roof height found from the roof points
• Velocity pressure qz at every level and qh at the mean roof height
• Wall, parapet, and roof design pressures for the north-south and east-west wind directions, for the positive and negative internal pressure cases and the minimum load of Section 27.1.5
• A summary and report with every coefficient and its figure or table reference

Adding the Section¶

1. Click the + beside the section tabs
2. Select MWFRS Directional

The section opens with one region, "Region 1", holding placeholder dimensions and a single level. The Close tab mark on the tab removes the section again.

Wind Regions¶

A wind region is one building, or one part of a building, that the directional procedure treats as a single block. Add Region at the top of the section adds another, and the three-dot menu (⋮) on a region's header duplicates or deletes it. The Region Description names the region in the summary and the report.

Building Information¶

Each wind direction uses one dimension as the width normal to the wind and the other as the depth along it, which sets the L/B ratio for the leeward wall coefficient.

Levels¶

The Levels table lists the elevations at which wall pressures are reported, typically each floor and the roof.

Add Level adds a row; the three-dot menu (⋮) at the end of a row deletes it. The velocity pressure qz is calculated at each level from Equation 26.10-1 (Equation 27.3-1 in ASCE 7-10).

Parapets¶

The Parapets table lists parapet tops, each with a description and its elevation above grade. Add Parapet adds a row. Parapet pressures are reported per Section 27.3.4 (Section 27.4.5 in ASCE 7-10) at each one, for the windward and leeward parapets. ASCE 7-10 and 7-16 give parapet pressures for flat, gable, and hip roofs only, so on those editions a parapet on a monoslope roof is outside the provision. ASCE 7-22 covers parapets on any roof shape in Figure 27.3-1.

Roof¶

Design Pressures¶

The summary on the right writes out the calculation for each region. The outline button at the top of the summary lists its sections and jumps to one.

• Building Information and General Configuration: the dimensions, the gust-effect factor, and the mean roof height
• North-South Wind Flow and East-West Wind Flow: for each direction, the velocity pressures by level, the external pressure coefficients Cp for the windward, leeward, and side walls and for the roof zones, and the design pressures
• Wall Design Pressures: a table per direction with qz, and the windward, leeward, and sidewall pressures at every level and parapet, for Case 1 (+GCpi), Case 2 (-GCpi), and Case Minimum
• Parapet Design Pressures: the combined net pressure on windward and leeward parapets per Section 27.3.4
• Roof Design Pressures: the pressures by roof zone for both internal pressure cases. Zones measured from the windward edge apply for wind parallel to the ridge and for roofs under 10 degrees; for wind normal to the ridge of a roof at 10 degrees or more, Figure 27.3-1 gives windward and leeward slope values instead

Case 1 and Case 2 are the positive and negative internal pressure cases. They are not the design wind load cases 1 through 4 of Section 27.3.5 and Figure 27.3-8 (Section 27.4.6 and Figure 27.4-8 in ASCE 7-10). Case Minimum is the minimum load of Section 27.1.5: 16 psf on the walls and 8 psf on the roof area projected onto a vertical plane normal to the wind, applied together. It is not a minimum pressure on the roof surface, and it does not apply to parapets.

Code Reference¶