Wind Loads and Pressure Paths

Wind Loads and Pressure Paths

Wind questions are easiest when treated as a controlled sequence rather than a search for one memorized pressure. For the July 2026 PE Civil: Structural exam, the reference lock is ASCE 7-16, IBC 2018, and, where a wood connection is checked, NDS 2018/SDPWS 2015 using ASD. Do not import ASCE 7-22 wind maps, tornado provisions, or revised coefficients.

The ASCE 7-16 workflow

Use this order so that every coefficient has a defined role:

1. Assign the building's risk category and obtain the corresponding basic wind speed, V, from the ASCE 7-16 map. Risk category selects a map; it is not an extra wind importance factor in the velocity-pressure equation.
2. Establish Exposure B, C, or D from the upwind surface roughness and exposure distance. Exposure is a site condition, not the building occupancy.
3. Determine directionality Kd, topographic factor Kzt, ground-elevation factor Ke, and the height-dependent exposure coefficient Kz or mean-roof coefficient Kh.
4. Classify the enclosure as open, partially open where applicable, enclosed, or partially enclosed. Then take the applicable internal pressure coefficient GCpi.
5. Compute velocity pressure in US customary units with the ASCE 7-16 form qz = 0.00256 Kz Kzt Kd Ke V^2, in psf. Use qh when the procedure calls for pressure at mean roof height.
6. Select the permitted analytical procedure and the external coefficient for the surface, wind direction, roof geometry, and effective wind area.
7. Calculate every required signed pressure case, then convert pressure to force on the correct tributary or effective area.
8. Apply the force to the correct system and trace it to ground.

The factors must come from the supplied standard or the problem statement. A common exam error is to use Kz at one height for every windward wall point even when the selected procedure requires qz, or to insert an old-edition importance factor into the ASCE 7-16 equation.

Preserve the pressure signs

For the directional MWFRS procedure, the pressure relationship has the form

p = q(GCp) - qi(GCpi)

where the external term may be written as q G Cp, and the internal term uses the velocity pressure specified by the procedure. Positive net pressure acts toward a surface; negative pressure acts away from it. Thus negative roof pressure is uplift. Never discard a minus sign just because the final connection force is reported as a positive uplift magnitude.

Internal pressure must normally be checked with both signs. Positive internal pressure pushes outward. It makes an already negative external roof pressure more negative because the internal term is subtracted. Negative internal pressure tends to increase inward pressure on a windward wall. The controlling sign therefore depends on the surface and external coefficient.

Worked pressure and uplift check

Assume an enclosed, low-rise building has already been evaluated under ASCE 7-16 and the problem gives qh = qi = 30 psf. For one roof zone, the given product GCp is -0.90; for an enclosed building, evaluate GCpi = +0.18 and -0.18.

The first case governs uplift. If a connection receives load from 6 ft × 10 ft = 60 ft², its nominal uplift demand from this pressure is 32.4 × 60 = 1,944 lb, or 1.94 kip, before applying the load combination appropriate to the design method. The coefficient -0.90 is an assumed problem input, not a universal roof value; roof angle, zone, procedure, and effective wind area can change it.

Now trace the 1.94-kip action: roof covering and sheathing transfer suction into fasteners; sheathing transfers it to joists or rafters; framing transfers it through clips, straps, or bearing connections to walls; wall anchorage carries it into the floor or foundation; the foundation supplies resistance to the ground. A strong rafter with a weak sheathing fastener or missing wall-to-foundation tie does not provide a complete wind design.

MWFRS versus components and cladding

The main wind-force-resisting system (MWFRS) stabilizes the structure as a whole: frames, shear walls, diaphragms, and their collectors and foundations. Components and cladding (C&C) transfer wind from local surfaces: glazing, panels, roof deck, purlins, girts, fasteners, and their immediate connections. Local corner and edge suctions can make a C&C pressure much larger than an area-averaged MWFRS pressure.

Do not add an MWFRS pressure and a C&C pressure as if they were simultaneous independent loads on the same piece. Instead, classify the element by function and follow the applicable ASCE 7-16 procedure. An element can participate in both roles, but each role requires the correct load model. For C&C, carefully identify effective wind area; it is a code-defined coefficient-selection parameter and is not always identical to a simple tributary area.

Exam-quality checks

Before accepting an answer, ask:

• Did I use ASCE 7-16 wind speed and coefficients?
• Did I evaluate every required wind direction and both internal-pressure signs?
• Is suction still negative in the pressure calculation?
• Am I checking MWFRS or C&C, and did I use that procedure's area and coefficient?
• Did I convert psf to force with the correct area and units?
• Does the load path continue through connections and anchorage to the foundation?

These checks catch most wind errors before detailed member design begins.