What is the difference between C&C and MWFRS wind loads?

MWFRS (Main Wind Force Resisting System) sizes the whole structure - frames, shear walls, diaphragms, and foundations - using wind averaged over large surfaces, so pressures are lower. C&C (Components and Cladding) sizes individual envelope elements like windows, panels, and fasteners over small effective areas, producing higher local peak pressures, especially at corners and edges.

Why are C&C pressures higher than MWFRS pressures?

Both methods start from the same velocity pressure, but C&C runs higher locally because a small panel cannot average out gusts the way a whole face does, so its GCp coefficient climbs as effective wind area shrinks. Flow separation and vortices at roof Zone 3 and wall Zone 5 corners and edges further spike local suction to the highest values.

Which ASCE 7-22 chapters cover C&C and MWFRS?

In ASCE 7-22, Components and Cladding are covered in Chapter 30 using GCp coefficients. MWFRS uses Chapter 27 (the directional procedure with Cp, Figure 27.3-1) or Chapter 28 (the envelope procedure with GCpf, Figure 28.3-1). Both procedures share the same velocity pressure basis and only differ in coefficient and effective wind area.

How is velocity pressure calculated for C&C and MWFRS?

Both procedures use the same velocity pressure equation: qz equals 0.00256 times Kz times Kzt times Kd times Ke times V squared. For buildings the directionality factor Kd is 0.85, and the gust-effect factor G is 0.85 for rigid buildings. Only the pressure coefficient and the effective wind area change between the two methods.

ASCE 7-22 · DESIGN METHODS

C&C vs MWFRS — two checks, one building

Q: Do I have to check both C&C and MWFRS for the same building?

Yes. ASCE 7 requires both checks on the same building. MWFRS pressures are lower and averaged and must never size cladding, while C&C pressures are local and never represent the whole-building load path. A complete design runs MWFRS for the structure and C&C for every envelope element such as windows, doors, and roof panels.

The structural frame carries the whole building's wind. The cladding fights it panel by panel. ASCE 7 makes you check both.

Ch 30C&C — GCp

Ch 27/28MWFRS — Cp / GCpf

0.85Kd · G (BUILDINGS)

−3.2ROOF ZONE 3 GCp @10 SQ FT

THE DISTINCTION

One system carries the whole load. One element takes the local hit.

MWFRS averages wind across large surfaces. C&C concentrates it on a single window, panel, or fastener.

SIDE BY SIDE

Same wind, two procedures

Both start from the same velocity pressure, then diverge on coefficients, areas, and where pressures peak.

Characteristic	MWFRS	C&C
What it covers	Frames, shear walls, diaphragms, foundations — the whole system carrying overall wind	Windows, doors, roof panels, fasteners, cladding — elements receiving wind directly
Coefficient	Cp (directional) or GCpf (envelope)	GCp
Effective wind area	Large — averaged over whole faces	Small — per component, by zone
Where pressures peak	Lower, averaged across the surface	Highest at corners & edges (Zone 3 roof, Zone 5 wall)
ASCE 7-22 chapter	Ch 27 Fig 27.3-1 (directional) · Ch 28 Fig 28.3-1 (envelope)	Ch 30

WHY C&C RUNS HIGHER LOCALLY

Small area + corner zones = peak pressure

Both methods use the same velocity pressure. C&C diverges because tiny areas at separation corners see concentrated gusts.

Smaller effective area

A single panel can't average gusts the way a whole face does, so its GCp climbs as area shrinks.

GCp ↑ AS AREA ↓

Corner & edge zones

Flow separation and vortices spike suction at roof Zone 3 and wall Zone 5 — the highest local coefficients.

ZONE 3 / ZONE 5

Peak suction values

Verified GCp peaks: roof Zone 3 −3.2 and wall Zone 5 −1.4, both at a 10 sq ft effective wind area.

−3.2 ROOF · −1.4 WALL

Shared basis. Both procedures use the same velocity pressure q_h = 0.00256 · K_z · K_zt · K_d · K_e · V², with K_d = 0.85 and gust factor G = 0.85 for rigid buildings. Only the coefficient and effective area change.

NON-NEGOTIABLE

You check the same building twice

MWFRS pressures are lower and averaged — never use them to size cladding. C&C pressures are local and never represent the whole-building load path. A complete design runs both: MWFRS for the structure, C&C for every envelope element.

NEXT STEPS

Run both checks the right way

Pin down your method, then size the structure and the envelope.

Calculate C&C and MWFRS Design Methods Guide