Short answer: not necessarily. ASCE 7-16 Section 30.2.3 permits you to design it with the MWFRS provisions instead. The catch is which area the threshold is measured on.
The provision
ASCE 7-16 Section 30.2.3, Tributary Areas Greater than 700 ft² (65 m²):
C&C elements with tributary areas greater than 700 ft² (65 m²) shall be permitted to be designed using the provisions for main wind force resisting systems (MWFRS).
It says tributary area rather than effective wind area, and it says permitted rather than required.
Why 700 ft² is a different number from the one on the figures
The Chapter 30 figures are entered with effective wind area. ASCE 7-16 Section 26.2 defines it, for C&C elements, as the span length multiplied by an effective width that need not be less than one-third the span. The commentary explains the floor as a better approximation of the load distribution on a long, narrow area; its effect is to reduce the average pressure on the member.
Commentary C26.2 makes the distinction explicit. Effective wind area equals tributary area except where the one-third rule is invoked, and then it can be larger. The enlarged area is used only to read GCp off the figure. The pressure is then applied over the actual tributary area. Effective wind area picks the coefficient; tributary area carries the load.
So a joist spanning 40 ft at 4 ft on center has a tributary area of 160 ft², but an effective wind area of 40 × (40 / 3) ≈ 533 ft². The figure is entered at 533. The Section 30.2.3 test is made at 160. Only the second number decides whether MWFRS is on the table.
Back to the question
A joist with 750 ft² of tributary area has cleared the threshold on the area that counts. Section 30.2.3 permits designing it with the MWFRS provisions of Chapter 27 or 28.
Whether you should is a judgment call. The C&C figures flatten out at large areas, so the two pressures often land close together. Where they diverge, the code is telling you a member this large behaves like part of the frame.
It cuts the other way too. Testing the 700 ft² threshold against effective wind area would send members to MWFRS that the code has not released. The joist above, at 533 ft² effective, is not there yet.
What Prose does
Prose enters the Chapter 30 figures with the effective wind area you give it and prints the coefficient reference next to the pressure. It never reroutes an element to MWFRS on its own. If you elect the Section 30.2.3 path, run the member in the MWFRS module and note the election in your calc.
Citations
- ASCE 7-16 Section 30.2.3, Tributary Areas Greater than 700 ft²
- ASCE 7-16 Section 26.2, definition of Effective Wind Area
- ASCE 7-16 Commentary C26.2, effective versus tributary area
- ASCE 7-22 Section 30.2.3 carries the same provision.
