MWFRS loads on gable end of open building, gamma = 90 deg 1

[1835Eng]

I'm trying to find a design value for MWFRS wind load applied to the gable end of an open (each wall greater than 80% open) PEMB structure.

ASCE 7-10 Figure 27.4-7 gives a C_sub_N value for "contributions from top and bottom surfaces" but what about the side surfaces? The Guide to the Wind Load Provisions of ASCE 7-10 by Mehta and Coulbourne skip past this in their open-building example and go straight to C&C loads on the truss.

There seems to be a gap in the code for finding loads on the <20% of the wall structure that isn't open and isn't roof. I have a narrow (2') strip of banding or fascia below the eave that is going to contribute lateral load to the rigid frames, and the side panels on the gable ends that will load the rod/cable bracing. Is it conservative to simply apply the velocity pressure (which in this case is greater than code-minimum 16 psf) to those areas, with no additional factors?

 

[ajh1]

ASCE 7-16 has added a new section that covers this. The basic equations came out of work that was undertaken by MBMA a number of years ago.

28.3.5 Horizontal Wind Loads on Open or Partially Enclosed Buildings with Transverse
Frames and Pitched Roofs. A horizontal pressure in the longitudinal direction (parallel to the
ridge) that acts in combination with the roof load calculated in Section 27.4.3 for an open or
partially enclosed building with transverse frames and a pitched roof (θ < 45o) shall be
determined by the following equation:
p = qh[(GCpf)windward – (GCpf)leeward] KBKS (28.43-3)
where qh = velocity pressure evaluated at mean roof height h using the exposure as defined in
Section 26.7.3;
(GCpf) = external pressure coefficient given in Fig. 28.43-1 for Load Case B where building
surfaces 5 and 5E shall be used to compute the average windward end wall pressure and
building surfaces 6 and 6E shall be used to compute the average leeward end wall pressure;
KB = frame width factor = {1.8 – 0.01B, B < 100 ft (30.5 m) or 0.8, B > 100 ft (30.5 m);
KS = shielding factor = 0.60 + 0.073(n – 3) + (1.25 φ1.8);
φ = solidity ratio = AS/AE;
B = width of the building perpendicular to the ridge, in ft (m);
n = number of frames but shall not be taken as less than n = 3;
AS = effective solid area of the end wall, i.e., the projected area of any portion of the end wall
that would be exposed to the wind (Fig. 28.43-2); and
AE = the total end wall area for and equivalent enclosed building (Fig. 28.43-2).
The total longitudinal force F to be resisted by the MWFRS shall be determined by the
following equation:
F = pAE (28.43-4)
Eq. (28.43-3) is applicable to buildings with open end walls and with end walls fully or
partially enclosed with cladding. For all cases, AE shall be the area that is equivalent to the end
wall fully enclosed. The longitudinal force, F, given by Eq. (28.43-4), represents the total force
for which the MWFRS longitudinal bracing shall be designed. The distribution to each sidewall
shall be based on force F applied at the centroid of the end wall area AE.
Fascia load need not be considered separately if fascia areas are included in the AS calculation.