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Wind Loads on Curved Roofs

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Frank Hartzell  wrote:

>>"Steel Structures" by William McGuire, Prentice-Hall, 1968 states that the
most comprehensive reference of wind shape factors,etc. is given by the
Swiss Building Code. McGuire presents portions of the code, including
coefficients for curved roofs, in his Appendix.

He references "Standards of the Swiss Association of Engineers and
Architects on Load Assumptions, Acceptance, and Supervision of Buildings",
Schweizerischer Ingenieur und Architekten Verein, Technische Normen No. 160,
1956.  There is very probably a more current version of the Swiss Standard.<<

These wind loads and diagrams from the Swiss Building Code were also 
published in ASCE Transactions, Part II, Vol. 126, 1961, pg. 1154-1165, and 
were also published in AITC's Timber Construction Manual, 1st and 2nd 
editions.  According to the ASCE paper, the Swiss Building Code wind load 
coefficients were from wind tunnel tests; one of very few wind tunnel tests 
that have been performed on low rise buildings.

The most comprehensive study of wind on a curved surface was probably ASCE's 
instrumentation of the Goodyear Zeppelin hanger (Civil Engineering, November, 
1930, pp 71-76).  A diagram of the wind pressures measured on the hanger 
(325-ft horiz. span; 197.5-ft rise) published in "Modern Steel Design," Vol. 
1, First Edition, 7th printing, 1956, by Linton E. Grinter, shows pressure at 
the ground of 37.5 psf on the windward side, changing to a suction of 150 psf 
near the windward side of peak.

One thing that should be remembered in using ANSI A58.1/ASCE 7 wind load 
coefficients is that pressures acting *toward* the surface are *positive*, 
and pressures acting *away* from the surface are *negative*.  Therefore, if 
you have a negative external pressure coefficient, and a positive internal 
pressure coefficient, the resultant pressure is a suction with a coefficient 
equal to the *sum* of the absolute values of the coefficients.  If both 
coefficients have the same sign, then the resultant pressure is the algebraic 
difference between the two coefficients.

Code required wind coefficients should be considered as only very rough 
approximations of what really happens.  The coefficients have been reduced 
since the 1985 UBC (and ANSI A58.1-1982), not because they were wrong, but 
because they generally showed extremely large suction pressures at roof 
edges and building corners.  Note that in the 1994 UBC, Table 16-H, that 
the pressure coefficient for Item 3, "Elements and Components ... at 
discontinuities," for "Wall Corners" is less than the pressure for Item 2, 
"Elements and components not in areas of discontinuities," for "Wall 
Elements" of partially enclosed structures.  The coefficients also do not and 
cannot take into consideration channeling or shielding.  There is one street 
intersection in downtown Tucson where the wind is always stronger than a 
similar intersection a block away.  This condition existed in 1946 and it 
exists today, even though some buildings have been demolished, new buildings 
built, and open space now exists on one quadrant.

I consider all buildings as partially enclosed unless they are constructed 
as bunkers without windows and with a security door.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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