Wind Pressure
The design of the structure's primary lateral force system, as well as its elements and components, shall be based upon wind pressures determined from the following equation:
where
P = design wind pressure in psf.
- Qs = wind stagnation pressure in psf at a height of 33'
above the ground.
- This wind force is a function of wind velocity:
- Qs = .00256 V2
where V = windspeed in mph found from UBC Figure 16-1.- The applicable wind speed map can be found as Figure 2-1 in the article Nature of Wind
- Can also find Qs using UBC Table 16-F.
- The minimum design speed is taken as 70 mph.
- Refer to Wind Speeds for information relating windspeeds in mph to observations.
- V is based upon the 50-year mean recurrence interval for the fastest-mile wind speeds.
- Qs = .00256 V2
- This wind force is a function of wind velocity:
- Iw = Importance factor taken from UBC
Table 16 - K.
- Essential facilities and hazardous facilities are designed to withstand higher forces by setting Iw = 1.15, which is approximately equivalent to a 100 year recurrence interval.
- Otherwise, Iw = 1.0
- CE = combined height, exposure, gust coefficient found
in UBC Table 16 - G.
- Wind pressure increases with building height:
- For the windward wall, UBC uses a stepped pressure diagram as f(height).
- For the leeward wall, pressure taken as constant over the
full wall height and is calculated using the mean roof height
for CE determination.
- Turbulence (gusting) = f(terrain),
is quantified in terms of an exposure
rating:
- B - urban, suburban, closely spaced obstruction the size of single - family residences.
- C - open country and grasslands that are generally flat.
- D - unobstructed flat terrain facing a large body of water extending inland 1/4 mile or ten times building height, whichever is greater.
- UBC recognizes only 3 of the 4 possible exposure classes.
- It has not adopted Exposure A - building sites in large city centers.
- Wind pressure increases with building height:
- Cq = Pressure coefficient.
- Found in UBC table 16 - H.
- Cq = f (structure or part of, analysis method, openings)
- Structure or part of:
- Cq for primary LFRS.
- Considering the whole structure as it resists lateral forces.
- Applied to horizontal diaphragms and shearwalls.
- Cq for elements and components:
- Accounts for higher pressures that occur locally.
- Taken at locations away from discontinuities, and
- At discontinuities.
- Cq for primary LFRS.
- Primary frame analysis methods:
- Method 1: normal force method
- A more accurate description of wind forces.
- Windward and leeward forces acting normal to all exterior surfaces simultaneously.
- Method 2: the projected area method.
- Carry over from earlier codes.
- Simpler than method 1.
- Generally more conservative.
- Cannot be used for gabled rigid frames or structures greater than 200' in height.
- Simultaneous application of horizontal pressures on the vertical projected area and vertical pressure on the horizontal projected area.
- Method 1: normal force method
- Openings:
- Open (known as partially enclosed in newer codes like the '94 and '97 UBC's) structures have higher outward pressures than enclosed or unenclosed structures.
- Doors and windows are considered openings unless protected.
- Account for the effect of openings in partially enclosed buildings
by using a bigger Cq coefficient according to footnote
1 Table 16 - H.
- This effect could be simply thought of as what happens to the surface of a balloon as it is being blown up.
- For primary LFRS, method 1: increase Cq for roofs according to footnote 1 of UBC 16-H.
- For elements and components not at discontinuity: select Cq according to the element under consideration, and if building is unenclosed, enclosed, or partially enclosed.
- Definitions, according to '97 UBC 1616, for enclosed, partially
enclosed, and unenclosed:
- Aoi = area of opening on projected side i.
- Ai = total projected wall area of side i.
- Aoj = area of opening on projected side j.
- If Aoi / AI ³ 0.85 for all sides, then structure is unenclosed.
- If Aoi / AI > 0.15 for side i
and if
S Aoj < .5 Aoi for all other sides j, then structure is partially enclosed. - All other cases, consider the building as enclosed.
