Design Pressures and Building Zones
Now that we know the basic parameters used to determine the wind loads, let's delve into two important ones: building zones and design pressures. In this post, we will understand how pressures act on a structure, and how we use this knowledge to calculate the minimum pressures a building designer must assign to a structure to make it safe for occupancy and use. We will define design pressures, building enclosure classifications and zones. This will lay the foundation for understanding how manufacturers design their products for high performance.
Did you know a building can be under pressure at any given time? In fact, a building is subject to pressure from the inside and the outside. For purposes of this writing, the design pressures referenced are those exerted on the building envelope (or exterior surrounding wall).
There are positive pressures exerted from the inside of the building to the outer walls. Design pressure is defined as the equivalent static pressure to be used in the determination of wind loads for buildings.
Cladding manufacturers provide performance information for their siding according to design pressures they can meet with a prescribed fastening schedule. These can be found in third party code evaluation reports, and in the manufacturer's published technical information.
There are also negative and positive pressures exerted on the outer walls; these can come from the outside, from wind or other physical factors. The terms 'positive' and 'negative' denote the direction in which the pressure is exerted on the outer walls and roof of the structure. The direction of these forces can be dynamic; that is, they can change depending on weather conditions. This changing nature complicates matters for the building designers. Figure 1 below shows the different directions in which wind can act on a building.
Figure 1: Wind Pressures Exerted on Building's Exterior Walls
As designers, we contend with wind suction on at least three of a building's exterior walls (negative), there are positive pressures on the wall encountering the prevailing wind, and there is uplift exerted on the roof.
The overall design load on a wall takes the following into account:
' Basic wind speed (discussed in more detail in Wind Load Series Part 1, defined in section 26.5 of ASCE7)
' Gust effect factor (as defined in section 26.9.2 in ASCE7)
' Natural frequency of structure (approximated with formulas according to section 26.9.3 in ASCE7)
' Structural damping
' Aerodynamic admittance
' Wind directionality factor (determined from table 26.6-1 in ASCE7)
' Topographic factor (determined from table 26.8-1 in ASCE7)
' Enclosure Classification (as defined in section 26.10 in ASCE7)
' Internal Pressure Coefficient (depends on Enclosure Classification and derived in tables 26.11-1)
These values are derived in a variety of ways. For simplification purposes, we will say that some of these are derived based on historical data, through fluid dynamics and vibrations tables that the engineer can reference in the SEI handbook for wind load designs: ASCE/SEI7: Minimum Design Loads for Buildings and Other Structures.
Love's Tire Barn with Nichiha KuraStone & VintageBrick - Enclosure Classification: Open = lower design pressures
There are three possible enclosure classifications for a building:
1. Open: a building that has each wall at least 80% open (no doors), for example a truck stop or tire barn, canopy.
2. Partially Enclosed: there are two different parameters for partially enclosed relating to pressure equalization.
3. Enclosed: is a building that does not fall into the above two other categories.
Wind load calculations are simplified for structures termed Low-Rise Buildings: buildings less than or equal to a mean roof height of 60 feet. The calculations get more complex and conservative for buildings above 60 feet.
Ever experienced the ultra-high winds while standing outside the top of a skyscraper? Get a windblown hair-do, without the expense of the salon!
Pressures also vary depending on height and where in the building they are exerted. ASCE7 breaks down buildings into zones. Zones 4 and 5 are located on a structure's exterior walls. If you deduced that Zone 5 regions experience the highest wind gusts, you are right! Zone 4 regions are not subjected to as strong pressures. The width of Zone 5 region, depicted by the letter 'a' in Figure 2 below, can be determined by a variety of methods and is dependent on the overall wind dimensions.
A good rule of thumb is that width (a) = 10% of the least building dimension or 40% of the height (h).
Why does anyone need to know how wide the Zones are?
1. The Designer/Engineer takes into account different factors such as gust effect (based on historical data, components and cladding manufacturers will typically provide differing fastening schedules for Zone 5 and Zone 4 regions in a building, especially in areas with high basic wind speeds (see Wind Load Series Part 1).
2. If there is a difference in installation instructions within the same wall of a building for differing zones, the installer should know this as well.
But how does a siding manufacturer know what the fastening schedules should be in any given building? Stay tuned for our next post in the series where we will explain the thorough tests we put our products through to determine performance and keep our customers safe!