Structural design for wind load involves a determination of wind pressures, analysis of the forces in the wall as a result of these pressures and the selection/design of members and components to resist the loads.
Table 4.2: Secondary loads
PRIMARY SECONDARY
Gravity or dead loads Wind Seismic Impact Guard Window washing Blast Sunshades Temperature, snow/ice
4-4
Determination of Wind Load
Wind loading is determined by either reference to relevant building codes or through specific modeling in a boundary layer wind tunnel (BLWT). The characteristics of each approach are summarized below:
Code Derived Wind Load – Features and Limitations
• NBCC provides relatively simple procedures for calculating wind load based on building height, shape and location.
• Total load to be resisted by wall is the algebraic sum of external and internal pressures.
• Formulae are provided to calculate external and internal pressures, with special factors included for the design of cladding elements. • Reference wind velocity pressure for cladding design is based on a
probability of being exceeded in any single year of one in ten. • Procedures only truly apply to regular rectangular shaped buildings.
Do not identify peak corner or hot spot load locations. Tend to overestimate positive pressures and underestimate negative pressures on high rise buildings. Do not account for special conditions such as crosswind deflection, vortex shedding or instability due to flutter or galloping.
Wind Velocity Pressures
With the rapidly escalating value of the contents of buildings the cladding of many newer buildings is designed for a more stringent, one in thirty, probability of being exceeded in any single year rather than the NBCC requirement of one in ten probability.
BLWT Derived Wind Load – Features and Limitations
• Given the limitations of code derived wind load usage of BLWT has increased steadily since the 1950s.
• Most frequently used on major buildings, building of unusual height or shape and buildings having special design features.
• Increase in number of BLWT available is reducing cost and studies now are done on lower rise buildings.
• By nature BLWT takes into account effects of local topography, surrounding buildings and wind direction.
• BLWT testing provides direct project specific information in lieu of generalized code values.
• BLWT testing provides far more detailed information than code derived procedures. Information allows for optimization of glass
Analysis/Design for Wind Effects
Once the wind pressures are determined, analysis can proceed to evaluate the forces that the metal and glass must resist. This analysis is carried out at several levels starting with consideration of the overall grid framework, its connections and anchors and then proceeding to the infill elements such as glass and panels and finally to the fixing of the infill in place in the gridwork. Wind load charts are produced for standard systems by most of the major curtain wall manufacturers, an example of which is shown in Figure 4.1. Charts are also available that consider in-plane dead loads.
Wind load charts can be useful particularly in preliminary design and in some cases can be used in final design. However, all charts contain several
limitations that should be considered.
Figure 4.2: Wind load charts
Wind Load Charts – Limitations and Cautions
• Charts are based on simple single spans and are most appropriate to design of horizontal rails.
• Chart values are unique to a specific aluminum alloy. At least three different alloys/tempers are in common use (see Chapter 3-
Components).
• Charts are usually based on most liberal deflection limit (L/175). • Frequently, charts do not consider lateral buckling of mullions (usually
not an issue with solid tubular members).
• Well prepared charts are based on specific code. Canadian and USA based manufacturer charts are not the same.
4-6
In most instances the horizontal rails are designed as simple beams transferring gravity and lateral loads to the vertical mullions. As such
windload charts are most appropriate to horizontal rail design. Any end fixity of the horizontal rails through their connection to the mullions is ignored. However, the horizontal rails do act as lateral braces to the mullions to increase their resistance to lateral buckling.
Wind pressure is typically applied to infill elements as uniform load. Analysis of most infill elements themselves involves a combination of bending and membrane theory discussion of which is beyond the scope of this document. However, resultant loads are transferred to the framing system based on tributary area concepts as illustrated below in Figure 4.3.
The vertical mullions are analyzed in one of three different ways depending on the position of the expansion or stack joint, as illustrated in Figure 4.4.
Figure 4.3: Resultant loads transferred to the framing system
Simple Span Continuous with shear connect
Continuous with moment connect • Span condition
dictated by sill rail elevation • Most inefficient structural condition • Most commonly designed condition • Mullion spigots provide shear connection but permit rotation • Least common design condition • Most structurally efficient condition • More difficult to install and fabricate
Figure 4.4: Vertical mullions
Pressure Equalization and Wind Load Design
Pressure equalization of a curtain wall has no bearing on the overall design wind load a wall must resist. However, the use of pressure
equalized design principles to reduce the wind load across the outer panel in a rainscreen assembly has been suggested. Standard wind load test procedures and computer modeling indicate a potential decrease in the maximum pressure across a vented outer panel and a very short duration to any maximum pressure.
Except in special cases, most prudent designers do not apply any pressure equalization reduced pressures to the strength design of a panel or its connections. However, applying the full wind load to a deflection analysis may be unduly conservative particularly with thin panels. Except where snap through, flutter or deflection induced disengagement is an issue, pressure equalization reduced pressures have been successfully used in deflection analysis.
4-8
In most curtain wall design it is stiffness that governs the design. Therefore structural analysis often focuses on determining a sufficiently stiff cross section first and then checking strength at critical joints.
For small single story buildings using standard tubular members design for wind load may be based on reference to wind load charts (see cautions above). As building size increases, the complexity of the analysis increases, as does the potential for greater economy in design.
With standard systems, design includes selecting a stock member that meets or slightly exceeds the minimum structural requirements. With a custom system, design the most efficient section that just meets these same structural requirements can be developed