Load Combination Spatial structures are subjected to various environmental loading conditions such as dead, live, snow, wind, earthquake, and thermal loads. Effects of the various combinations of these loads on the spatial structures should be considered.
Dead Load
For a general discussion of dead load, click here.
Cladding weight used for spatial structures ranges between 2 psf (corrugated deck) to 10 psf (sloped glazing) to 15 psf (built-up roofing). The weight of aluminum spatial structures is typically about 1-2 psf (high strength/weight ratio). The weight of steel spatial structures is about twice that of their aluminum counterparts. Obviously, these are approximate values and may vary based on the specific configuration, loading, support conditions, and locations. Using high-strength steel, strength/weight ratios of approximately equal to that of aluminum can be achieved.
Live Load
For a general discussion of live loads, click here.
Spatial structures are mostly used as roof systems, therefore, roof load is considered as the live load. For locations where snow loading should be considered in the design of spatial structures, it is generally considered as the roof live load.
Snow Load
For a general discussion of snow load, click here.
Snow load is the governing load for spatial structures design used as roof systems in nearly any weather condition outside of tropical climates. The snow load acts vertically on the horizontal projected roof surface area. In the case of flat spatial structures, snow load is considered as uniform and allows for drift based on the governing building code requirements.
Snow Drift
Due to the shape of barrel vaults they are subjected to non-uniform balanced snow loads. For these structures, wind can cause snow to be blown away from one side and result in unbalanced snow loading.
Snow Accumulating on a Double-Layer Braced Barrel Vaults
Snow Drift on a Double-Layer Braced Barrel Vault
Wind Load
For a general discussion of wind load, click here.
Flat spatial structures used for roofs are susceptible to wind suction (uplift) when wind passes over. Due to the light weight of these structures, the lower layer may be subjected to compression forces (stress reversal), which can control the design. This happens when the wind uplift is larger than the roof self-weight.
The wind loading on barrel vaults is more complex than flat roof systems. Depending on the slope, part of the roof is subjected to pressure and the remainder subjected to suction (uplift).
Wind Over a Flat Double-Layer Grid
Wind Over a Double-Layer Braced Barrel Vault
For irregular shape roof systems, due to the complex nature of the action of wind on the structure, wind tunnel testing is used to estimate the wind load.
Earthquake Load
For a general discussion of earthquake load, click here.
Due to the large strength/weight ratio of spatial structures, in most cases, earthquake load does not control the design.
Thermal Load
For an introduction on the effects of
changes in temperature and resulting stresses, click here.
Spatial structures are mostly used for roofs, canopies, atriums, curtain walls, etc. which make them susceptible to large fluctuations of temperature and therefore susceptible to axial forces (if restrained). When the spatial structure is restrained against horizontal movements, both the structure and its support system have to be designed to accommodate the effects of large temperature variations. There are different design strategies when considering this problem:
Completely restrain the supports and design the structural member for the generated thermal forces.
Design some supports to be of a free roller type such that thermal loads are only applied in one direction. This support condition can be achieved by using slotted or oversize expansion holes for bolted connections between spatial structures and their supports.
