It's hard to interrupt this season with talk of snow, especially after the winter of '97. But experts say if you're constructing or reconstructing a building, you should be thinking about snow. Snow load, more specifically.
In the upper midwest, snow load is the biggest weather threat to roofs. It simply means the weight of snow collected on a building. Some structures are engineered to better withstand higher levels of snow load.
Last year, when many areas in the region received two to three times the average snowfall, many people saw what snow can do to buildings. More than 5,000 structures, many of them agricultural buildings, collapsed under last year's great white weight.
While there may not be historic snows every year, the region usually gets more than its fair share. That's why it's important to plan for snow now.
John Dierauer, a structural engineer with Lester Building Systems, provides some answers to important questions about snow load.
Each building is different, so these are some general recommendations. Check with your local builder for more exact details.
How can I minimize the chance of my building collapsing under the weight of snow?
Proper design for roof snow load capacity is the key. Designers consider the building type and factor how much snow is expected to accumulate on the roof, how much will drift to certain areas of the roof and how much will blow off.
Whether building for the first time or rebuilding, make sure you ask specifically about roof snow load because it is easy to be confused when other types of load capacities are quoted.
Where you build matters, too. Sometimes buildings near larger structures, such as other buildings, trees or silos are more likely to collect drifting snow that increases snow load. Let builders know the surroundings so they can make proper design adjustments.
How many pounds of snow should a building be able to hold?
That varies. Snow load capacities depend on the building type, location, occupancy, and primary use. For non-commercial, agricultural buildings, snow load designs can range from 12 to 40 pounds per square foot. Commercial building snow load designs are dictated by local building codes.
What are the key engineering elements that will help my building carry snow load?
The roof pitch and material of the roof can influence how much snow stays on the roof. Lester ag buildings typically have a 4:12 pitch (four feet raise to every 12 feet in length) and steel sheeting that allows snow to more easily slide off the roof.
A truss is the backbone of a building. Trusses should interlock with the walls, creating a stable roof. Bottom chord bracing that runs the entire length of the building and sets into heavy metal pockets provides additional stability.
Remember, the building is only as strong as its weakest link. That's why it is so important that individual elements be designed to fit together as a system. Make sure all the building components (purlins, fasteners, wall girts, bottom chord bracings, etc. ) are designed to stand as one unit against snow load and other environmental stresses.
Which designs are more vulnerable to collapse?
Any building that is not designed as a system is more prone to collapse.
Special design considerations are required for buildings
with multiple roof lines because they tend to collect snow. For instance,
if you have a 14-foot high shop connected to a 10-foot high office, snow
drift can be a problem. So design adjustments must be made. The same goes
for buildings with varied architectural treatments such as peaks, valleys