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Develop a Proactive Severe Snow Plan

As part of your building maintenance plan, Needham DBS recommends developing a Severe Snow Plan to limit the impacts on your structure.  The plan should contain a list of contacts:  owner, building manager, local testing agencies, building engineer of record, local consulting engineers.  It's a good idea to establish an "on call" local testing agency and local engineer since travel from other regions may be difficult during an event.  The plan should list the design snow load for the building, the snow density used, the depth of the balanced snow load and what drift regions were included in the design with magnitude of load.  

Determine the Potential for Loss based on the following high risk factors:  Large Open Space Roof, Flat or Low Slope Roof, Parapet Projections, Tower Projections, Internal Roof Drains.  Any of these increase risk of failure during a severe event, a combination of these increases the risk further.

Monitoring Snowfall and Snow Density:  Density of the snow and ice can change based on the snowfall intensity as well as the temperature variations throughout the event.  Since the design basis is 16.6 pcf, it's a good idea to monitor temperatures and the density of the layers.  If snow melts and refreezes as ice it can double or triple its density.  Therefore, depth alone cannot be used in determining loading.  Use a 12" x 12" x full depth sample to check weight per square foot.  This can be done with the assistance of a testing agency if needed.

Snow Removal Considerations:

  • First, remove snow accumulation around drains and prevent area from freezing.
  • Second, remove large accumulations due to drifting and unbalanced snows (typically due to high wind in combination with snowfall).  Pay particular attention to parapets, projections, roof features.
  • If snow removal is expected - prepare the roof ahead of the snowfall by identifying protrusions such as hatches, drains, scuppers, vent pipes, utilities.
  • Do NOT remove all snow down to the roof covering, avoid damaging the membrane.
  • Prepare for snow removal when half of the design height has been reached.  This allows for the loads for the equipment and personnel to be within the capacity of the roof design.
  • Remove snow in strip patterns about 1/3 of the column spacing width starting mid-span of the columns.  A diagram should be included in the plan.
  • No all snow and ice must be removed from the perimeters.  Take care to NOT damage flashings and membrane.
  • Protect and barricade areas on the ground where snow will be dumped.
  • Do not dump snow on to canopies or other ancillary roofs.
  • Do not stockpile snow on the roof.
  • Do not use sharp tools to remove ice.
  • Do not block egress of the building when dumping or stockpiling snow on the ground.
  • Experienced and knowledgeable employees or roof contractors should be used for snow removal.

Structural Considerations:

The intent of the severe snow plan is to handle the maintenance of the building so that no structural damage occurs.  In the event that the snow cannot be cleared in a timely manner and there are any concerns that the structure may be unsafe, contact the on call structural engineer for guidance.

Signs of overstress would be:   severe roof leaks, ripples and bends in steel, rolling of members, loud popping noises, water ponding, obvious deformities on the roof, excessive deflections (greater than 2 inches).  If any of these items are observed, clear the area and contact the engineer of record and the on call structural engineer.

 

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