Nature seems to be getting more
violent in recent years with frequent earthquakes, increased numbers of
hurricanes and record breaking snowstorms.
Insurance damage claims have increased considerably. The International Building Code has revised
upward its wind and snow loading requirements for some areas of the U.S.
Each year there are reports of
greenhouses that have been damaged by weather and natural events. Greenhouse design is different than
conventional farm buildings in that the structural profile has to be small to allow
maximum light to reach the plants. Most farm buildings are over designed to
handle severe weather conditions.
Damage to greenhouses can
include racking of the frame, bending of the hoops, broken glass or torn
plastic and uplifted foundation posts.
Preparation ahead of time can minimize the damage.
Wind loading
Wind forces that act on a
greenhouse are influenced by numerous factors including the basics wind speed,
building orientation, exposure, height and shape of doors or vents that may be
open. The wind passing over a greenhouse
creates a positive pressure on the windward side and a negative pressure on the
leeward side. These can combine to
create a force that wants to collapse or overturn the building. An 80 mph wind can produce a pressure of 16
pounds per square foot (psf). For
example, the 10’ by 100’ sidewall of a gutter-connected greenhouse would have
to resist a 16,000 pound force.
Wind can also create a force
similar to an aircraft wing that wants to lift the greenhouse off the
ground. An 80 mph wind blowing
perpendicular to the side of a 28’ x 100’ hoophouse can create a lifting force
of 220 pounds per foot of length or 22,000 pounds of uplift on the whole
structure. When you consider the total
weight of materials and equipment in the greenhouse is about 6000 pounds, the
foundation must have a withdrawl resistance of about 300 pounds each. This is why building inspectors frequently
require that the posts be surrounded by concrete.
Although you have no control
over the force or direction of severe winds, here are a few tips to help
minimize storm damage:
- Check the area for loose objects. Anything that can be picked up and
hurled through the glazing should be secured or moved indoors. Metal chimney (stove pipe) sections
should be secured with sheet metal screws.
- Inspect for dry or weak tree limbs that could fall on
the greenhouse.
- Close all openings including vents, louvers and
doors. The effective force of the
wind is doubled when it is allowed inside the building. The wind on the outside puts a pressure
or lifting force on the structure.
The wind inside tries to force the walls and roof off.
- On air inflated greenhouses, increase the inflation
pressure slightly by opening the blower’s intake valve. This will reduce the rippling
effect. Check to see that the
plastic is attached securely and that any holes are taped.
- Disconnect the arm to the motor on all ventilation –
intake shutters and tape the shutters closed. Then turn on enough exhaust fans to
create a vacuum in the greenhouse.
This will suck the plastic tight against the frame.
- Windbreaks can reduce the wind speed and deflect it over the greenhouse. Conifer trees (hemlock, spruce, pine, etc.) in a double row located at least 50’ upwind from the greenhouse can reduce the damaging effects of the wind. Wood or plastic storm fencing can be used as a temporary measure.
Snow loading
Snow that accumulates on a
greenhouse can put significant weight on the structural members. Snow loads vary considerably from 0 along the
southern coastline to more than 100 pounds per square foot in Northern
Maine . Local building codes specify the design snow load.
Snow can be light and fluffy
with a water equivalent of 12” of snow equal to 1” of rain. It can also be wet and heavy with 3” equal to
1” of rain. Snow having a 1” rain water
equivalent will load a greenhouse with 5.2 psf.
This amounts to 6.5 tons on a 25’ x 96’ greenhouse.
The following are a few pointers
to consider before the next snow season:
- The foundation piers or posts should be large enough
to support the weight of the building including crop and equipment loads.
- All greenhouses should have diagonal bracing to keep
it from racking from the weight of the snow or force of the wind.
- Collar ties and post connections should have adequate
bolts or screws. This is a weak
point in some greenhouse designs.
- Allow 10’ to 12’ between individual greenhouse for
snow accumulation and to prevent sidewalls from being crushed in.
- When building new hoophouses, consider using a gothic
design that sheds snow easier. In
hoop shaped houses, install 2” x 4” posts under the ridge every 10’ when
heavy snow is predicted.
- The heating system should be large enough to maintain
60ºF to melt snow and ice. It takes
250 Btu/hr per square foot of glazing to melt a wet snow falling at a rate
of 1”/hour. Heat should be turned
on in the greenhouse or under the gutter several hours before the storm
begins.
- The plastic should be tight and inflated to at least
0.25” water pressure. This can be
checked with a monometer. Any cracked or broken glass should be replaced.
- Energy screens should be retracted to allow heat to
the glazing.
- A standby generator should be available with adequate fuel for the duration of the storm to power heaters, fans and blowers.
Selection of greenhouses that
meet the International Building Code and good construction techniques are
important considerations when building new greenhouses. A little preparation before a storm can
minimize damage from severe weather events.
(by John W. Bartok, Jr.)
(photo by Ted Carey, KSU Extension)
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