Before You Buy
Critical Design Factors to
Consider
Not all structures are created equal and
because of this we are often asked, "What makes your product
different?" and "What should I look for when comparing products?"
We understand building designs are difficult to compare and so is
making decisions around which one will provide you with the best
long term value for your purchase, as well as being the safest and
most functional. When comparing products, the following information
will enable you to ask the right questions, ensuring you have
purchased a product that meets all your requirements, and that your
product is safe, minimizes your liability, and is insurable.
In order to compare designs and ensure a safe, long lasting
product is purchased, it is increasingly important that customers
understand basic engineering topics that relate to fabric covered
steel buildings.
Steel Strength
Significant factors influencing the safety and longevity of a
building are the quality, thickness (gauge) and the total amount of
steel used in a structure. One of the results of inadequate steel
is called "web punching". Web punching occurs when the truss webs
punch through the walls of the truss chords if the steel in the
truss chord is too thin. Thin steel can lead to truss chord
buckling at the web connection location. This process is called
"chord plastification".
Many manufacturers use the least steel
possible when designing their buildings and often use thin 14 gauge
steel for their truss chords. Light gauge steel like this permits
excess flexing of the truss and buckling at low web forces.
Norseman Structures recognizes that undersized
steel has the possibility to fail in this manner.
To combat this, our designs require the use of larger diameter
webs as well as thicker gauge steel in our truss chords, thus
preventing any "punch through".
An effective way to compare building
structures is by weight of the structural steel in the building. As
it is unlikely a competitive design could be safely engineered
using ten percent less steel than our buildings, it is likely the
competitive design does not meet all building code requirements.
Norseman Structures also utilizes heavy gauge steel in order to
meet the needs of our customers.
This picture shows an example of the dangers of "web punching". As
you can see, the truss webbing has 'punched' through the wall of
the thin gauge steel likely causing a ripple effect throughout the
rest of the structure. The end result of this unfortunate
occurrence is at best an unusable building and at worst, a
catastrophic failure causing a collapse.
Web punching occurs when the truss webs punch through the walls of
the truss chords if the steel in the truss chord is too thin.
Bracing
Bracing is required to give the structure multidimensional
strength against wind and snow load forces. It must also be strong
enough to be able to counteract the force of the fabric under
tension. Bracing systems include the use of purlins (square or
round tubes fixed between truss sections) and cable systems that
can crisscross multiple trusses.
Many manufacturers underestimate the forces on
the building and use too few and too slender roof purlins and wind
bracing.
Norseman Structures use larger diameter roof
purlins and heavier cables to resist the fabric tension loads, as
well as to provide stability bracing for the roof trusses.
Snow & Wind Loads
Building codes require engineers to consider the impact of wind
and snow accumulation on building designs. However, some
manufacturers treat these as separate factors and ignore the
possibility of the effects of 'combined' snow and wind loads using
the theory that wind will blow snow off the building.
By not considering these factors together,
your building could be under-engineered allowing for the use of
lighter steel and/or wider truss spacing as well as reducing the
strength and cost of the building. Unfortunately, due to these
factors, the structure will likely be unsafe in certain conditions
exposing users to undue safety risk.
Norseman Structures understands wind will not
blow snow off buildings under certain temperature and humidity
conditions and considers all combinations of combined snow and wind
loads required by the Building Codes.
Snow Shedding Qualities
Some manufacturers make an assumption that a sloped roof and
slick fabric properties will allow snow to be shed automatically,
reducing the effect of potential snow loads and allowing for a
lighter and less costly structure. This, "snow shed" design theory
does not always hold true.
Snow shed decreases over time. As fabric
ages and becomes rougher, the possibility of snow accumulation
increases, decreasing the safety factor of the building.
Certain temperature and humidity conditions
also contribute to greater snow and ice adhesion to fabrics and
eliminate the ability of the fabric to shed snow efficiently.
Norseman Structures always designs to the
strict snow load requirements identified in local building codes.
Our designs include larger diameter and thicker steel than
competitors, in order to provide safe, long lasting structures.
Use and Occupancy
Most building codes allow for the strength of building designs
to be adjusted based on the level of human occupancy - the more
human use within the structure, the higher the engineering
requirements or "hazard" rating. The choice of hazard ratings used
in the design can have a dramatic impact on the overall strength,
and therefore safety of the structure.
To reduce costs, some manufacturers use a
Low Hazard importance factor for all buildings. The use of
sub-standard importance factors allows a building to be
under-designed in order to save on steel. The end result is a more
inexpensive product for the customer, but a building that may only
be safe under certain weather conditions.
Norseman Structures only uses the Low Hazard
rating for low occupancy buildings such as cold storage facilities
and agricultural storage buildings. A Normal importance factor of
1.0 provides 25 percent more structural capacity than Low Hazard
importance of 0.8. The chart below outlines the conditions a
building needs to meet in order to be classified as Low Hazard. It
is important to note if you are unsure about the classification,
Low Hazard should not be the default.
Importance Categories
Importance
Category |
Use & Occupancy |
Factor |
Low Hazard
(Category 1 USA) |
Buildings that represent a low hazard to
human life in the event of failure, including:
- Low human-occupancy buildings where it can be shown that
collapse is not likely to cause injury or other serious
consequences
- Minor storage buildings
|
0.8 |
Normal
(Category II USA) |
All buildings except those listed in
importance categories Low, High, and Post-disaster |
1.0 |
High Hazard
(Category III USA) |
Buildings that are likely to be used as
post-disaster shelters, including buildings whose primary use is:
- As an elementary, middle or secondary school
- As a community center
Manufacturing and storage facilities containing toxic, explosive or
other hazardous substances in sufficient quantities to be dangerous
to the public if released |
1.15 |
Fabric Bracing
Some manufacturers rely on the fabric to give the building trusses lateral strength. This allows them to reduce the size of the truss members and the amount of bracing in the structures. This in turn reduces the cost and strength of the building.
Competitive Design
Some designs rely on the fabric cover to brace the outer truss sections. This does not work if the fabric is damaged or if snow/wind loads are uneven, which is often the case.
Competitive Design
Ripping of the fabric cover may cause failure of the truss system.
Norseman Structures' designs do not rely on fabric to brace the truss chords. Our buildings are designed to meet all wind and snow load requirements based on adequate internal bracing.
Norseman Design
Norseman's designs use strong steel bracing systems to secure the truss - not the fabric cover.
End Truss Strength
As truss sections at each end of the building do not have
bracing on both sides, unlike other trusses internal to the
structure, there are different load demands on them to be
considered.
Many manufacturers do
not adequately consider all loads on the end trusses when checking
the effects of fabric tension. This can lead to failure of the end
truss, which will lead to failure of the entire structure.
Norseman Structures
considers both vertical and horizontal fabric tension in
combination with design snow and wind loads on the end trusses.
Exposure Factors
Shelter from winds by adjacent buildings, trees or terrain can
have a significant impact on the wind and snow load requirements of
buildings. Correctly choosing the exposure factor is very
important!
If the proper exposure factor is not
selected, a building may use substandard bracing or truss and web
steel and may not meet the requirements for the specific location.
For example, a building located in an area defined as "Open
Terrain" may require a 90 mph wind rating, but may only be rated
for 80 mph if the importance factor used did not meet the design
requirements for the site.
If the building is sheltered from wind by
adjacent structures or trees, it should be designed for an
increased snow load, since less snow will potentially be blown off
the roof. Adjacent structures can also cause drifting of snow which
increases snow loads on the building.
Norseman Structures only uses wind and snow
load reductions for sheltered or exposed conditions in strict
accordance with building codes. Generally, Norseman Structures uses
the 'Open' classification in order to ensure your building meets
all current, as well as future needs. The classifications, as well
as brief explanations, are as follows:
Rough Terrain (Exposure C -
Canada, Exposure B - USA)
Suburban, urban, wooded areas, center of large town. Although this
generally depicts 'rough' terrain, there are areas of 'open'
terrain. For this reason, 'open' terrain should be selected if
there is any question about the type of terrain.
Open
Terrain (Exposure A - Canada, Exposure C - USA)
Few trees, scattered trees, open water. Buildings classified as
'open' require as much as 30% more capacity and strength than those
classified as 'rough'.
Steel Specifications
Many manufacturers use the steel strengths advertised by their
steel supplier, instead of the steel strengths stated in the
building codes and steel specifications. Use of steel strengths
higher than those specified by the ASTM or CSA specifications
should not be used, due to lack of consistency of strength and
elongation requirements. This could result in a 30 percent under
capacity in some cases.
Complete Structure Engineering
In order to have a fully engineered building, every
component of the building must also be engineered - from screws, to
cables, to trusses, to doors. In order to ensure you have a
properly engineered product, it is important to ascertain whether
all building components are engineered. At Norseman Structures, we
use exclusively engineered parts. It is important to note that any
piece retrofitted on site or incorrectly engineered will affect the
structural integrity of your engineered product.
Enclosure Category
A building with significant openings in the fabric such as
air vents, fabric doors, or overhead doors not designed to resist
the specified wind loads is subject to significant wind pressures
or suctions. This means any overhead doors or man doors must be
rated to the design wind speed. Failure of a door or window could
result in wind forces as much as 50 percent higher than the design
wind forces. Norseman Structures design all door frames and
components to resist the design wind speeds. In buildings with
large openings or fabric doors, Norseman Structures designs the
building structure to resist the additional wind pressures caused
by the openings in accordance with the building codes.
Thermal Factor
Some manufacturers use a thermal factor of 1.0 which is only appropriate for heated buildings, or even 0.9, which is only appropriate for heated greenhouses, to reduce the design snow load on the building. Norseman Structures uses the correct thermal factor of 1.2 for the design of unheated fabric buildings.
Summary
We are confident that if these suggestions are taken into
consideration, our customers will have the knowledge that allows
them to ask the right questions when purchasing a fabric covered
steel building.