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A Norseman Structures Inc. White Paper
Fabric Covered Steel BuildingsCritical Design Factors to Consider
0296-11 04232012
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 diffi cult 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 these
basic engineering topics that relate to fabric covered steel buildings:
• Steel strength
• Bracing
• Snow & wind loading
• Snow shedding
• Use & Occupancy
• Fabric as Bracing
• End truss strength
• Exposure factors
• Steel specifi cations
• Complete Structure Engineering
• Enclosure Category
• Thermal Factor
WHITE PAPER
Fabric Covered Steel Buildings Critical Design Factors to Consider
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 20296-11 04232012
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 3
Steel StrengthSignifi cant factors infl uencing 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 plastifi cation”.
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 fl exing 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.
The picture below is 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 PunchingTruss webbing punching through the thin guage outer chord of the truss
Webbing
Webbing
Outer Chord
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.
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0296-11 04232012
BracingBracing 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 fi xed 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 LoadsBuilding 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 SheddingSome 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 effi ciently.
Norseman Structures always designs to the
strict snow load requirements identifi ed in
local building codes. Our designs include larger
diameter and thicker steel than competitors, in
order to provide safe, long lasting structures.
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Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 4
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0296-11 04232012
Use and OccupancyMost 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 classifi ed as Low Hazard.
It is important to note if you are unsure about
the classifi cation, Low Hazard should not be
the default.
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 5
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Importance Categories for BuildingsImportance 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 suffi cient quantities to be dangerous to the
public if released
1.15
0296-11 04232012
Fabric as BracingSome 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.
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 6
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Fabric Stress
Truss Cross
Section
Truss Cross
Section
Fabric
Lower Brace
Lower Brace
FabricFailure
OutwardForce
OutwardForce
Fabric
Cross Bracing
Competitive Design
Some designs rely on the
fabric cover to brace the
outer truss sections
Competitive Design
Ripping of the fabric
cover may cause failure
of the truss system.
Norseman Design
Norseman’s designs use strong
steel bracing systems to secure
the truss – not the fabric cover.
This does not work if the fabric is
damaged or if snow/wind loads are
uneven, which is often the case.
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.
0296-11 04232012
End Truss StrengthAs 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 FactorsShelter from winds by adjacent buildings, trees or
terrain can have a signifi cant 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 & web steel and may not meet the
requirements for the specifi c location. For
example, a building located in an area defi ned
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’ classifi cation in order to ensure
your building meets all current, as well as
future needs. The classifi cations, 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.
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Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 7
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0296-11 04232012
Open Terrain(Exposure A – Canada, Exposure C – USA)
Few trees, scattered trees, open water. Buildings
classifi ed as ‘open’ require as much as 30% more
capacity and strength than those classifi ed as
‘rough’.
Steel Specifi cationsMany manufacturers use the steel strengths
advertised by their steel supplier, instead of
the steel strengths stated in the building codes
and steel specifi cations. Use of steel strengths
higher than those specifi ed by the ASTM or
CSA specifi cations should not be used, due to
lack of consistency of strength and elongation
requirements. This could result in a 30 percent
capacity reduction in some cases.
Complete StructureEngineeringIn 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 retrofi tted
on site or incorrectly engineered will affect the
structural integrity of your engineered product.
Enclosure CategoryA building with signifi cant openings in the fabric
such as air vents, fabric doors, or overhead doors
not designed to resist the specifi ed wind loads is
subject to signifi cant 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
to resist the additional wind pressures caused
by the openings in accordance with the building
codes.
Thermal FactorSome 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.
SummaryWe are confi dent 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. What was once a very complicated
process, is now something we can all understand
and use as an evaluation tool.
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 80296-11 04232012
Norseman Inc. has been manufacturing and selling building enclosure systems
since the 1960s and is a major supplier of fabric building and drilling rig enclosure
systems to the Oil and Gas and commercial markets. Norseman Structures now
operates over 200,000 square feet of quality controlled manufacturing space. This,
combined with an extensive network of corporate offi ces in Alberta, British Columbia,
Ontario, Saskatchewan, the USA and UK, positions Norseman Structures to be a world
leader in the manufacturing and distribution of steel-framed, fabric covered buildings.
Contact Us:
CANADA3815 Wanuskewin Road
Saskatoon, SK S7P 1A4
Toll Free: 1.855.385.2782
USA2851 South Pike Ave
Allentown, PA 18103
Toll Free: 1.855.385.2782
UNITED KINGDOM1-8 Enterprise Glade
Bath Yard, Moira
Swadlincote
Derbyshire DE12 6BA
UK: 0800 3891490
Outside NA & UK: +44 (0)1283 554120
www.norsemanstructures.com
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider0296-11 04232012 NORSEMAN & NORSEMAN AND DESIGN ARE TRADEMARKS OF NORSEMAN GROUP LTD. 05/11
Building Model
Building Size (width x length)
Chord Gauge - What gauge of steel is being used in the chords?
Chord Diameter - What is the diameter of the chord steel?
Web Gauge - What is the gauge of steel being used in the web?
Web Dimension - What are the dimensions of the web steel?
Truss Depth - What is the truss depth?
Purlin Diameter - What is the diameter of the roof purlins?
Wind Bracing - How many bays are wind braced?
Cable Size - What size are the cables being used?
Snow & Wind Loads – Does the building design take the effects of
‘combined’ wind and snow load into account? See Page 4YES
Snow Shedding – Does the building design assume that snow will be
shed from the roof of your building? See Page 4NO
Use & Occupancy – Does your building design assume the appropriate
occupancy rating? Remember, if your building is to be occupied, it
needs to have an occupancy factor of at least 1.0. See Page 5
Exposure Factors – Does the building design take the proper exposure
factor into account? Remember, Open terrain should be used when
building exposure is in question. See Page 7
Use of fabric in the design – Does the building design use the fabric to
give the building trusses lateral strength? See Page 6NO
End Truss Strength – Does your building design consider both vertical
and horizontal fabric tension in combination with snow and wind loads
on the end trusses? See Page 7
YES
Steel Strength – Does your building design account for web punching
or chord plastifi cation? You may require larger web material. See Page 3YES
Bracing – Does your building design use adequate bracing? Compare
purling and cable sizes. See Page 4YES
Steel Specifi cations – Is the steel strength used in the design
specifi ed by the manufacturer or a regulatory organization ie. CSA
or ASTM? See Page 3
YES
Enclosure Category – Does your building design include doors and/or
windows? Have these additional wind pressures been accounted for in
accordance with the building code? See Page 8
YES
Thermal Factor – Is your building unheated? If so, is it using a thermal
factor of 1.2? See Page 8
WHITE PAPER
Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | Appendix 1
Compare Before You BuyAll buildings are engineered, but are they equal? Co
mpe
tito
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Com
petito
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0296-11 04232012
WHITE PAPER
Contact Us:
CANADA3815 Wanuskewin Road
Saskatoon, SK S7P 1A4
Toll Free: 1.855.385.2782
USA2851 South Pike Ave
Allentown, PA 18103
Toll Free: 1.855.385.2782
UNITED KINGDOM1-8 Enterprise Glade
Bath Yard, Moira
Swadlincote
Derbyshire DE12 6BA
UK: 0800 3891490
Outside NA & UK: +44 (0)1283 554120
www.norsemanstructures.com
Fabric Covered Steel Buildings: Critical Design Factors to Consider0296-11 04232012
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