Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: tornado alley

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Prescriptive codes are not necessarily a bad thing. Here in Australia, the
majority of housing is "designed" to the timber framing code AS1684. This
contains tables for spans, tie-down, and bracing. Selection of suitable
tables is based on simplified wind classification to AS4055. There are 6
non-cyclonic classifications N1 to N6, and 4 cyclonic classifications C1 to
C4. The wind speeds for the cyclonic classifications are the same as those
for non-cyclonic, but other criteria apply to cyclonic regions: buildings
considered open, and fatigue issues. Timber framing in the main is
"designed" by timber estimators, and approved by building surveyors. The
timber framing code talks about nominal connections and specific
connections. The nominal connection for conventional rafters is skew
nailing, for trusses the nominal connection is straps.

Our wind loading code differs from ASCE7-05 in that we apply the
terrain/exposure multipliers and topographical multipliers to the wind speed
before calculating the pressure. We can thus make an assessment of the wind
speed at a site, by adjusting the basic regional wind speed. Using AS4055
drafters and timber estimators can obtain the site wind speed using simple
descriptive tables, without calculation. Using AS1170.2 a more refined
estimate can be obtained, which typically shows operating at lower end of
the wind class. Most builders however use wind speed maps, which indicate
the wind class for the site. The wind classification permits design and
specification of standard products: for example roof trusses simply ordered
for wind class N2, as are windows.

Compare the WFCM construction guide for 130 mph and exposure B. The
implication is that it is not suitable for 85 mph and say exposure C, or
exposure B with topographical. There is more than one way to get a given
reference pressure qz, by adopting a wind classification system, either
design wind speed Vz, or design pressure qz, can be adopted and listed for
sites. With colored wind speed maps, owners and builders can easily see what
the design wind speed is for a site: the basic regional wind speed may be
low, but exposure or topography or something has increased the speed at the
site. The required uplift forces and racking forces can be looked up in
tables, and systems which can supply that resistance can then be looked up
in other tables. Basically only houses in wind class N1 use the nominal
connections, and they are mostly existing houses. Engineers can deviate from
the wind class maps, by using AS1170.2, but there is little benefit unless
can achieve a lower wind class, because no one is going to pay to have all
the structure engineered for a lower wind speed to AS1170.2, not when they
can get all the structure from the timber framing code in a few minutes.

As for straps and framing brackets, builders seem to have varying opinions
about the cost. Whilst the materials are relatively low cost, they argue
that installing is time consuming and costly. Others argue framing brackets
make their job easier and faster than skew nailing and use all the time.
Whilst those from cyclone country don't see any hardship in providing full
height tie-down rods on all houses. Cost is a relative thing: and relative
to the individual.

As for probability: the 50 year mean return period is a
permissible/allowable stress reference, with design factors applied
elsewhere: roughly a load factor of 1.5 to get to the ultimate limit state
load. The ultimate limit state speed typically taken to have a 500 year mean
return period, which roughly provides a 10% probability of the design speed
being exceeded over a 50 year life-expectancy. At the ultimate limit load
the building is on the brink of collapse, and has under gone permanent
deformation and potentially irreparable damage: the building is no longer
serviceable after the event and may no longer be habitable. After the
allowable stress wind speed event, the building should be in a better state
of repair (eg. The load experienced was 1.5 times lower than designed for).
There are multiple states-of-nature, and each one has its own acceptable
levels of performance. Allowable stress wind speeds could be considered as
another limit state.

Our houses are not the place to take shelter during the ultimate limit state
event. From another perspective here (SA) our basic regional speed is 45m/s
[162km/h , 100mph], the bureau of meteorology issues severe weather warnings
at average speeds of 63km/h [39mph] and gusts at 90km/h [56mph], I take this
to be the life safety issue: at these speeds it is dangerous to be outside
and need to take shelter. The higher resistance installed is more to do with
economic loss and loss of amenity. The real life safety issue is not about
resistance of the structure but the behaviour of the structure under extreme
load: noting that the design load can be exceeded. {For SA R=50 Vr=39m/s for
life of 50 years risk=63.6%, R=500 Vr=45m/s for life of 50 years risk=9.53%}

Here a basic premise is that our wind loading codes task is to ensure a
building remains anchored to the owners property, not necessarily intact and
useable: that is we reduce airborne debris. Also our code no longer has an
importance factor, but importance level, and beyond that we have some scope
to adjust design-life and risk.

It shouldn't be necessary to design the whole of a building to the same
importance level, or same risk and design-life. There is a certain core of a
building which is important for recovery and minimizing spread of disease
after a disaster: such as kitchens and bathrooms. So if a specified core of
a building is made highly resistant, then all those games rooms,
entertainment areas can be considered luxury and the economic loss and loss
of amenity is of minor concern. If there is a "safe" core where the
occupants can take shelter, then the luxury rooms can collapse in a
relatively "safe" manner when their ultimate load is exceeded and yet still
remain tied to the property.

Buildings could be designed with energy absorbing crumple zones, and
pressure release vents. It's not necessary to put resistance into a
structure to make it safer, it could be taken out. Design a vent panel which
fractures and opens when the pressures get too high: like they have in the
chemical industry.

Most of our wind loading codes are based on relatively linear flow of air
over a building, which causes turbulence. A hurricane is large enough
relative to a building that the flow can be considered linear. But a tornado
is a tight spiral that can encompass a whole building. Debris is not simply
picked up and thrown at a building, it can move through the spiral,
hammering its way in and out off a building. Drag forces on architectural
decoration can be the cause of holes ripped in the building fabric. Also the
pressures within a tornado are huge: so it is difficult and impractical to
design a building which can with stand impact with a tornado. Fortunately in
Australia, tornados mostly occur in remote unpopulated areas.

It seems to me that for tornados it is better to change the environment
around the house, providing structures which can remove debris from the
tornado, and avoid becoming debris: which may be equally impractical as
providing resistance in the house. But a house with an isolated and highly
resistant core, and energy absorbing perimeter rooms may minimize the
consequential impact of a tornado. The perimeter rooms can be battered and
flattened against the ground, but have enough tensile resistance to avoid
being dragged from the site. With any luck (probability), only part of the
perimeter will experience the impact, and the core avoids contact

Another more radical approach is to have parts of the building mechanically
retracted, or otherwise alter its profile. {eg. Windward walls have positive
pressure, wind ward roof slopes typically experience suction: temporarily
remove the vertical wall: tilt it over.} Wonder what the geometry of the
tornado spiral is for say first 10m above ground level. So that can locate
core of house at appropriate centre from perimeter. Tornado travel path may
still be through the core, but at contact with core it will have less
energy. Still further what can the building do to destroy the tornado? I
thinks it's a low pressure upward spiral: hot at ground level. So lower the
temperature: change the earths albedo? The other option is also to change
the wind exposure, and reduce the fetch. Build more houses, plant more
forests: this will also change the albedo and create microclimate:
potentially creating an environment less prone to tornado formation. As far
as I am aware tornados don't form in suburban streets, usually form in wide
open spaces and may or may not travel towards populated areas. So like
coastal defence against waves provide barriers/obstructions to dissipate
energy of tornado as it approaches populated areas. Baffles like they use in
wind tunnels, to smooth flow. It may be as simple as an earth mound, or cool
wet land used as part of stormwater drainage. The mound stripping debris
from the tornado rather than the tornado going up and over. If shift focus
from loads and resistance I'm sure there are all kinds of possibilities more
acceptable by the community: or less acceptable pushing them towards being
more accepting of higher resistance than tradition. But acceptance is all
about risk, and likelihood.

Cars are designed for limited impact by cars, but not for collision with
locomotive. Should we design cars for such situation given increased
incidence of such collisions?

Live safety is not really about magnitude of load, or resistance of
structure. The quest for ductility and robustness is not about magnitude of
resistance, but behaviour of the structure under load. Ductile deformation
of a structure provides visual and possibly audible warning before ultimate
collapse: people have some opportunity to escape the situation. Some what
difficult however if have a staggered work day to get people into the
building in the first place.

It seems to me that the concepts of quality robust design haven't yet
infiltrated too far into the building and construction industry. Variation
in all its forms is not thoroughly considered throughout the whole system
being designed, nor differing levels of acceptable performance.

For example many framing brackets require pre-drilled holes and hand
hammered nails. The builders don't waste time drilling holes, therefore
resistance is less than quoted by manufacturers, but builders build houses
unsupervised most of time and experience no problems. Others use shot fired
nails, and Miitek gangnail has tested some and identified the need to fire
the nails through the metal not the holes. In which case the brackets don't
need to be made with holes, but then don't have guide to number of holes and
pattern. Also most nail guns are too large to get close enough to put nails
into brackets, so nails are every where except in the brackets. Nail guns
need making smaller and the brackets making larger. Change the components
reduce the variation in the quality of the assembly. Change the tooling and
reduce the variation in the quality of the assembly. Or reduce the allowable
resistance of the installation. For example 5th percentile resistance of
installation only 80% of expected resistance: either improve quality of
workmanship and get it right or we keep reducing the allowable resistance,
rather than increasing design load.

Also should try and keep design simple, and avoid unnecessary variation in
the form of variety. If skill base not available to achieve quality
demanded, then simplify design to match quality of skill available.
Reduction of variety can increase levels of automation and permit faster
supply. With fast supply can recover from disaster faster. Reducing variety
can also increase resistance whilst lowering cost of supply. For example
houses may be designed as wind class N2, and site classified as wind class
N2 with a design speed of Vzu=40m/s whilst the AS1170.2 design speed for the
site is only around Vzu=37m/s, which is greater than N1 Vzu=34m/s. So the
simplified system puts more resistance than needed by AS1170.2 into the

There is a limited range of structural sections available, and they do not
have a uniform increase in resistance. So if change of load doesn't change
the size of section, why waste time checking for that load. Determine the
limitations of the sections. It seems to me better to know the limitations
of a section for a given application, than to crunch lots of formulae and
decide adequate for an individual case. If know limitations then have a
better feel for the consequences of variation. Does it really matter where
the span is measured from? If at the limits of span it does, if not then a
minor detail. When are back of envelope calculations sufficient and when is
more detail required?

Framing brackets have published resistance, if they speed up construction
and have greater resistance than skew nailing, then that becomes the norm:
the low end of acceptable resistance. If skew nailing is faster, it remains
the norm, and the limitations of its suitability need to be determined.
Lower wind loading is of no concern for such residential construction. As
wind loading increases then skew nailing becomes unacceptable, and
proprietary framing brackets become more suited, but this is a quantum
increase in resistance, there is a upper limit on the suitability of the
bracket. For wind loads less than the limit of the bracket there is no real
need to get into the details of the design.

It is not simply a matter of designing the product: building. More attention
needs to be given to the processes: from design, through fabrication and
construction, operation, maintenance and disposal. The construction
techniques and tooling need to be developed with knowledge of structures.
Make the builders life easier and more likely to get the desired resistance
into the structure. Also with increased attention to how a building is used,
operated, and then can improve design for life-safety by means other than
added resistance. Increasing trend towards large open space, indoor/outdoor
living areas: the house could be considered a free-roof with no walls.
During construction many houses are just free-roofs. What value are the
walls? Brick veneer is expensive cladding. If the structural value of the
walls differs from one room to the next, then maybe the walls can be made
from different materials: like paper. Paper or textile walls are potentially
less hazard than brick walls. What is the basis of the big bad wolf not
being able to blow the house of bricks over?

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********