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RE: STRESS AND FORCE

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William,

We get lots of clients turn up, declaring such and such over engineered and
they cannot compete. I usually explain I cannot remove any of the
engineering, only add more. That things are over-sized due to a lack of
engineering, or undersized due to a lack of engineering. That I cannot
guarantee reducing the size of anything, further more their competition may
be using product based on faulty design: that there is far too much
inconsistency in what building departments will approve.

I also explain that manufacturing and construction economics is far more
complex than simple costing a few parts. That the larger enterprises they
are competing against have a greater volume of sales and therefore lower
units costs, thus they can distribute the cost of engineering across more
projects, plus they can afford to, and are willing to, cover the costs of a
few failures. That it does not make sense to expend effort reducing
resistance, and increasing risk of failure. From a mechanical perspective we
would not expend time and effort machining a shaft to a smaller diameter
unless really needed a smaller shaft. Such is the expenditure of energy to
throw materials away: and that is also throwing resistance away.

Those builders who insist on reducing resistance/size typically don't know
much about their industry, and have little interest in quality. Those who
understand their industries and work processes are less concerned about
material costs and more concerned about labour costs. If material costs are
of concern then usually relates to economies of bulk purchasing and reducing
variation in materials used.

Here in Australia residential timber framed construction is mainly sized up
by timber estimators and building designers, with no engineering input. Our
timber framing code AS1684, is based on a structural model presented in part
1, AS1684.1, and is based on our loading code AS1170, and timber structures
code AS1720. We also have a simplified wind classification system AS4055.
The timber framing code, contains span tables for natural timber, there are
also resistances given for various connection methods and bracing systems.
Manufacturers of framing brackets also publish resistances, whilst
manufacturers of LVL, Glulams etc, publish span tables compatible with the
timber framing code. Steel manufacturers, and the steel industry have also
started to produce span tables for use of steel in residential. Most of the
span tables are based on the wind classes. Wall bracing is also sold as an
off-the-shelf product, the manufacturers providing tables of resistances.

If I was to do a wind assessment to AS1170, it would be found that the
reference pressure qz, is at the lower end of a wind class. It could
therefore be argued that economy can be achieved by individually engineering
the timber construction. But the engineering is going to take far longer
than the 30 minutes or so it takes a timber estimator to use the timber
framing code. So generally the span tables are used for member sizing.
However the builders may look to a wind assessment to reduce tie-down and
bracing requirements, since some consider it to be a labour intensive
exercise. Others however consider that using the higher capacity framing
brackets makes construction faster and easier than simple nailing, and are
willing to ware the cost of the brackets. It all depends on the cost
structures used by the individual enterprise: and here, most builders pull
their cost estimates out off thin air. Subcontractors for residential also
tend to be on day rates, so reducing construction time by an hour or so,
doesn't make any difference to costs, unless it can drop out a day.

Also optimum costs tend to be relative to a specific enterprise rather than
an individual project. So one builder may not have any concerns using twice
the number of nails on their projects relative to another builder. But a
builder may be concerned if current project requires double the nails of the
previous project.

My point about the 4" or 6" nail spacing was really more focused on
unwarranted variation which leads to on site construction errors. If the
builder has got into a ritual of 4" spacing on some 80% of projects what
real benefit is there changing to 6" spacing for the odd project? The cost
saving unlikely to be passed onto the buyer. If got into a ritual of 6"
spacing, then likely to have many defects on project requiring 4" spacing:
thus requiring a higher level of inspection and rework. The cost of defects
have to be covered somehow. The expected quality of the work is a simple
matter of understanding human behaviour. The use of pre-printed bracing
sheets, change the job ritual to placing fastener at the blue dot: and there
is no variation in mark up of the sheets which are printed in a factory by
machine. If you can see any blue dots on site then there is a defect.

So clients do expect a certain level of consistency. But each client has a
different focus, and on different design parameters. Such demand for
consistency assists in working out pre-engineered solutions between
projects, and which answer problems across a range of projects. Feeding such
information back to the clients, demonstrates an interest in the client and
their business, and helps them to develop improved cost assessments. Further
more the clients, if interested will request you to investigate other issues
which may help reduce costs or improve quality. Though the builders may
simply be lumbered with the cost of engineering due to regulations: in which
case even more important to demonstrate value of engineering services beyond
code checking.

As for building departments: What they want is not necessarily the same as
what they need or the codes require. In South Australia builders and owners
are always complaining about how long it takes to get approval. New systems
will be introduced next year, yet another attempt at speeding up approval. I
doubt it will have any affect. We have no requirement for architects or
engineers to be involved with a project. Owners and builders cannot draw and
cannot assess structural adequacy: their applications get delayed, and a
request for engineering calculations is made. Builders tell their clients
that council (AHJ) is inconsistent and always changing its mind. Not true.
Individual councils are highly consistent, different councils are highly
variable in their understanding of the building code.

If a manufacturer submits a range of designs to the AHJ, then will typically
be informed they need to get them certified by an independent engineer: one
who didn't produce the designs. Typically however, it is more efficient to
get the individual project certified, to make sure a proposal complies with
the pre-engineered solution. If it works for the manufacturers, I don't see
why it cannot work for engineers and other designers.

Publish pre-engineered solutions; get them certified by another engineer.
Possibly share the use of such solutions, and lodge or otherwise make
available to building departments. Cut down on the amount of paper being
shuffled and checked. Do it once, and get it right!

What is the difference between what the engineers do and manufacturers do?
Building departments need the manufacturer's catalogues of section and
material properties, safe load tables for girts/purlins, resistances of
framing brackets and much more besides. Plus all the codes and design
manuals. So a few extra manuals from local consultants shouldn't be too much
of a problem. And if local consultants get together could probably produce a
single manual of pre-calculated solutions for common situations, and have
much smaller manuals for their individual practices.

There is more than one way to do things. And in building, civil/structural
engineers seem to just do, rather than think about what they do and why.
Just some ritual they go through: because they believe that is what the
building department wants, and the building department believes that is what
they require. The principal task is ensuring the proposal complies with the
code of practice, and having adequate evidence of such compliance. There is
no requirement the calculations have to be delayed until a project comes
along. The more design-solutions which can be stockpiled as it were, prior
to projects, will enable the projects to be implemented faster.

Further more the more range finding and limit finding carried out by
industry associations, the more efficient the industries will become.

If a steel fabricator has just pushed a button on an automated beam-line for
a hospital, school, car park or whatever, then they have to be asking, how
far can they transport such steel work, and what demand exists for the exact
same structure? To go back to the drawing board and design something else is
wasteful. Further more every minute spent designing and building a needed
hospital is one minute too long. The hospital is little value if half the
patients have died waiting for it. Supply of buildings needs to be faster,
that doesn't mean cutting the design process, just displacing when design
occurs.

Also modern buildings are not exactly dependent on insitu construction from
local resources. Resources are transported in and often that also includes
the labour. What would happen if the building industry could mobilise like
the military? Set down any where on earth and build a town. Apparently one
billion people in need of housing, how many hospitals, schools, power
stations, water filtration plants and the likes is the world short off? Also
note that a car is the first source of private personal space most people
acquire. What additional facilities does a house provide and how often are
such facilities needed? What would happen in the global market place if the
car industry changed the reason for having a car: that is changed the
emphasis from transport to shelter? Could the car industry supply 1 billion
dwellings faster than the building industry? Further how does the
proof-engineering for a car differ from that for a building: the approach
that is? To mobilise the building industry or any workforce increasingly
requires providing the comforts of home away from home. Which would tend to
promote the benefits of mobile homes, and residential hotels and motels.
Which diminishes the need for buildings negating the need to mobilise the
building industry. Dynamic equilibrium of adaptive systems.

In any case a manufactured product roughly has a resistance in force terms
equal to the resistance of its weakest link. Such can be compared directly
against an applied load or in more complex systems against the effects of
the applied loads. The manufactured product embodies more information about
the design-solution than the drawings and calculations: the product isn't an
abstraction of reality.

Manufactured products are part of the evolution of the industrial system and
they can be built upon, simplifying the development of more complex systems.
The bulk of the building industry however seems to be dormant and reluctant
to adapt to change and the potential of new technologies. Thus we have
global building shortages. Though sometimes by deliberate intent to push
prices up.

What would happen if the computer industry or car industry deliberately
curtailed supply? Their main problem is finding demand, and that typically
means finding a design with appropriate features to better meet the needs of
the market. Whilst they may be aiming for a more construction industry
approach of providing custom design. The issue is getting batch size for a
given model correct. Small batch sizes minimise risk on a particular model,
but design of a model which meets the needs of more people is better. Design
it once is preferable.

Whilst there may be significant variation in the architecture of buildings,
it doesn't mean such is reflected by variation in engineering solutions.
Such is reflected by the large variation in houses permitted by the timber
framing code AS1684. Residential construction is not the only area in which
this is the case. Other pre-engineered solutions are possible. Which in turn
permit more attention to be given to tooling requirements for fabrication
and construction: and improving quality and productivity in construction.

Builders are not interested in stresses or forces. Their concern is the
fabrication and construction processes. If section sizes or fastener sizes
change, then they may need new tooling in the workshops, or new equipment on
site. By assessing a range of design-solutions equipment with appropriate
capacity or versatility can be selected in the first place. Meet lots of
builders who tell me things are not possible, when what they mean is they
don't have the right tools.

Admittedly I am biased towards manufacturing. I also deal with manufacturers
who have a long history of asking civil/structural engineers the wrong
questions, and consequently getting less than they really need.
Pre-engineered manufactured buildings should be superior quality than on
site one-off construction. That they are perceived as otherwise indicates
something is wrong!

Part of the perception seems to relate to the complexity of the codes, and
an inability to understand relationships between characteristics of a
system, and the sensitivity to variation. In other words if numbers are not
pushed through the code formula on a project by project basis then the
design is perceived as being some how defective. Whilst that which is
defective is considered acceptable because checking is not required by the
code. Strange industry!

For a principal point of things could be done faster and more efficiently, I
guess I wrote far too much!


Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia




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