Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
When is it an advantage to go design and
construct on prestressing?
On the surface this seems like a
good idea as the prestressing contractor benefits directly from an economic
design and is theoretically more likely to refine the design more to save money.
The reason projects go design and
construct on prestressing is often on the advice of the structural
engineer. The project structural
engineer tries to convince the project manager that this will somehow save time
or produce a leaner design. The fact is
it never saves time and will only produce a leaner design if the project
engineer is incompetent or inexperienced at prestress design. The main result of such a decision is for the
project structural engineer to save a portion of the design fee, which they
rarely offer back to the client.
Mal has carried out prestressed
designs on all types of buildings for consulting companies as well as
prestressing companies such as VSL and APS so he has seen both sides of the
picture. Typical D&C projects are complex
with load transfers on every floor and the preliminary design given by the
original consultant almost never works without amendments (sometimes
major). The reason the design does not
work is generally because the original designer has only guessed many the slab
and beam sizes after running calcs on a few typical design strips.
Following are some of the usual
problems resulting from this delivery method:
Designs are amended so late that formwork needs to change after it has
been constructed.
Long lead time items such as studrails are required and the pour is held
up waiting for the supply.
The time that the project manager
thought he had gained is lost because the stressing companies take far longer
to produce their stressing drawings.
Assumptions the original designer
made about moments in columns, loads to footings and even overall building
movements and stability have to be revisited after design changes. The situation can become quite messy especially
when footings are already poured.
Design responsibility has become
split between the consultant and the prestressing company so that when a design
problem arises the client is unsure who is at fault.
A typical
example of the last point would be a slab where the client needs a waterproof solution. The design consultant may for example detail
connections to lift cores and stairwells that restrain the slab to such an
extent that no reasonable amount of prestressing will prevent shrinkage
cracking. The client may have passed the
waterproofing design responsibility to the stressing company on the original
consultant’s advice. By the time the
calcs are finished on a D&C job the cores may have already been jump formed
so all the stressing company can do is stress to over say 2MPa and write a letter
explaining that he can no longer carry design responsibility for this aspect of
the work. If the original designer had
stressed the project the problem would have been evident from the start
especially if they had modelled the whole structure as part of the design - ..\Prestressing\Best
prestressed Concrete Design Software.htm.
Perhaps the most ironic case we know
of was when an interstate consulting firm convinced a project manager to go
D& C on a major building project.
The project was stressed out of the same firm’s
The lesson
to be learned from this is to pay suitable up front fees and use the best
structural consultants you can find.
This way you will have the design sorted out from the start, competitive
prestress tenders and provide the best value for the client.
If you need
the full job done by a team that details the whole package give Mal Wilson from
Advanced Structural designs a call on Ph 02 61612171.