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Re:Large Span Structures

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Sehdev,

That is a tough question to answer.
Where is the project? In India or abroad?

There are too many factors to consider.

1)In India labour is cheap while material is expensive
but in Europe and USA it is the other way around.

2)We in India freely use field welding while it is
avoided as far as possible abroad and high strenth
bolting is preferred.

3)Structural fabrication abroad is more accurate than
in India and done in well equipped shops and not in
open yards at the project site as in India. This
influences the choice of the system.

4)In India, we have very few rolled I sections
available to choose from (about 11 ISMB sections are
readily available though many more are listed in the
tables). In USA several hundred W sections are listed
to choose from.

5)The total quantity of steel is very relevant. It may
influence the decision on whether to use heavier
rolled sections with minimum fabrication or adopt
built up optimized sections with more fabrication work
involved.

6)Erection feasibility and access for erection may
also influence the decision.

7)The capabilities of the contractor are also
relevant. Does he have the specialized equipmnent
needed for lifting in one piece? 

8)Aesthetics is sometimes relevant. I have known
clients who prohibited open-web trusses and insisted
on "sleek" roof rafters for aesthetic/architectural
reasons even though they were advised that for the
spans and loads in question roof rafters involved
double the steel quantity. The sleek looking roof
rafter with depth considerably less than a truss would
have thick flanges and webs but the client may not
care.

9)The total superimposed load is also a consideration.

10) You finally need to decide  what to reduce while
not compromising on functional and safety
considerations. Is it quantity of steel, time for
fabrication and erection, overall cost of steel
structures only or overall cost of the project
including civil and other works. (The percentage of
structural cost in relation to overall project cost
will also be relevant).

In general only a deep study by a competent and
experienced engineer equipped with all facts, data and
preferences of the client, functional needs and
awareness of ceiling on costs  can settle this
question. 

(And finally when he takes the best technical
decision, some non technical person higher up may veto
it!)

Normally in India while designing routine roof
structures for Industrial buildings, I used Rolled
sections for spans upto 12 metres, I used fabricated
sections for spans from 12 to 18 meters and roof
trusses for spans above 18 meters upto 40 meters.

For spans above 40 meters, special structural
arrangements will be necessary if the quantity of
steel is to be minimized. Space structures will be
needed.

I have no experience with suspension cables.
I hope this helps.
Regards
Vish 
(G Vishwanath, Bangalore)


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