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Re: factory-made roof trusses

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There are two types of trusses that I specify. 

One has wood chords with metal web diagonals, with the whole truss having ICBO
approval. In our area ICBO approval is required by most building departments.
Call manufacturers in your area and ask for manuals with information on design
specifications their products are based on and make sure they conform with
your standard. Talk to technical representatives who will give you his time
for questions on the quality, reliabililty and competitiveness of their
products. The manuals should also have enough information on details of
recommended connections and shear transfer from their product to the building
which you will have to show on your structural plans. Chose one brand and
specify it in your plans.

Remember, the truss  manufacturer will provide the layout and structural
calculations for their procuct and you have to review this to make sure they
conform with the structural plans and with your specifications and design load
conditions. The truss engineer should be licensed in the state where the
building is located. Trusses are special building components. If you do not
have any experience, do not design it. Leave it to the experienced, competent
and reliable truss manufacturers in your. area.

The second type of truss are those made up of regular 2x chords and diagonals.
Usually, the truss members end connector are the only item that requires ICBO
approval. The truss members are just checked for allowable stresses and
deflection. Connections are similar to typical wood framed construction. The
truss manufacturer also provide calcs and layout.

Which type of truss to use depend on a lot of factors such as:

 1. Cost-The cheapest  one which conforms to your minimum standard.
Manufacturers will claim that theirs is the cheapest but don't worry about
this since the contractor usually give the job  the lowest bidder and not
necessarily follow the brand you specify on the plans. Just make sure that
when you review the truss mnufacturer's submittals,   they conform to the
plans and specifications and have the required approval.
 2. Shape of the roof-Some trusses can incorporate the roof profile in the
shape of their truss thus eliminating the extra framing time and cost.
 3.Technical support-You should be able to discuss with the truss
manufacturer's technical representative your concerns about their product, and
get them to assist you in the preparation of the structural plans. Ask if they
are willing to assist you when there are field questions and problems during
construction. This will help in case there are field revisions or ommissions
that requires additional engineering time working on calculations to get the
trusses to work after they have been installed. 

Check with the building department if they will require a signed truss layout
and calculations before issuing permit. If yes, you might have to request that
the truss manufacturer spend time and effort to provide calcs and shop
drawings for you job even if they do not have the contract to supply the
trusses. Usually, the building owner send the plans for bidding after it is
approved by the bldg dept and the winning contractor will chose his truss
supplier. But if the bldg dept requires signed truss plans and calcs before
approving the plans, and teh truss manufavturer is not willing to do it uless
he gets the contract,you're in a bind. The owner might blame you for this
potential delay in construction.

In reviewing the truss calcs and drawings, make sure they include all
miscellaneous roor loads such as mechanical equipment, screens, water storage
tanks, etc.

These are all based on my experiences.

Ernie Natividad