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RE: asd vs lrfd

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Roger,

I agree that the current LRFD codes have some shortcomings,
much as any developing code would.  I would like to address
a couple of you comments as to why not to use LRFD.

>Reasons not to use LRFD:
>
>1.	Service load criteria, i.e., deflection, drift, vibration,
>etc., frequently control the design, not stresses.
>

Unfortunately, I can't argue here.  

>2.	When the material strength is so variable, as with wood, that
>LRFD does not give any better indication of the factor of safety than
>one would get with ASD.
>
Granted.  However when LRFD is used throughout a project in a consistent
manner, such as that presented in the AASHTO LRFD Specification, I
personally
have a better "feel" for the level of safety provided.  The AASHTO spec
uses the
same load factors for all elements regardless of material and then has a
variety
of resistance factors that should produce a consistent factor of safety
throughout
the structure.  This not only makes your analysis easier since different
load
factors aren't being applied for different materials, but makes it
easier to compare
strengths between components of differing materials (i.e. is my steel
beam stronger
than my concrete column).

>3.	When live loads are of a different type of load than dead
>loads (concentrated, varying, stepped, etc.) such that the different
>load factors seriously affect the points of inflection, maximum
>moments, etc.
>

Once again, can't argue here.  As far as determining points of
inflection
or maximum forcesso that cover plate or bar cutoffs can be determined
see my comment below about variable permanent load, load factors.

>4.	In foundations when subject to partial uplift.

This is why the AASHTO LRFD doesn't give a fixed load factor for
permanent
loads (soil loads, dead loads, ...).  They give a minimum and maximum
value so
that conditions such as this can be addressed properly.  For instance in
the design
of rigid frames (a bridge where the abutment is monolithic with the
superstructure)
the code lists a minimum and maximum load factor (0.9 and 1.35) to be
applied
to the soil loads to protect against the variability in soil loads.



Chris Serroels