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Re: ASD vs. LRFD

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I indicate only the ultimate load.  Where there are several combinations of
loads that may require multiple design checks, I design the connection myself.

Generally, I design all connections that are not simple beam-to-column or
beam-to-beam connections.  This means I generally design all braced frame
connections, all hanger connections, all chord and diaphragm connections, all
base plates, all moment connections, etc.  This requires a significant amount
of work on the front end, but I've found it saves enormous time during shop
drawing checks; if the connection is not as I designed it, it is not acceptable
without explanation from the detailer/fabricator/contractor.

The DL/LL argument came up early in my career when I and another junior
engineer convinced the boss to use LRFD (1st ed. - sucked!) on a rare
earthquake governed project in NE Arkansas.  For that project, I recall
specifying all connections as full depth because the boss couldn't get his
brain around the separation of DL and LL with separate "load" factors.  I don't
see this as a major problem with anything except the rarest beam-to-beam or
beam-to-column connections that have some special load.  The boss also had
several qualms about time spent (I've since developed a greater appreciation
for his apprehension, though I still favor LRFD).

Notation generally looks like this:

    23k         W16x31           20k
 |-|--------------------------------|-|
        8     |   2   |      8
              |       |
              |       |

Of course, it can be shortened for beams with no studs and equal end reactions.

None of this is new.

-Keith Fix, PE
-Little Rock, AR

--- michael.barbetta(--nospam--at)swec.com wrote:
> 
> 
> Keith,
> 
> How does the fabricator's engineer know which LRFD load factors to apply to
> your
> end reactions? Do you indicate both DL & LL reactions at the ends of every
> beam
> and girder?
> 
> Thanks.
> 
> Mike Barbetta, P.E.
> Stone & Webster
> Cherry Hill, NJ
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Keith Fix <kefix(--nospam--at)yahoo.com> on 12/19/2001 09:37:59 AM
> 
> Please respond to seaint(--nospam--at)seaint.org
>                                                                              
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>   
> 
> 
>                                                               
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>                                                               
>  To:      seaint(--nospam--at)seaint.org                                   
>                                                               
>  cc:      (bcc: Michael Barbetta/Civil-Structural/SWEC)       
>                                                               
>                                                               
>                                                               
>  Subject: Re: ASD vs. LRFD                                    
>                                                               
> 
> 
> 
> 
> 
> 
> 
> 
> --- "Mitchell J. Sklar" <MJS(--nospam--at)bala.com> wrote:
> > I have been doing some cost comparisons between ASD & LRFD design. My
> > LRFD designs are consistently lighter that ASD (No Big surprise). The
> > GC/Fabricator are insisting that due to the connection design and safety
> > factors involved that it is cheaper to go with an ASD design. Is the
> > something that I am missing. I have just recently attended the AISC
> > Connection seminar and must of step out of the room when pricing was
> > discussed.
> >
> > Besides, most (almost most) fabricators use computerized software to do
> > their designs, so where is the extra effort. If I was going to really
> > design all my connections, I would also buy or make a spreadsheet to
> > automate this. In addition, if my specs call out full depth double angle
> > or shear tab connections, and lets say I have a W16 w/ 4 bolts. What is
> > the difference whether it is ASD or LRFD? Four Bolts will give me a
> > certain capacity either way.
> >
> > Has anybody heard of special safety factors when doing schools? Say in
> > New Jersey?
> >
> > Mitchell J. Sklar, P.E.
> > Senior Structural Engineer
> >
> > BALA Consulting Engineers, Inc.
> > (mjs(--nospam--at)bala.com)
> > p 610-649-8000 x345
> > f 610-649-8475
> >
> 
> About half my career has been in ASD, the second half in LRFD.  Generally I
> prefer LRFD, if for no other reason than that the code is written better.
> 
> I began putting all my beam reactions on my drawings beginning two or three
> years ago.  I began doing this so that I could check shop drawing connections
> more quickly (it works!).  I have not had any complaints, besides the usual,
> "We usually design with ASD....".  Since I put reactions on my drawings, none
> of my connections are designed as "full depth".  If a fabricator/detailer
> insists on using them, its his profits down the drain.
> 
> If you design with LRFD, I recommend you stick to you guns.  LRFD is not
> "visible" to the public, and saves ME time and effort.  I find ASD a time
> waster, and am glad I no longer work for men than insist on its use
> (something
> to point out when hiring).  All that said, I still use ASD for masonry
> design.
> 
> ;-)
> 
> As for schools and extra safety factor: I recently designed an expansion to
> Children's Hospital in Little Rock.  The only "safety factor" I needed was
> the
> thought of something going wrong with dozens of kids in hospital beds.  My
> school building design has been simalarly governed.
> 
> -Keith Fix, PE
> -Little Rock, AR
> 
> __________________________________________________
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__________________________________________________
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