Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Section 1633.2.6 & 1612.4

[Subject Prev][Subject Next][Thread Prev][Thread Next]
>
> If the collector is WOOD (with steel braced frames) then omega is
> 2.8 and the load duration factor of 1.33 MAY be combined with the
> 1.7 allowable stress increase, so the above equations yield
> the following:
>
This doesn't seem right to change the Omega factor due to the material of
the drag.  If the lateral load resisting system and R factor do not change,
why would the Omega?

> which is a factored or ultimate strength design force.  If allowable
> stress design is to be used for the design of the braces, most
> engineers would immediately divide 0.196 by 1.4 to get a base shear
> of V=0.14W, which is the same as the 94 code.  This base shear will
> give you ASD demands on the braced frames.
>
> Now, suppose you start analyzing the building with the 0.14 base
> shear coefficient (ASD), and you have a collector force, F, for which
> you need to provide a steel member, let's say it is a TS section.
> The tension or compression demand will be omega times Eh which is
> 2.2(1.4)(F), where 1.4(F)=Eh.  DON'T FORGET THE 1.4 FACTOR!!

If you multiply back through with the 1.4 factor, you have reconverted your
forces to ultimate levels.  Could you explain further why you think that 1.4
factor is necessary if you are designing to ASD?

Mike O'Brien





> -----Original Message-----
> From: Swingle, Mark [mailto:Mark.Swingle(--nospam--at)dgs.ca.gov]
> Sent: Friday, September 10, 1999 2:33 PM
> To: 'seaint(--nospam--at)seaint.org'
> Cc: 'mswingle(--nospam--at)earthlink.net'
> Subject: Re: Section 1633.2.6 & 1612.4
>
>
> I am surprised that this question has not yet received a response (at
> least I didn't see one on the web site), so I will offer my comments.
>
> I believe your interpretation is correct.  In the 1997 UBC, the demand
> Em to be calculated for a collector is omega times Eh.  The capacity
> to be calculated is the allowable stress design (ASD)
> capacity times 1.7,
> which may ALSO be multiplied by an ADDITIONAL 1.33 for wood members,
> but only for stresses to which the LOAD DURATION FACTOR (C sub D)
> applies, such as tension, compression, and bolt bearing on wood (but
> not for compression perp to grain).
>
>
> Now let me add some editorial comments regarding the following items:
> 1) Em, 2) combined stresses, 3) rho, 4) C sub D, and 5) collectors
> in buildings with wood structural panels.
>
>
> ----ITEM 1)----
> Do not forget to "put the 1.4 factor back in" before calculating Eh,
> if allowable stress design (ASD) is used.  Let me explain what I mean.
> Most designers, after calculating the governing base shear equation,
> will IMMEDIATELY reduce the base shear coefficient by the 1.4 factor.
>
> Following is a lengthy example, so bear with me.
>
> Suppose one has the following building: steel ordinary braced frames
> (R=5.6, omega=2.2), Zone 4, I=1, soil type S sub D, rho=1, Na=1; and
> one is using ASD for steel.  The governing base shear equation for
> this short period building is V=2.5(Ca)(I)(W)/R, which yields
> V=0.196W,
> which is a factored or ultimate strength design force.  If allowable
> stress design is to be used for the design of the braces, most
> engineers would immediately divide 0.196 by 1.4 to get a base shear
> of V=0.14W, which is the same as the 94 code.  This base shear will
> give you ASD demands on the braced frames.
>
> Now, suppose you start analyzing the building with the 0.14 base
> shear coefficient (ASD), and you have a collector force, F, for which
> you need to provide a steel member, let's say it is a TS section.
> The tension or compression demand will be omega times Eh which is
> 2.2(1.4)(F), where 1.4(F)=Eh.  DON'T FORGET THE 1.4 FACTOR!!
>
> The allowable capacity from the AISC ASD book may be multiplied by
> 1.7, but may NOT be multiplied by an additional 1.33 allowable
> stress increase, since the 1.7 is INSTEAD OF 1.33 for steel members.
>
> Let's compare the 94 UBC with the 97 UBC for THIS CASE ONLY.  In
> the 94 UBC, the analysis would yield the same collector force of F,
> also at an allowable stress design level (actually slightly less,
> since the base shear coefficient was 0.1375 vs 0.14, in insignificant
> difference of less than 2%). However, the 94 UBC had no requirement
> to increase the force on collectors, although many engineers did this
> anyway (the old 3Rw/8) for all collectors, recognizing their
> importance in the LFRS.  The 94 code did allow a one-third increase
> in allowable stresses for this condition.  Therefore, using the
> notation Cap for the allowable capacity we have (for this case only):
>
>                            demand < capacity
>              94 UBC:          (F) < 1.33(Cap)
>              97 UBC:  2.2(1.4)(F) < 1.70(Cap)
>
> which is an EFFECTIVE increase in the required
> capacity (97 UBC vs 94 UBC) of: (2.2)x(1.4)x(1.33)/(1.7) = 2.4
>
> If the collector is WOOD (with steel braced frames) then omega is
> 2.8 and the load duration factor of 1.33 MAY be combined with the
> 1.7 allowable stress increase, so the above equations yield
> the following:
>
>                            demand < capacity
>              94 UBC:          (F) < 1.33(Cap)
>              97 UBC:  2.8(1.4)(F) < 1.33(1.70)(Cap)
>
> which is an EFFECTIVE increase in the required
> capacity (97 UBC vs 94 UBC) of: (2.8)x(1.4)/(1.7) = 2.3
>
>
> ----ITEM 2)----
> If the collector resists gravity loads as well, don't forget to
> check the combined bending and compression due to 1.2D + fL + 1.0Em
> and 0.9 +/- 1.0Em.
>
>
> ----ITEM 3)----
> If rho is greater than 1, don't forget that you can "take it back
> out" before checking collectors, if your collector force is based
> on E, which already includes the rho factor.
>
>
> ----ITEM 4)----
> If your steel braced frame building has wood collectors such as
> straps and blocking, don't forget that the load duration factor
> does not apply for compression perpendicular to grain stresses,
> according to the NDS.  So, do NOT multiply the capcity by 1.33(1.70),
> but only by 1.70
>
>
> ----ITEM 5)----
> Finally, don't forget that this ENTIRE DISCUSSION does not apply
> to buildings braced by wood structural panel shear walls, whether
> with wood studs or steel studs (Exception to paragraph 1633.2.6).
> My guess is that there is virtually no known failure of a collector
> in a properly-designed wood building.  Can you imagine the outcry
> if all double top plates (and their connecitons), as well as all
> straps in residences had to resist 2.3 times the 94 force?
>
>
> I welcome any and all comments.
>
> Mark Swingle, SE
> Oakland, CA
>
> Disclaimer:  These are my own opinions.  They are
> subject to change due to the ravages of time and
> after being subjected to reasoned criticism.
>