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Re: EBF's

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On Fri, 05 Sep 1997 09:24:55 -0800 Bruce Hopper <Bruce(--nospam--at)uskh.com> writes:

>>EBF requires the link beams to yield first and absorb energy.  It is
>>commonly known that Mill Test Reports (MTRs) provided by the
>>steel mill generally show that steel has considerably higher
>>yield strength than the nominal yield strength (Fy).  I am curious 
>how you design to ensure that yielding will occur at the link beam 
>first?
>
>>Y. Henry H
>
>The link beam is selected based upon the seismic loading.  Thereafter, 
>the other components are selected such that they have a capacity of 
>1.5 times the strength of the link beam.  By using the same grade 
>steel throughout the EBF, the ratio of actual yield to nominal yield 
>should be relatively close for all the elements.  If the actual yield 
>is higher than the nominal yield for all the steel sections in the 
>system, the link beam is still the weakest element in the system.
>
>Forgive me for doing this, but this is where the thread forks.   
>Implied in your message is that the lateral system in this building 
>will require more seismic load to reach first yield than it was 
>designed for.  Should we be using a number which better approximates 
>the actual Fy in the design of EBF's rather than the nominal Fy?  
>Doing so would force the system to respond at force levels consistent 
>with the design.
>
>Is the increase in Fy significant?  Seems to me that the EBF is an 
>in-line fuse that protects the seismic circuit; just in case we get a 
>short circuit in our seismic load theory and the real loads exceed 
>what we think they are.
>
>The other branch to the thread is that sometimes we have to shoot the 
>engineer and get on with production.  This is not meant to flame 
>anyone, just that I enjoy how discussions on the list cross back and 
>forth over the fence between theory and practicality.  I know 
>engineers who reside on one side or the other of the fence and never 
>cross, but I like those who can walk the top of the fence or hop from 
>side to side as they go.
>
>Bruce Hopper, P.E.
>
>USKH, Inc.
>2515 A Street
>Anchorage, AK  99503
>
>907.343.5231
>
>bruce(--nospam--at)uskh.com
>                                         
If the nominal strength is 36 ksi for link beam and for bracing,
increasing 50% the force in bracing is similar to thinking that the link
beams have equivalent strength of 36ksi x 1.5=54 ksi.  It is not
inconceivable that the steel supplied for the link beam could be dual
cert or A36 having actual strength higher than 54 ksi, since ASTM do not
have upper limit for Fy.  Should this be the case then the link beam may
not yield first as designed.

Y. Henry Huang