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re: amusement structures

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Rho accounts for both redundancy and reliability.  Codewriters had a 
certain level of redundancy and reliability in mind when setting R 
values.  Rho was added to the code because many designers weren't 
providing the kind of structures originally envisioned by 
codewriters.  When the R values for nonbuilding structures (in Table 
16-P) were set, a different level of redundancy and reliability was 
envisioned.  Therefore, no additional factor was deemed necessary.  
(You will notice that the R values in Table 16-P are generally lower 
than those in Table 16-N.)

Mark pointed out the key item for your consideration.  According to 
Section 1626.3 seismic effects must still be considered.  The code 
does NOT indicate that a simple base shear check is sufficient.  The 
distribution of the base shear (to various elements) is not the same 
for both wind and seismic.  Even if every force in every element is 
larger for wind loads, most member detailing and connection design 
can still be governed by seismic (due to prescriptive detailing 
requirements, overstrength factors, requirements for connections to 
develop the capacity of members, etc.).  The key point is that the 
design has to "earn" any R value greater than 1 by means of 

If you read and correctly apply the code, seismic considerations 
may control some features of the design in lower seismic zones.  
As Roger noted, improper lateral force design (neglecting seismic 
effects erroneously) is very common in low and moderate seismic 


> From:          Aaron Burkhardt <aaron(--nospam--at)>
> Subject:       re: amusement structures
> Date:          Tue, 4 Jan 2000 13:51:45 -0800

> Thanks for the responses. I think I should have been a little clearer in 
> what I was looking for. My main concern is the UBC's stipulation that these 
> type of structures can be designed with a rho=1.0. I don't understand where 
> the redundancy in the cantilevered column system is. If it were to have to 
> be designed with a rho=1.5, then seismic would govern the design in my 
> case.
> Aaron J. Burkhardt, P.E.
> KPFF Consulting Engineers
> Portland, OR

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Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
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