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Seismic Design in FCAPS
Briefing for BSSC PUC
February 27, 2013
Standard Under Development
• Pultruded Fiberglass Reinforced
Composite structural shapes (FRP) under
development at ASCE, supported by
composite manufacturers, fiber
manufacturers, FRP shape manufacturers,
and users
• Among the common uses: cooling towers,
minor structural elements in wastewater
treatment plants…
Design Basis in FRP
• Design strength = φλRn
• Nominal strength Rn = R0*CM*CT*CCH
• Reference strength R0 = 5% value, set at
lower value of 80% confidence band
• Expected strength will be roughly 150% of
R0, thus Rn, assuming no bias in Ci
– Last statement is based upon a coefficient of
variation of about 20%
Design Basis in FRP, continued
• Time effect factor λ = 1.0 for loads with
seismic
• Resistance factor φ =
– 0.65 for rupture in tension or flexure
– 0.70 for member buckling or shear
– 0.80 for local buckling
Current Basis of Seismic Design
• Complex problem to get the force effect:
– Dynamic response, approximated as static
– Collapse limit state permits local damage
• Linear static force effects reduced by
1.5R, the response modification factor
– 1.5 is there for historical compatibility with R
factors developed in 1970’s under a different
concept of the basic criterion
“Design motions” = (2/3) MCE motions
Current Basis for Seismic Design
• Important constituents of R
– It is for a structural system
– Ductility, but FRP itself has very little
– Overstrength, FRP has significant amount
– Damping, where different that 5% of critical
• Methodology for validation of an R factor
set forth in FEMA P695 (the ATC 63
project) looks at all, but focuses on
ductility and overstrength
Design Basis for Seismic
• Generic FRP structure:
– Ductility =1 (elastic brittle), thus no real
contribution to R
– Damping, most are bolted, but not necessarily
all; no real basis for anything different than
standard 5% at this time for the generic FRP
structure, thus no contribution to R
– Overstrength is approximately 1.5/φ for a
member; this would be a reasonable lower
bound for a structural system
Design Basis for Seismic
• Generic FRP Structure continued:
• Code equation:
– Demand = E/R = MCER/(1.5R) < φRn
• Recall the objective of 10% failure rate at
MCER and ductility is essentially nil, thus
we can say MCER < Rn (the 5% strength)
is conservative (implies mean collapse
margin ratio on the order of 1.5)
• Rn’/(1.5R) < φRn
Design Basis for Seismic
• Therefore, the seismic response
modification factor, R, for the generic FRP
structure would be
R = (Rn’/Rn)/(1.5φ)
(Rn’/Rn) is somewhat greater than 1.0
(1.5φ) ranges from 0.98 to 1.2
• The higher values of 1.5φ actually have
some ductility, thus
Set R = 1.0 for generic FRP structure
Design Basis for Seismic
• Deflection amplification factor, Cd
Per FEMA P695, where damping is 5%
set Cd = R = (=1.0)
• System overstrength factor Ω0
– Intention is to protect against brittle failures in
a load path from sabotaging system
performance dependent upon ductility in a
yielding element
– ASCE 7 would apply Ω0 to collectors and
diaphragm transfers
Design Basis for Seismic
• System overstrength Ω0, continued
– Could argue for Ω0 = 1.0 based upon lack of
ductility
– However, the factor is used as a useful
penalty for certain types of irregularity, which
we should not inadvertently subvert, therefore
Ω0 = 1.251.5
• This corresponds with the penalty for light
framed wood in ASCE 7
• Limitations on height in high SDCategories
Defined Seismic System #1
• Cooling tower braced frames
– Essentially all column lines are braced
• Redundancy is good
– Columns are continuous full height, and brace
connections are the controlling element
• Single connection failure would not cause collapse
– Design basis for connections in FCAPS is
somewhat more conservative (safety index of
4.5 versus 3.5 for member limit states)
Defined Seismic System #1
• Actual cooling towers using the defined
“full” braced frame:
– The seismic mass is currently defined
conservatively by including the weight of
water film
– The fill and the water will result in structural
damping well above the standard assumed
5% of critical
• 15 to 20% of critical damping is likely
• Needs to be validated by testing
• Proposed R = 2
Future Defined Seismic System #2
• Braced frame with ductile connections
– Rectangular hollow shapes connected with
bolted stainless steel straps
– Proportioned to preclude (based on reliability,
not absolute) brittle failures of connections
•
•
•
•
Strap buckling
Strap yielding
Fasteners allowed to yield
Bearing on FRP cannot control (bushings)
– Testing and analysis will be required
– R factor likely to be in vicinity of 4
Future Defined Seismic System #3
• Sheathed shear panels on “light” FRP
framing (4a shear walls; 4b diaphragms)
– FRP panels with screws – this will have to be
the source of some ductility (is it reasonable)
– Hold-downs and end posts (chords) will
required design methodologies
– Assembly tests will be required
– System analysis (NLRH) will be required
Schedule
• Draft standard has gone through many
ballots; resolution has been contentitious.
Seismic provisions are currently out for
their first ballot
• Industry supported testing for damping of
cooling towers is currently being
negotiated