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572 R. KLEMENCIC, J. A. FRY AND J. D. HOOPER

Copyright © 2006 John Wiley & Sons, Ltd. Struct. Design Tall Spec. Build. 15, 571–579 (2006)

DOI: 10.1002/tal

Table 1. Ductile core wall system projects designed by MKA

Number of Project name stories Height

Millennium Tower; Seattle, Washington 21 241 feet

Terry Avenue Apartments; Seattle, Washington 26 245 feetAsian Star Building; Manila, Philippines 23 292 feetKey Center; Bellevue, Washington 22 305 feet1700 Seventh Avenue; Seattle, Washington 23 320 feetElliott Grand Hyatt Hotel; Seattle, Washington 30 330 feetSeventh at Westlake; Seattle, Washington 31 358 feet300 Spear Street; San Francisco, California 43 400 feetIDX Tower; Seattle, Washington 35 450 feetWashington Mutual Tower; Seattle, Washington 42 544 feetPacific Plaza Towers; Fort Bonafacio, Philippines 46 617 feetOne Rincon Hill; San Francisco, California 57 625 feet

Figure 1. Rendering of One Rincon Hill in San Francisco, California



• Height limits

• Selection of the response modification factor, R

• Selection of ground motions

• Shear demands

•Higher mode dynamic effects

• Foundations

• Detailing

• Peer review process

2. HEIGHT LIMITS

One of the first and most contentious issues has been surpassing the specific height limits detailed in

Table 16-N of the Uniform Building Code for buildings in Seismic Zones 3 and 4. This table limits

the height of shear walls in building frame systems to 240 feet, and limits shear walls as part of bearing

wall systems to 160 feet. The lateral systems for buildings reaching above these heights are limited

to be moment frame systems or dual frame systems.

In each of the buildings listed in Table 1, some form of DCWS was developed. As an example, theprimary lateral-force-resisting system for the One Rincon Hill project is depicted in Figures 2 and 3.

While this system includes concrete walls, these walls are arranged in the form of a perforated struc-

tural tube. Coupling beams above the core wall penetrations provide the first line of energy dissipa-

tion in the longitudinal direction of the core. The coupling beams (activated by shear lag), together

with supplemental buckling restrained braces (BRBs), provide the first line of energy dissipation in

the transverse direction of the building.

A qualitative comparison between this system and a planar wall structure dominated by shear behav-

ior suggests the possibility of a more ductile response. However, current prescriptive building code

provisions do not differentiate between planar shear walls and coupled walls.

In researching the origin of the 160- and 240-foot height limits, one learns the first mention of this

limit is made in the early 1950s in an ASCE committee paper that suggests moment-resisting frames

PERFORMANCE-BASED DESIGN OF TALL RC DUCTILE CORE WALL SYSTEMS 573


DOI: 10.1002/tal

Figure 2. Plan view of the lateral-force-resisting system in One Rincon Hill



The design of tall buildings is generally controlled by drift limitations and not the strength of

the lateral elements. Therefore, it is contradictory that a relatively stiff system such as a wall

assemblage is assigned a higher basic strength requirement ( R = 4·5 or 5·5) than a much more

flexible moment frame system ( R = 8) as the resulting design forces are used to assess building

deformations.

In addition, it is inconsistent that all walls or wall systems be assigned the same value of R. It is

easily understood that a coupled wall dominated by flexural behavior will have superior ductility when

compared to a planar wall dominated by shear deformations. However, current building code provi-

sions do not reflect this fundamental difference in building response. It should be noted that the

National Building Code of Canada does include provisions which recognize these different behaviors.

Finally, if a capacity design approach is implemented for a structure such as One Rincon Hill, where

the link beams, BRBs, and base of the core wall are selected as the primary areas of energy dissipa-

tion, assigning a high strength requirement to these components will significantly increase undesir-

able shear demands on core walls, diaphragms, and foundations. Based on this fact alone, consideration

of a higher value for R is warranted.

Unfortunately, influenced by the bounds of the current Building Code, most building officials and

peer reviewers have insisted on the selection of R = 5·5 or, in the case of San Francisco, R = 4·5. Aswas observed directly in the design of One Rincon Hill, shear demands on the core walls, diaphragms,

and foundations were increased to nearly intolerable levels.

A reassessment of R values which are more reflective of actual building behavior is warranted, such

that desirable behaviors (flexural yielding) are promoted, and undesirable demands (shear) are mini-

mized. For very tall buildings, or for buildings in aggressive wind climates such as the Philippines,

perhaps it is more appropriate to allow wind demands to determine the basic strength requirements,

with the seismic design primarily focused on ductility and robustness.

4. SELECTION OF GROUND MOTIONS

Selection of appropriate seismic ground motions and response spectra with which to evaluate the build-

ing design is critical. Generally, three levels of demand have been assessed:

• Serviceability earthquake (SE)—50% probability of exceedance in 30 years (43-year return

period)

• Design basis earthquake (DBE)—10% probability of exceedance in 50 years (472-year return

period)

• Maximum considered earthquake (MCE)—2% probability of exceedance in 50 years (2,475-year

return period), with a deterministic limit in appropriate locations

Common practice in design is to begin with an assessment of the building when subjected to the DBE,

reduced by the selected R factor, as shown in Figure 4 for the One Rincon Hill project. For most

tall buildings, the fundamental building period will range between 4 and 8 seconds. Performing a three-

dimensional elastic analysis of the primary lateral-load-resisting system subjected to this level of demand determines the basic strength requirement for the building.

Affecting the design of tall buildings are the minimum base shear equations. Equation 30-6, required

in all seismic regions, represents the ‘3%’ base shear value that has been in the code for decades. In

regions of high seismicity, a lower-bound limitation is placed on the basic strength requirement, as

represented by Equation 30-7 of the Uniform Building Code (seen in Figure 4).

Equation 30-6 V C IW = ⋅0 11 a



DOI: 10.1002/tal



Equation 30-7

As in the case of San Francisco, the lower bound limitation on base shear is significantly greater

than the site-specific-design DBE spectrum, reduced by R, would otherwise suggest. The One Rincon

Hill project adhered to the lower bound strength limitation of Equation 30-7.

An assessment of the One Rincon Hill project at the SE indicates the critical coupling beams meet

the ‘no-yield’ criteria as suggested in recently published design procedures by the Los Angeles Tall

Buildings Structural Design Council (LATBSDC) and the Department of Building Inspection in San

Francisco (see Figure 5). However, a close inspection of Figure 4 indicates that without the lower

bound limitations of UBC Equation 30-7 the SE would control the strength design of the coupling

beams.An assessment of collapse prevention performance was investigated for each of these projects as

well. Selecting appropriate ground motion records for the nonlinear time history analysis (NLTHA)

is more difficult than might initially appear. A large database of strong ground motion records which

will excite the building structure in the period range of interest is not available. Furthermore, the

fundamental period of vibration for tall buildings is generally very long (4 to 8 seconds). Scaling an

MCE spectrum, which is applicable in this period range, without overpredicting short-period demands

is a challenge. The duration of selected ground motion records is also important as several cycles of

V ZN I

RW =

⋅

0 8 v



DOI: 10.1002/tal

Figure 4. Comparison of design spectra for One Rincon Hill



motion are generally required before amplified building response is observed. Lastly, choosing records

that also excite the higher modes of vibration of the tower is important; however, this is difficult to

accomplish without overestimating ground motions in the lower period range.

Summarizing MKA’s experience:

(1) Given a 43-year return interval for the SE, the ‘no-yield’ acceptance criteria may be too strict, and

some modest amount of yielding may be acceptable. An appropriate assessment of building drift

is a more important parameter.

(2) The application, or lack thereof, of Equations 30-6 and 30-7 will likely control the basic strength

requirements for the DBE.

(3) A careful selection of appropriate ground motion records for the NLTHA, which properly test the

design, is critical.

5. SHEAR DEMANDS

As previously noted, shear demands are driven by the selection of the basic strength requirement for

the flexural elements. Great care should be exercised in selecting the basic strength requirement,thereby balancing stiffness and strength with the implications of possible undesirable increased shear

demands.

Great debate among consultants and building officials has occurred regarding the assessment of

shear demands. While all agree shear ‘failure’ is undesirable, there is a clear lack of consensus on how

to properly assess demands, what constitutes a ‘failure’, and how to properly assess shear capacities.

Based on MKA’s experience, it would seem reasonable to assess shear demands consistent with

something larger than the average demands predicted by a NLTHA. Some have suggested shear



DOI: 10.1002/tal

Figure 5. Coupling beam serviceability forces versus provided capacity for One Rincon Hill



demands be assessed as one standard deviation greater than the average demands predicted by a suite

of seven NLTHAs. Others have suggested that shear must be assessed at the peak response predicted

by a suite of seven NLTHAs. While it is possible to satisfy either of these two criteria with properly

selected ground motions, shear demands equal to one standard deviation above the average predicted

from a suite of seven NLTHAs appears adequate.

6. HIGHER MODE DYNAMIC EFFECTS

Most of the structures cited at the beginning of this paper are dominated by first mode behavior, where

higher mode dynamic effects played little role in the outcome of the design. In the case of One Rincon

Hill, however, higher mode dynamic effects played a much more significant role. The combination of

absolute building height (625 feet), slenderness, and site-specific ground motions produced significant

higher mode effects. The result of these higher mode effects was a significant reduction in the effec-

tive moment arm of the lateral force distribution, creating a significant shear demand at the base of

the building as well as high flexural demands near the building’s mid-height. A detailed study of this

effect indicated the shear and flexural demands were nearly independent of the basic flexural strength

assigned to the base of the core walls. Careful assessment of the elastic DBE analysis can give cluesof the potential importance of higher mode dynamic effects. NLTHAs, with properly selected ground

motions, will confirm the building’s behavior.

7. FOUNDATIONS

Very little guidance is provided in the Building Code or published literature regarding the appropri-

ate design of foundations for seismic demands. Common practice is to design building foundations

for the basic strength requirements of the DBE, with no consideration of the possible over-strength of

the superstructure.

In the case of the buildings listed at the beginning of this paper, including One Rincon Hill, the

foundations were designed considering demands imposed by the full over-strength capacity of the

superstructure. In most cases, this resulted in foundations nearly two times thicker than those tradi-tionally designed.

Furthermore, in the case of One Rincon Hill, where a deep mat foundation (12 feet thick) is

employed as the tower’s foundation, shear reinforcing was provided where shear stresses exceeded

in accordance with recent research by Michael Collins and American Concrete Institute (ACI)

discussions regarding shear critical behavior of thick, one-way systems.

8. DETAILING

Detailing of reinforcing steel is critical to promote the behavior predicted by analysis. Two areas of

particular note are highlighted below.

8.1 Coupling beams

The confinement of diagonal reinforcing in coupling beams specified by current ACI provisions is

nearly impossible to construct in the field. Compromises in field installation are common, suggesting

the resulting behavior may not be as expected. As an alternative approach, the entire coupling beam

section may be confined per ACI 21.4.4, relieving some of the constructability issues. A code change

proposal allowing such detailing has recently been presented to ACI and is currently making its way

through the committee approval process.

1 f c′



DOI: 10.1002/tal



8.2 Confinement of vertical wall reinforcing

A comparison of strain demands predicted by NLTHAs with laboratory results indicates that predicted

tensile strains agree well with test results. Compressive strains, however, may be underpredicted by

as much as 100%. Great care must be exercised in specifying confinement of vertical wall reinforc-

ing based on these results.

9. PEER REVIEW PROCESS

Because these unique building designs fall outside of the prescriptive language of the Building Code,

detailed and rigorous peer reviews have been the norm. In general, the process has been positive and

has produced improved building designs. However, there have been many inconsistencies between

reviews despite similarities in systems and design methodology. These inconsistencies are based pri-

marily on the personal biases of the individual reviewer and not any ‘industry standard’. Further, there

has not been an effective means to resolve differing opinions when disagreement has arisen. Rather,

the norm has unfortunately fallen toward accepting the whims of the reviewer in favor of an expedi-

ent review process. In the future, the engineering community should work diligently toward stan-

dardizing the scope and authority of peer reviews.

10. CONCLUSION

Despite numerous technical challenges, tall buildings with unique structural systems can be designed

to meet or exceed the performance objectives of the current Building Code. Given the importance of

these structures, great care and due conservatism are warranted. The Building Code was not written

with tall buildings as its basis. Therefore, appropriate interpretations are important and should not only

be permitted, but required. Building officials should allow engineers to use rational engineering

methodologies and proper engineering mechanics to demonstrate that a proposed design meets or

exceeds Building Code performance expectations, and not tie the engineers’ hands by limiting designs

to the prescriptive requirements of the Code. As a prominent engineer recently said, ‘Codes were

written by mere mortals’; therefore, they are not all-knowing.



DOI: 10.1002/tal

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