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7/27/2019 Displacement LR

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Displacement-Based Design

Displacement-based design (DBD) has evolved as a way to implement PBSE in an

easy way and has been comprehensively studied for different researches. Each one has

suggested different ways to use this approach focusing on different aspects and types of

structures as explained next. The basic assumption behind the displacement-based methods is

that structural damage is better characterized by deformations. Thus, the deformations of the

structure (drift or displacement) are the starting point of the design and not the end product as

in the traditional force-based design methods. Then, the limit states established by the owner

and engineer are related to displacement or drift to design the structure.

Panagiotakos and F ardis (2001) proposed a modified displacement-based seismic design

(DBD) procedure for reinforced concrete buildings. Their approach differs from most other

DBD procedures mainly in that the displacement-based seismic design is integrated with the

ultimate state and while other loads such as winds are linked to the serviceability limit state.

Also, local seismic displacement and deformation demands are used directly for member

proportioning and detailing, without conversion to strength demands. They compared the

results of their designs with the Eurocode 8 (EC8). The main differences between a building

designed with the DBD procedures and design codes such as EC8 is that the seismic

reinforcement is concentrated only where it is needed to meet seismic deformation demands,

rather than being placed indiscriminately in all members.

Sul l ivan et al (2003)studied the limitations and performance of eight different displacement

based design methods applied to five different buildings. These methods include:

Freeman (1998)-Capacity Spectrum

This method is best for the assessment of existing structures. It uses a capacity spectrum of

the structure superimposed with demand spectra at different ductility/damping levels. This

approach is good for irregular structures, when a pushover analysis is used to compute the

yield displacement. It does not recommend a procedure to design new structures and develop

demand spectra for different damping levels.

Panagiotakos and Fardis (1999)-Initial Stiffness Deformation Control


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This method involves the calculation of an expected maximum displacement for an

already designed structure. This method was developed for frame type structures since it

provides factors to compute the elastic and inelastic chord rotations of this type of system.

Sullivan et al (2003) found this method appropriate also for wall type structures. The method

checks the target ductility at two different design levels, which could appears restrictive in

performance based design. The model recommends the use of an uncracked stiffness for the

elements in the structure in the initial elastic design. This causes that the design shear to be in

general higher that in the other methods.

SEAOC (1999)-Displacement Based Method a

This method designs the structure for target drift values while ductility demands are

not controlled. Four different risk scenarios and drift limits can be considered. Target

displacements are based on prescribed factors and assumed ductility demands. The method

cannot be used for flexible structures and has some limitations for wall structures.

Aschheim and Black (2000)-Yield Point Spectra

This procedure permits the design of a structure considering several levels of

performance in a relatively quickly way. The method defines permissible design regions

using a yield point spectra based on target drift and ductility values. After that the yield

displacement is determined, the strength of the structure to reach the selected ductility and

drift levels is obtained. This method relies on a good estimation of the yield displacement. It

is less accurate for irregular structures, since the yield displacement obtained from the spectra

is very sensible. Small changes in the yield displacement can cause large changes in the

design base shear.

Priestley and Kowalsky (2000)-Direct Displacement Based Design

This method designs the structures to satisfy pre-defined drift levels in a direct

manner. Code drifts and inelastic rotation capacities of the structure are part of the design

process. It also uses displacement profiles of the structures to determine system displacement.

These profiles have not been developed for irregular structures. However, Sullivan et al

(2002) proves the effectiveness of this method for the design of irregular structures.

Browning (2001)-Initial Stiffness Iterative Proportioning


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This method imposes a limit on the maximum displacement of the structure. Thus,

changes to the structural system are made to attain this limit by an interactive process. This

method was developed for regular reinforced concrete frame structures. It does not provide

any recommendation to obtain design base shears. Also, it uses uncracked stiffness properties

for the elements in the structure and acceleration amplification factors, which can result in

large design strengths.

Chopra and Goel (2001)-Inelastic Spectra

The method is based on the work done by Priestley and Calvi (1997) to determine the

target displacement and design ductility. Then, it uses an inelastic displacement spectrum to

obtain the period and initial stiffness of the system. This approach does not provide

recommendations for structures other than SDOF oscillators and does not recommend a

procedure to distribute the shear in the structure. Also, it is not appropriate for structures with

flexible foundations. The accuracy of this method depends on an accurate estimation of the

initial stiffness.

Kappos and Manafpour (2001)-Advanced Techniques with Time History

This method is complex in the sense that it requires the development of a detailed

model in which members are able to exhibit inelastic behavior. Then, the model is subject to

two different time history analyses corresponding to two earthquake hazard levels. Target

limits for these levels are checked and detailing for plastic rotations is provided. It is

recommended for irregular structures or when the inelastic response appears to be difficult to

predict because it is a time consuming method.

Abderrachid and Ahmed (2010) presented a comparison of the displacement based design of

reinforced concrete structures using spectra provided the Algerian seismic code. The study

was conducted on regular reinforced concrete frames, which consisted of four frames in each

direction and a maximum number of storeys taken equal to three. The study proposed the

need for appropriate displacement spectra for design purposes.

Gij i et al (2012)reviewed six displacement -based seismic design approaches for reinforced

concrete moment resisting frames, and compared their relative performance. The methods

were applied in the design of three reinforced concrete building frames (4-storeyed, 9-

storeyed and 15-storeyed). The performance of each method were assessed by comparing the

actual design parameters with parameters obtained through time history analysis. The paper

identifies the direct displacement based design (DDBD) proposed by Priestley and Kowalsky

(2000) to be the most promising.


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From above methods, this study is going to employ the direct displacement-based

method for frames developed by Priestley and Kowalsky (2000) and fully described in

Priestley et al. (2007) for the seismic design of reinforced concrete frame buildings.

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