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12th European Conference on Earthquake Engineering
Paper Reference 820
REVIEW OF SEISMIC STRENGTHENING GUIDELINES FOR
R. C. BUILDINGS IN DEVELOPING COUNTRIES
D. D’Ayala1 and A. W. Charleson2
1
Department of Architecture and Civil Engineering University of Bath,
Claverton Down, Bath BA2 7AY, UK
2
School of Architecture, Victoria University of Wellington
Wellington, New Zealand
ABSTRACT
In the aftermath of recent major destructive earthquakes in Turkey and India there is increased
awareness for the need to evaluate and improve seismic performance of existing reinforced
concrete buildings in these and other developing countries. The paper first reviews current
guidelines from India, USA, Europe and New Zealand from the perspective of a developing
country, and then it applies USA and New Zealand evaluation guidelines to a moment frame from
a typical Turkish apartment building. Findings of the evaluations of the frame are summarised,
followed by comments regarding applicability of the guidelines.
Keywords: reinforced concrete structures, vulnerability, strengthening, developing countries,
guidelines.
INTRODUCTION
The recent earthquakes in Turkey and India have highlighted the structural inadequacy of those
building stocks with respect to seismic loads. Building owners and occupiers are now aware of
how vulnerable their buildings are and may wish to undertake strengthening work. In Turkey
there are thousands of apartment buildings vulnerable to severe damage in a moderate or larger
earthquake [1]. Typically three to seven storeys high, they consist of relatively poorly detailed
and constructed reinforced concrete frame members infilled to various extents by unreinforced
masonry walls [2]. In many cases structural framing systems do not follow a rational layout from
the perspective of resisting lateral loads. Column orientation is more suited to internal space
utilization than achieving readily identifiable moment resisting frames in two orthogonal
directions. Commonly observed configurational problems include soft-storeys, caused by a
combination of increased ground floor interstorey heights, weak columns and strong beams, and
masonry infill walls at first floor and above.
In this paper seismic strengthening or retrofit guidelines from four areas, India, USA, Europe and
New Zealand are reviewed from the perspective of their applicability to developing countries. In
the context of reinforced concrete structures, seismic strengthening that can provide reliable
performance is a considerable technical challenge. If the often poor performance of reinforced
concrete (frame) buildings in developing countries indicates the relative difficulty of achieving a
reliable process of design and construction of new buildings, then the challenge of achieving
sound retrofit buildings is clear.
REVIEW OF EVALUATION AND STRENGTHENING DOCUMENTS
India
The Indian Standard [3] focuses on providing guidance on reinstating damaged or weak elements
by rebuilding or strengthening. General principles as well as some common strengthening
techniques are discussed. For example, details of encasing reinforced concrete members to
improve strength, and methods of improving floor and roof diaphragm action are provided. The
Standard appears to concentrate on reinstating or upgrading gravity load paths of reinforced
concrete members, rather than improving seismic resistance. Its value therefore lies in effecting
rapid repairs, probably in most cases to non-engineered structures. While the lack of emphasis on
the need for engineering evaluation, analysis and design of seismically deficient structures may be
addressed in a future revision, it might be argued that much repair and retrofitting will be
undertaken without professional engineering advice. This highlights the value of practical and
detailed documentation in the form of manuals for contractors who can then retrofit a limited
range of common building types. Such a document as been recently produced by a NGO for the
Gujarat reconstruction [4].
U.S.A.
In the 1990s, triggered by several damaging Californian earthquakes, a vigorous programme of
seismic code development was undertaken. Two outcomes are the publication of FEMA 310 and
FEMA 356[4,5]. FEMA 356, based on extensive theoretical and practical research advocating a
displacement based method and non linear push over analysis, is intended to become a nationally
recognized standard. It will lead and shape current and future earthquake retrofit practice due to
its technical and procedural rigour and breadth. Selected aspects are now discussed from the
perspective of application with reference to Turkey.
Building information, evaluation and retrofit objectives
Although FEMA 356 can be used as an evaluation tool, the more traditional evaluation resulting
from the application of FEMA 310 may be more appropriate for a developing country. Both
documents take a rigorous approach to determining existing structural conditions by specifying
the as-built information required, including exposure of primary reinforced concrete connections
to ascertain the standard of reinforcement detailing. Uncertainties associated with minimum data
collection are accounted for in the analysis by application of a Knowledge Factor. This approach
might be redundant for buildings whose existing structures are discounted completely, due to
serious constructional deficiencies, and in whom additional structures are inserted to resist lateral
loading. The documents also outline potential geological hazards and provide guidance on
assessment and mitigation.
Assuming the evaluation process recommends retrofitting, FEMA 356 requires that the “design
professional” discuss with his/her client the retrofit objectives. For an appropriate earthquake
hazard level, such as a 450 year return period event, the target building performance level is
agreed upon. For typical Turkish apartment buildings, a performance level between the Life
Safety Performance Level and the Collapse Prevention level might be appropriate. This implies a
design standard somewhat less than the current Turkish code with associated cost savings and
may need approval by Turkish authorities before being accepted. The design professional also
has to explain that in order to achieve the agreed performance, the quality of design, detailing,
construction and supervision will have to be very significantly better than current practice. The
contractual and cost implications of a far more rigorous approach will be considerable.
Building Evaluation
FEMA 310 is aimed at identifying vulnerabilities and deficiencies in non-damaged buildings. It is
structured in three tiers of increasing analytical detail and decreasing conservativism towards
safety. Building assessment is performed with respect to compliance to certain criteria that are
deemed sufficient to resist earthquakes. In the first tier criteria are mainly qualitative, while the
few quantitative ones are based on fully elastic performance of the structure. Compliance of the
system is considered first, followed by each of its structural and non structural components. If the
building does not comply with one or more criteria then the professional has the choice to either
perform a more sophisticated (Tier 2) assessment, or if the building is deemed to fail the
assessment, to propose a strengthening scheme, thence reducing the cost of the assessment stage.
Tier 2 first requires a quantitative elastic analysis of the entire structural system, either dynamic
or static equivalent, compulsory if the building is in the highest risk zone. Compliance criteria are
again laid out by structural element with reference to the rules of capacity design and assumed
ductile behaviour. If there is no compliance at this level then a full non linear analysis, for
instance the push-over method, should be performed. Reference is made to FEMA 356 for the
detailed application of Tier 3.
The limitation of FEMA 310 is the a priori assumption of ductility levels and hierarchical
performance of structural elements, which may not necessarily occur in reality, and for which no
alternative provisions are considered. Also, in the event of non compliance, no suggestion is
provided for strengthening strategies to be pursued in order to realise such compliance.
Rehabilitation methods and analysis
For concrete moment frames, with or without masonry infills, the Systematic Rehabilitation
Method, involving consideration of non-linear response, is required. In developing countries it is
likely that designers will prefer the Non-linear Static Procedure. This procedure requires all
primary, secondary (defined in the document) and non-structural components (if their lateral
stiffness exceeds 10% of the total storey initial lateral stiffness) to be modelled mathematically.
Stiffness and strength degradation is also to be included. Unreinforced masonry infill walls may
not be neglected. In fact, analysis may indicate these elements are beneficial, reducing the extent
of new rehabilitation construction.
FEMA 356 requires this procedure to be “reviewed and approved by an independent third-party
engineer with experience in seismic design and non-linear procedures.” This peer review
requirement is crucial as it acknowledges important factors: the innovative nature of the
analytical approach, its technical complexity, the need for consistency, and the maintenance of
construction and professional standards. Some consultants and clients will react negatively. The
document also outlines a Construction Quality Assurance Plan, involving the contractor, design
professional and building officials. The emphasis is on the importance of detail, so often
relegated to non-professionals in developing countries, and on the structural consequences of
poor detailing. For example, “For beams and columns in which p erimeter hoops are either lapspliced or have hooks that are not adequately anchored in the concrete core, transverse
reinforcement shall be assumed not more than 50% effective in regions of moderate ductility
demand and shall be assumed ineffective in regions of high ductility demand.” ATC -40 [6]
comments on an equivalent clause: “This severe recommendation is made with the understanding
that shear failure of poorly confined columns commonly is a cause of column failure and
subsequent structural collapse.” Observed damage to Turkish apartment buildings emphasizes the
relevance of these clauses.
The FEMA approach is thorough and its correct application can be expected to achieve building
performance as close as possible to that desired. Within and outside the US these documents
represent a significant step forward. Design professionals have to consider many new aspects,
especially displacement based design. For developing countries the documents’ content
represents a giant leap. Strategies need to be developed to bridge the gap between current
practice and the high standards necessary for successful rehabilitation. In this respect local
engineering bodies have a role to introduce and support the gradual uptake of the documents by
undertaking a number of measures including:
1. Testing of local materials, foundation conditions and masonry infill wall properties, to
provide ranges of engineering properties and appropriate default values.
2. Sensitivity analyses that result in simplification of some of the provisions and provide
guidance on overcoming common mathematical modelling difficulties.
3. Case studies of several typical building retrofits illustrating analytical methods and different
retrofit strategies.
4. Disseminating lessons learned from peer reviews to build up local technical expertise.
European Code
Eurocode 8: Design of Structures for Earthquake Resistance – Part 3: Strengthening and Repair of
Buildings is currently being developed. Comments made in this paper are based upon Draft No. 1
(June 2001) [7] and an earlier Draft for Development [8] which includes Annex G, Details for
Concrete Structures.
While comment is not offered on technical details due to the draft nature of these documents,
some general discussion is warranted. Overall, it seems that the code may not be of great
usefulness to designers in developing countries. The main difficulty is a widespread lack of
explicit guidance. Principles are given, but without any specifics. Designers are left without
specific quantitative guidance on many issues. For example, when undertaking a simplified
estimation of stiffness and resistance “model correction factors” may be used, but no values are
given and designers are told that values should be “conservatively chosen, taking into account
available technical literature and local experience.” Other difficulties include the need to keep
referring frequently to other code documents and an unclear document structure. This general lack
of explicit guidance limits the usefulness of the document.
New Zealand
The most up-to-date document [9] has been in draft form since June 1996. It assumes a life
safety performance level and begins with a rapid evaluation procedure based on the visual
screening approach of ATC 21 [10]. Approximately fourteen structural criteria are assessed and
demerit points are awarded for features likely to impair seismic performance. The three most
significant parameters to determine evaluation outcome are the level of site seismicity, the
presence of significant torsion and a weak storey. The “score”, intended to relate to the
building’s damage ratio under a current code earthquake, is then combined with the gross
building area (to reflect the number of occupants and potential casualties) to confirm whether a
more detailed analysis is warranted. This evaluation procedure, if adapted to typical building
types in developing countries, may be very suitable given its simplicity.
For reinforced concrete moment resisting frames, with or without masonry infills, designers can
choose either a force or displacement based approach. Both procedures are outlined in flow charts
and elaborated upon in step-by-step explanations. Less experienced engineers will prefer the
force method. For an identified collapse mechanism and probable member and joint strengths,
assuming no degradation of shear strength, the lateral force capacity is compared to the code
response spectrum to ascertain the minimum acceptable level of structural ductility. Checks are
then made to ensure member ductility capacity exceeds demand, and that, given member
curvature ductilities, degraded shear strength capacities allow the development of member
flexural capacities. If previous check outcomes and ductility and shear capacities are adequate,
seismic improvement is not required.
This procedure is suitable for application in developing countries: it is based on a first principles
approach, with a clearly set out methodology and very few coefficients necessary to account for
hidden complexities. The designer understands the process and therefore has better control, while
advice is provided by commentary on assumptions and limitations. An evaluation can be
undertaken without reference to other documents, hence making the process as straight forward as
possible. Analytical procedures are expressed in terms of current design standards: this assists in
achieving uniformity of approach and enables better control by building officials. It also enables
easy comparison between the evaluation and design of retrofitted buildings with new buildings.
For example, the document suggests that it is generally appropriate for the strength of ‘at-risk
buildings’ to be just 67% the strength of an equivalent new building. Although this reduced
strength level effectively increases the risk to existing buildings between two and three times, the
lower standard makes earthquake improvement more financially viable.
The displacement based approach is also presented. Considerable analytical simplifications are
possible once the collapse mechanism is determined. Member plastic hinge rotation and (joint)
shear capacities are checked against the structure displacement demand which is determined from
displacement spectra, easily generated from typical code spectra, and effective natural period and
damping appropriate to the assumed inelastic mechanism. Again, this is a first-principles
approach and although a pushover curve may be used, it is feasible to use hand methods. If either
of the analytical approaches indicates serious structural deficiencies that might compromise the
life safety performance objective, guidance is then provided on how structural performance may
be improved.
MOMENT RESISTING FRAME CASE STUDY
FEMA 310 and the NZ guidelines are applied to a typical apartment building from the Dinar
region in Turkey [11]. This six-storey building is evaluated in the stronger longitudinal direction
only. Its rather irregular framing pattern is rationalized and simplified so that it can be assumed
that longitudinal loads are resisted on four identical frames. The most irregular feature of the
frame (Fig. 1a) is the orientation of the column on grid G, bending about its weaker axis.
Materials properties and reinforcing detailing are based on a general description of similar typical
buildings [2]. The frame is first assessed assuming no unreinforced masonry infill walls, and then
reassessed assuming two bays are completely infilled (Fig. 1b). Assuming a natural period of
0.5secs for the bare frame, the appropriate elastic lateral design coefficient from the latest Turkish
code [12] is 1.0 (Seismic Zone 1, Importance factor = 1.0 and Spectrum Coefficient = 2.5). This
figure is then reduced by a Structural Behaviour Factor (R), based on the system ductility factor,
which, for a high ductility frame is 8.
250
600
Column
cross-section
(steel 1%)
500
4φ16 +4φ10
150
500
2φ16
200
Beam cross-section
at column face
Stirrups φ10/250
( none through
beam-column
joint )
f` c=15 MPa
fy=220 MPa
a)
b)
Fig. 1 Diagram of analysed frame with typical cross-section and steel reinforcement
NEW ZEALAND GUIDELINES
Rapid Evaluation
Assuming maximum New Zealand seismicity is appropriate in areas of Turkey, the building
scores 100 demerit points, corresponding to an expected 100% damage ratio in a code earthquake.
This result correlates with building damage reports from the Koaceli earthquake. The conclusion
is that a detailed assessment be undertaken.
Detailed Assessment
The most advantageous reliability of information factor, 1.0, is used rather than the appropriate
value of 0.5 for this situation where there is little knowledge of component details, in order not to
exaggerate any potential structural deficiencies. After probable member and joint strengths are
calculated assuming no degradation of shear strength, calculation of a ‘sway potential index’ at
first floor level suggests a beam inelastic mechanism. This result may not apply higher in the
building where column dimensions, and strengths, are reduced. The capacity of the beam sway
mechanism is 0.13g, compared to a ground floor column sway mechanism capacity of 0.17g
(where a soft ground floor is caused by masonry infills above).
The R value required from the building, determined by dividing the elastic demand (1.0g) by the
structural capacity (0.13g) is 7.7. Due to the combination of ineffective tie anchorages, large tie
spacing and small tie diameters, a maximum structural ductility factor of 2.0 is permitted. This
equates to an allowable reduction of elastic response of less than 2.0. For this building, an
unachievable reduction factor is required for it to survive a code earthquake.
Some key assumptions made above are invalidated by the presence of masonry infill walls. Using
provided formulae, infill wall strengths for the two infilled bays (Fig.1) are found to be
significant. Wall thickness is 200mm, f’m=2.0MPa and the ratio Ec/Em is taken as 2.0.
Compressive strut failures will occur at 0.40g, while mid-height wall sliding and associated
column flexural hinges occur at 0.28g. However, as the authors are unaware of this type of
failure mode occurring during the Kocaeli earthquake this second mechanism will be neglected in
this discussion. An elastic frame analysis indicates that at about the load level at which
compressive strut failure occurs, infills withstand 75% of the base shear. Therefore, if masonry
infills are present at ground floor, the elastic strength of the combined frame and infill is 0.55g. If
the ground floor is open, the ratio of first floor strength to ground floor strength exceeds 3.0
causing an inevitable soft storey.
FEMA 310 EVALUATION
For the site seismicity FEMA 310 uses 2/3 of the spectral design value, and an earthquake with
2% probability of being exceeded in 50 years. This assumption is stricter than the Turkish code,
(10% probability of exceedence is assumed), and onerous when considering that the useful span
life of an existing building could be 30 years or less. However the value obtained compares well
with the design coefficient of the 1998 Turkish Code, assuming a ductility factor of 2 for the
frame, realistic in light of the lack of basic ductility requirements.
Tier 1
Assuming high seismicity requirements for Dinar, the performance level to be satisfied is life
safety. There are four compliance checklists: basic structural system and supplemental structural
system aimed at identifying load paths, lateral force resisting system and integrity of connections;
geologic site and foundation checklist and non-structural system checklist. Reinforced concrete
frames without and with infills are assessed as distinct typologies. Given the lack of basic
information for the two last lists, only the structural system is assessed.
The basic structural list aims to identify a continuous load path for lateral forces in two
orthogonal directions: this building does not comply, as frames can be identified only in one
direction. The next issue is the identification of weak or soft storeys and abrupt geometrical
changes. This frame would not comply as the strength and stiffness of the ground floor columns
are more than 20% greater than the above ones. This criterion seems illogical and certainly too
conservative. The basic check list for the lateral load resisting systems requires the identification
of sufficient redundancy, defined as the presence of more than one bay in any given complete
frame, absence of poorly connected infill walls, and a check on column shear stress. The two last
conditions are not satisfied, having assumed a class C15 concrete.
Completion of the supplemental structural checklist would reveal further non compliance: a
strong column-weak beam condition at the upper storeys and shear failure of columns at all
storeys except at the top one. Furthermore, on the basis of evidence collected in situ during the
Kocaeli earthquake [1] it is prudent to assume that reinforcement detailing requirements are not
met, from prevention of brittle failure to length and position of lap-splices, to spacing of stirrups
and proper hooks. If the outcome of the Tier 1 check is non-compliance then for concrete frames
and concrete infilled frames a Tier 2 complete evaluation is always required.
Tier 2
The Tier 2 evaluation phase uses a displacement based lateral force procedure and ‘m’ factors
(component ductility related factor) on an element by element basis. This approach is particularly
suitable for non-conforming systems. As the building height H< 33 m, and there is no assumed
mass, stiffness or geometric irregularity, the Linear Static Procedure (LSP), is used. Calculations
are based on the Pseudo Lateral Force calculated for Tier 1. The LSP obtains the displacement
required by the design earthquake. As the procedure is elastic the corresponding component
forces will be higher than the actual maximum forces experienced due to non-linear behaviour.
The evaluation is based for each component on the compliance of the elastic seismic effect
divided by the appropriate m factor, with prescribed criteria. There is confusion on terminology,
equivalence between linear and non-linear displacement and corresponding forces. The
parameters in the checklists are the same as Tier 1, only the acceptance criteria are quantitative,
based on the bending and shear capacity of the elements. It is also recognised that the shear
demand is a function of the bending capacity of the beam, and that bending capacity of columns
should be greater than that of beams.
From this assessment the beams, except at the two upper most floors, need a minimum required
ductility ranging between 3.5 and 5.4, but at best they can sustain ductility of 2.5; the external
columns are satisfactory but internal columns require ductilities ranging from 2.60 to 4.1,
compared with expected capacity of 2. First floor columns are the worst off. If failure would
occur by bending, these columns would be the first to fail, followed by the first floor beams.
However in absence of proper confining action by stirrups and of contribution of steel to the shear
capacity, both beams and columns will fail in shear before bending failure occurs, the beams of
the first floor proving the weakest elements.
If the infill masonry is considered as an alternative load path for lateral loads, according to Tier 1
it fails to comply with the shear strength criterion and geometric proportion criterion. Moreover,
the detailing criteria relative to the connection of the walls to the diaphragm and to the frame are,
on the basis of in situ evidence, in general not satisfied. Tier 2 refers to the strut model but does
not provide criteria to quantify the cross sectional area of the strut; hence for geometric
proportion no compliance there is no provision either to quantify its ability to contribute to the
frame seismic capacity, or to quantify the collapse load factor for the out-of-plane mechanism.
DISCUSSION ON STRENGTHENING STRATEGIES
The results of the two evaluations agree on the inadequacy of the analysed frame and the need for
improvement strategies. Possible approaches are here discussed:
1. Provision of new shear walls: this option will normally result in good seismic performance,
however its effect on planning and its relatively high costs are disadvantages. Often existing
foundations under the new walls need upgrading and this is expensive. If the walls are relatively
stiff they may protect infills from significant damage and out-of-plane failure. In selected
locations, ductile coupled shear walls might ease planning disruption.
2. Upgrade the existing frames: this would involve jacketing the columns, beams and joint
regions to improve flexural and shear strength, and concrete confinement. To avoid soft storey
mechanism and prevent out of plane failure of infills, it is necessary to separate them from the
frame. Reliable face load resistance can be provided by two vertical floor to ceiling steel
mullions attached to wall and structure. In some buildings, depending on the configuration of
infill panels, it might be possible to avoid frame upgrading on the top one or two floors by
resisting lateral loads entirely by the existing (face load upgraded only) infill walls and frame. In
order to limit ductility demand on the open frames below it would be essential for them to form a
beam sidesway mechanism. Significant foundation work may not be required.
3. Upgrade existing masonry infills: Existing masonry infill frames may be significantly
stronger than bare frames, suggesting the improvement of strength and ductility of infills as a
good strategy. New ground floor walls could be tied into strengthened infill walls above.
Shotcreting might be a suitable solution. A single layer, of reinforced shotcrete, say 75-100mm
thick, might not only provide sufficient lateral resistance together with the existing wall, but also
prevent out-of-plane infill failure. If the strengthened infills are well distributed in plan,
foundation upgrading will be minimized. Face load support for non-strengthened infills might be
required.
CONCLUSIONS
The review shows clearly that some of the reviewed documents are unsuited for potential use in
developing countries. In one case the approach to evaluation and strengthening lacks a
sufficiently well developed analytical methodology. In another, the document’s structure and
lack of detailed advice to designers severely limits its usefulness. Lack of transparency in the
detailed procedures of some documents is also of concern, as it limits a designer’s ability to
understand what is going on. If a high level of understanding is absent, mistakes are more likely
and any potential educative value is limited. Although FEMA 310 and 356 are thorough
documents, in some areas they lack the transparency to upgrade designers’ understanding. The
FEMA system should definitely be used as a checklist to ensure all aspects of an evaluation are
included and is most valuable for structural systems that can not be clearly categorized as either
frames or shear walls. For example, it appears to be the only document that provides explicit
information and requirements on vertical foundation deformation under seismic load. The
advantages of the New Zealand document are its transparency, its first principles and nongeneralized approach, and its clear indication of the modeling to be assumed. Its rapid evaluation
procedure might be useful to other countries.
The seismic evaluation case study results confirm prior expectations that the structural frame
considered to be typical of many Turkish apartment buildings is inadequate for expected seismic
demands. Although masonry infill walls contribute to its poor seismic performance, possibilities
of using their inherent strengths, in this case approximately 300% of the reinforced concrete
frame strength, need to be further explored and developed. Clearly, infill walls should not be
neglected in any seismic structural analysis unless they are separated from a frame. This is an
area that will benefit from further research, and certainly will need full inclusion in any
evaluation or strengthening and repair codified document.
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