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10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
HISTORICAL REINFORCED CONCRETE
HIGH-RISE BUILDINGS
C. Chesi1 , M. A. Parisi2 and V. Sumini3
ABSTRACT
The seismic risk of a country is significantly affected by the presence of reinforced concrete
structures built without reference to a seismic design code. This problem has a worldwide
extension. In Italy approximately 60 percent of the building stock, with a large fraction of
reinforced concrete buildings, is located at sites that were not considered seismic at the time of
construction. Additionally, many reinforced concrete structures were built in the reconstruction
period after WWII and are now at the end of their conventional life cycle, which could imply
decay of the mechanical properties of their elements. Some of these buildings, at the same time,
are now listed as part of the national cultural heritage.
The case of the first reinforced concrete high-rise buildings, dating back to the 1950’s and
reaching a height of 30 or more stories making use of normal strength concrete, is of particular
interest. Design was based on an allowable stress approach without consideration of the ductility
resources and, in the Italian case, without any requirement of seismic capacity. The design,
however, is often based on very effective intuition in terms of structural concept.
This point is well reflected in two high-rise buildings constructed in Milano, Italy, at that time:
the Pirelli building and the Velasca tower. They present two different structural schemes,
innovative for the time and very effective with respect to seismic action: the Pirelli with a shear
walls scheme, the Velasca tower with a tube-in-tube one.
The re-analysis of the two high-rise buildings, performed with today’s computational means and
with reference to present code requirements, has shown high capacity resources and very
effective behavior characteristics. As a conclusion, the seismic safety of existing reinforced
concrete high-rise buildings necessarily implies their re-analysis and the consideration of many
different aspects related to the period of construction and to the effect of time elapsed. Yet, from
the structural view point, these buildings, designed with analysis tools not comparable to the
present ones, were an engineering challenge and, as a result, are often found to reflect great
wisdom and intuition in the structural concept.
1
Professor, ABC Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
Associate Professor, ABC Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
3
Doctoral student, DAStU Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
2
Chesi C., Parisi M.A., Sumini, V. Historical reinforced concrete high-rise buildings. Proceedings of the 10th
National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
Historical Reinforced Concrete High-Rise Buildings
C. Chesi1, M.A. Parisi2 and V. Sumini3
ABSTRACT
Non-ductile reinforced concrete structures built in seismic areas before modern design codes were
introduced are an important fraction of the building stock worldwide and strongly condition its
vulnerability. Some of these buildings, for their architectural and historical characteristics, are now
considered part of the cultural heritage of their countries. As a consequence, special considerations
may apply in assessing their vulnerability and, particularly, in defining possible intervention
criteria for their seismic improvement. The case of two high-rise buildings of particular value in
Italy, the Velasca Tower and the Pirelli Building in Milano, Italy, are examined. The design data
available for the two buildings made possible to evaluate the response to loads and seismic action
currently imposed by design codes, permitting to assess their adequacy and to develop some
general considerations on historical concrete buildings in seismic areas.
Introduction
Reinforced concrete structures built without reference to a seismic design code in areas that have
subsequently been included in a seismic zone may significantly increase the seismic risk of a
country. At present, in Italy approximately 60 percent of the building stock, of which reinforced
concrete buildings are a large fraction, belongs to sites not considered seismic originally.
Furthermore, many reinforced concrete structures date back to the post-WWII period and are
now at the end of their conventional life cycle, which could imply problems of degradation of the
mechanical properties of their elements. Several of these buildings, at the same time, may be -or
are already- considered part of the national cultural heritage and protected by special regulations.
As such, they may require special criteria for vulnerability evaluation and, particularly, for
interventions aimed at improving their seismic behavior.
Within a research program, now in progress, the reference design codes of the period, the
typologies and mechanical properties of the construction materials used, the construction practice
of the time as well as the possible consequences of decay on the structural response are
considered. In this context, the case of the first reinforced concrete high-rise buildings, dating
back to the 1950’s and reaching a height of 30 or more stories making use of normal strength
concrete, is of particular interest. Such buildings were simultaneously erected in Europe and in
North-America. Design was based on simplified calculations, with an allowable stress approach
1
Professor, ABC Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
Associate Professor, ABC Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
3
Doctoral Student, DAStU Dept., Politecnico di Milano, 20133 Milano, Italy, [email protected]
2
Chesi C., Parisi M.A., Sumini, V. Historical reinforced concrete high-rise buildings. Proceedings of the 10th
National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
without consideration of the ductility resources and, in the Italian case, without any requirement
for seismic action.
The design, however, is often based on very effective intuition in terms of structural
concept. This point is well reflected in two high-rise buildings constructed in Milano, Italy, at
that time: the Pirelli building and the Velasca tower. They present two different structural
schemes, both now recognized as very effective with respect to seismic action but totally
innovative then: a shear walls scheme for the Pirelli building, a tube-in-tube one for the Velasca
tower. The latter is one of the first applications of the concept, which will have great
development in the following years particularly in the USA.
It is not by chance that the structural designer has been for both Arturo Danusso, who as a young
engineer in 1908, in the period following the Messina-Reggio Calabria earthquake, had been
involved in the committee defining first requirements for developing resistance to horizontal
actions for the reconstruction of the two cities and subsequently devoted his activity to the
development of structural dynamics from its beginning.
The re-analysis of the two high-rise buildings, performed with nowadays computational
means and with reference to current code requirements, is presented in this work, together with
some considerations on historical reinforced concrete buildings.
Non-ductile reinforced concrete buildings
Non-ductile reinforced concrete buildings constitute worldwide a significant source of seismic
risk, because of their vulnerability and of their high number. Failure in these structures may
result in the global collapse of the building, with high economic losses and possible loss of
human life. The severity of the problem is increased by the difficulty and limited capability in
sorting out critical situations that may trigger collapse in the different building typologies, the
high costs of interventions and the uncertainties related to their effectiveness. Many research
units internationally are devoting their efforts to this problem. The need is to define or update
vulnerability assessment procedures with different levels of accuracy depending on the type of
application: synthetic vulnerability assessment tools not requiring numerical analyses, for a fast
recognition of critical situations, up to the development of more sophisticated indicators
comprising nonlinear analyses to be applied in a predefined and standardized form [1].
The problem of assessing the behavior of reinforced concrete buildings designed for
vertical loads has been the object of several studies in Italy for over a decade. In particular [2]
reports the results concerning reinforced concrete buildings of the city of Catania, in Sicily, as
part of a multidisciplinary study on the seismic conditions of the city. This and other works seem
particularly interesting as a contribution for defining vulnerability assessment procedures
oriented to the characteristics of Italian buildings.
Reinforced concrete before seismic codes in Italy
In order to quantify the amount of structures concerned, the census data of year 2001 [3] have
been examined. The data quantify the different typologies of residential buildings, the different
ages, the seismic zone at the site if applicable, and in such case the year when seismic zonation
started for the area. Information is too generic for supplying details or structural parameters, still
some major characteristics like the structural typology and the age are given. Results have been
summarized in diagrams shown in the following. In order to avoid misunderstandings, data and
diagrams refer to dwellings, while a single building may correspond to more dwellings.
The ages of buildings have been grouped here depending on the years when a new
design code was issued. In order to interpret correctly the graphs, the first national design code
with provisions for seismic areas was issued in 1962, and a specific code for seismic design, in
more extended and modern form in 1975 (statically equivalent force distribution similar to the
first mode, with possibility of using modal response spectrum analysis). Yet, before then,
specific but local provisions were usually issued in towns and areas hit by an earthquake.
Seismic zonation has been extended to large areas of the country at the beginning of the 1980’s
and covers the entire territory from the beginning of this century. Therefore, the case of buildings
that were not considered in seismic areas at construction time but are now is fairly common. The
first graph, fig. 1 (left), reports the number of dwellings as a function of materials and, thus,
typology. The prominence of reinforced concrete stands out, with about 13 millions dwellings,
while dwellings in masonry buildings are about 10 millions.
Figure 1. Left: dwellings by structural typology; right: concrete dwellings versus building time.
Figure 1 (right) indicates the number of dwellings in reinforced concrete buildings as a
function of the year of construction. About 8.5 millions reinforced concrete dwellings,
amounting to 60% of the total, have been built when the reference design code was still that of
1939, using procedures, computational methods, and over all practice that may be deemed
inadequate with respect to current safety requirements.
Figure 2. The graph shows the percentage of dwellings in seismic zone of 20th century.
Figure 2 is a global picture, on the national level, of the reinforced concrete dwellings in
seismic areas in year 2003, designed with or without seismic provisions.
According to the analysis of data, 60% of the dwellings in seismic areas were not built
for seismic action; yet with the upgrading of seismic codes, especially after the introduction of a
performance based design code with very strict capacity design and ductility requirements, many
buildings designed according to more primitive seismic provisions may also not satisfy current
levels. The problem is, thus, wider than it appears from information elaborated from census data.
Heritage reinforced concrete buildings
The issue of reinforced concrete buildings defined as part of the cultural heritage has been little
explored so far. A number of non-ductile concrete buildings may, indeed, be recognized in this
category because of their architectural, historical, or engineering value. Buildings, like the two
case studies summarized here, are already classified as protected. In Italy as in other countries,
this classification implies that any refurbishing or strengthening intervention must be previously
examined and approved by the relevant Authority. For Italy, a law of 2004 [4] imposes that any
public building, including churches, 70 years or older belongs automatically to this category. To
date, the main focus has been on older masonry buildings, yet this time is now elapsing also for
many reinforced concrete structures. For many of them, considering the new seismic mapping of
the national territory, seismic improvement has to be planned, within the indications and
restrictions implied by their heritage status. Modalities, however, are more difficult to define
with respect to other materials and typologies, like masonry buildings, for which more
experience is available and guidelines have been defined. Case studies appear important as a
starting point for highlighting issues and addressing the definition of suitable criteria and
procedures. The Torre Velasca and the Pirelli building, erected between the 1950’s and early
1960’s, have been analyzed in this perspective. Currently, other high-rise buildings outside Italy
are under study by the authors.
The Velasca tower
The Velasca Tower was built in 1956-58 and is protected as cultural heritage building since 2011
[5]. The BBPR Studio (architects G.L. Banfi, L. Belgioioso, E. Peressutti, and N. Rogers) was a
prominent design firm in the country. The group, after WWII, had directed their research toward
developing innovative projects that could be nevertheless inserted harmonically in traditional
environments, in opposition to the so-called “International style”. The aspect of the Tower, with
a larger top, fig. 3, is due to urban as well as historical reasons, making it immediately and
naturally perceived as part in its own right of the urban landscape of mainly low-rise buildings.
Yet, the major interest is here the structural concept, based on the concrete walls of the stairs and
elevators core located in the central area, associated with the concrete frames located at the
façades and resulting in a lattice enveloping the entire building. The system is one of the first
realizations of the tube-in-tube scheme, which will have great development in the following
years particularly in the USA. It is worth remarking that the Tower is one of the first reinforced
concrete buildings reaching a height of 106 m. The structure was conceived by Arturo Danusso,
professor of structural design at Politecnico di Milano, who had actively participated to the
development of structural dynamics and of seismic design concepts and to their spreading in
professional practice in Italy.
Figure 3. The Velasca Tower and a detail of struts at the expansion area.
Building layout
The building has 29 stories, two of which underground, and is multi-use, with shops at the base,
offices to story 10, mixed use to the 18th, corresponding to the base of the enlargement, one story
for services and appliances, and residential use up to the top. Plans in fig. 4 show the continuous
concrete walls of the inner core and the outer frames. The scheme, and the double symmetry,
will prove effective for seismic response. Traditional brick infill walls were used inside and
precast panels outside. The inclined struts at the top expansion apply compression to the lower
floor where they connect, and tension at the upper one. Beams with pre-tensioned cables have
been used in the tension zone, while the compressed slab is thicker than regular ones.
Figure 4. Velasca Tower plan at different elevations
The current state
Refurbishing interventions concerning façade elements are now in progress according to a
maintenance plan. Although these works do not concern structural elements, easier access to data
on geometry and the state of materials has resulted. Indications of engineers in charge of the
current interventions are that no significant decay has been observed on structural elements, the
conditions of reinforcement being still of full protection by a well-conserved and efficient
concrete cover. Information on design issues is available from publications of the time, and so
are the characteristic values of the materials used, tested at the Politecnico Laboratory, and the
design loads, which did not include seismic action. Information and blueprints of details have not
been accessible for this work, and have been supplied only very recently. On this basis only an
elastic analysis has been performed, still permitting to classify the present situation of the tower
structure. Further considerations based on nonlinear analysis will be possible in the next future.
Modeling and analysis
From the available documents, an accurate estimate of the dead load has been performed. The
weight of the building amounts to approximately 400 000 kN, divided as follows: concrete
contributes for 65.5%, brickwork for slabs 3%, steel 2.5 %, infills and cementitious decorative
apparatuses 11.5 %, finishing layers, floors, etc. 7.5%, and various additional overloads for 10%.
Columns hold an axial load of over 10 000 kN with a cross-section of 1.1 m2. Concrete works up
to about 8 MPa, a high stress level that was allowed after testing had confirmed the high quality
of the concrete used. In all the tests the failure limit has always been over 30 MPa.
The load analysis and the detailed knowledge of geometries has permitted to develop a
structural model suitable for modal analysis (fig. 5) [6]. Modal analysis has pointed out the
regularity of the structure in plan as well as in elevation, deriving from an accurate distribution
of stiffness. As a consequence, the increase of mass in the upper part has little effect on the
global response. Table 1 shows results and points out the good isochrony in the two main
horizontal directions (X is the longer side), which have very similar periods and, thus, stiffness;
also, the translational modes, occurring as first and second, are totally separate from rotational
ones, the third is torsional only. The first three mode shapes are shown in fig. 6.
Figure 5. Model of the Tower for structural analysis and detail of the expansion zone.
Table 1. Results of modal analysis
Mode #
Period (s)
1
2
3
4
5
6
3.23
3.21
1.90
0.64
0.60
0.51
X-direction
Modal mass (%) Total mass (%)
67.53
67.53
0.00
67.53
0.04
67.57
17.37
84.94
0.00
84.94
0.31
85.25
Y-direction
Modal mass (%) Total mass (%)
0.00
0.00
67.28
67.28
0.00
67.28
0.00
67.28
18.33
85.61
0.00
85.61
Figure 6. Mode shapes, from left: first, X-direction, second, Y-direction, and third mode.
The response spectrum analysis has been carried out according to the present design
code. The effects of 1) the wind action of the original project, 2) the wind action now required,
and 3) the seismic action have been considered according to current required load combinations
from the Italian National Code, corresponding to Eurocode 8 [7]. The wind action today is only
slightly higher than at the design time. For the earthquake, the area is of moderate to low
seismicity: the local design spectrum with a 475 years return period has been considered with a
behavior (reduction) factor of 2. This prudential value, quite below code assumptions for existing
buildings, has been assumed because of the lack of information on reinforcement details. The
maximum spectral acceleration was just below 0.1g.
In the comparisons wind has been assumed in the least favorable direction, considering as
well possible torsional effects from non uniform pressure in plan. Wind, also reaching moderate
maxima in the area, has resulted to be the more demanding action. Even considering a structural
life over the standard 50 years, the result would not change, because the characteristics of local
seismicity present a slow increase of PGA, not proportional to the return period. These actions
may be absorbed by the structure still in its elastic field.
For a synthetic view of results, Table 2 reports the maximum base shears and top sways
for wind in the worse direction and for earthquake in each principal direction, comprising the
contribution of the orthogonal one according to the code.
Table 2. Summary of results
Wind, Y-dir.
Earthquake, X-dir. (main)
Earthquake, Y- dir.(main)
Base shear, kN,
(weight percent)
X-direction
44510 (11%)
13350 (3.3%)
Base shear, kN,
(weight percent)
Y-direction
65430 (16.3%)
14490 (3.6%)
48310 (12%)
Max. top sway,
(mm)
ratio
sway/height
67
44.5
45.7
0.63x10-3
0.42x10-3
0.43x10-3
The Pirelli building
The Pirelli building [5, 8], known for long in Milano as “the Pirelli skyscraper”, was built
between 1956 and 1961 for the Pirelli rubber industry and in some way symbolized the vital and
energetic post-war industrial character of the region. Prominent architects and engineers were
chosen to work at the project. The architectural design is by Gio Ponti, the slender and elegant
structure was conceived by Pierluigi Nervi and developed by Arturo Danusso.
Building layout and data
The building is 127 m high, with 3 underground floors and 31 above ground. It was at the time
the tallest reinforced concrete building in Europe. Its plan dimensions, of 70.4 m by 18.5m
account for its slenderness, which posed statics problems. The 18.5 m across for a height of
127m was an unusual ratio in high-rise buildings. Figures 7 and 8 show the building plan, with
the characteristic tapered extremes, the “tips”, and the elevation. The statics challenges posed by
geometry brought to design a mixed structure, where the area of the tips presents stiff boxed
vertical structures, that contained also stairs and elevators. Additional load bearing structures
were the 4 concrete walls positioned two by two transversally at a distance of 24 m. The walls
actually had openings that permitted the use of the floor as open space.
Figure 7. Pirelli building plan.
Fig. 8. View of the Pirelli building elevation and structural model.
Contrary to the Velasca case, reinforcement and details were available, together with a
good knowledge of the materials properties at the time of construction and also more recently.
The Politecnico Laboratory had performed the testing of concrete and steel. The two types of
concrete used had an average failure stress of 26.9 and 38.1 MPa respectively. Tests on steel
specimens are also available and show appreciable ductility of the material. The meticulous
design and care of the structural engineer may be appreciated by the fact that rebars were not
overlapped, but joined with threading and a cuff. First forms of deformed bars were also used to
improve adherence.
More recent tests were carried out in year 2000, in order to control the state of the
structure and possibly plan extraordinary maintenance operations. The current modulus of
elasticity was evaluated. The decay of concrete cover, generally modest, was measured at various
locations. The building underwent, however, forced rehabilitation in 2002, after a small airplane
out of route crashed into it in April 18th 2002.
The small-scale model
At the time of structural design, traditional computational methods were perceived as insufficient
to guarantee the safety of a tall structure with the slenderness characteristics of the Pirelli,
particularly with respect to exceptional wind action. A criterion typically adopted at the time was
experimental testing on reduced scale models. During 1955-6 various scale 1:15 models had
been prepared and tested at the ISMES Laboratory, a facility specialized in modeling and testing
large structures. From documentation, modeling of the rather soft soil was one of the problems
that were faced in scaled model preparation. Results were obtained for vertical as well as for
wind loading, including lateral sway and an estimation of the fundamental period for the model.
Scaling permitted then to evaluate the corresponding measures for the full scale case,
approximating real values. The fundamental period for the full scale resulted of 3.8 s. A finite
element modal analysis has now been carried out for the model in full scale and has supplied a
very close value of 3.94 s.
Full scale analysis
A numerical model of the building has been developed in order to analyze it according to current
design codes. Modal analysis has indicated a fundamental period of 2.84 s corresponding to a
bending mode for the structure in its thinner direction. Similarly to the case of the Velasca tower,
modes are either translational or torsional, with more differentiated periods in the lateral modes
as may be expected from the difference in plan dimensions. The period is in full agreement with
approximate formulas for lower building in Eurocode 8 (2.95 s) and UBC (2.81), but lower than
the value obtained from the scaled model and denoting a probable overestimation of stiffness.
The higher deformability of the physical model is likely due to a more accurate modeling of soil
effects.
A first response spectrum analysis had been carried out with a code-suggested value of
3.6 for the behavior factor. All the main elements passed the check for the actions resulting from
the seismic load combinations. Subsequently, nonlinear push-over analyses were carried out,
indicating as appropriate a lower value of the factor, close to 2. Spectrum anlyses were repeated
with the updated reduction, with checks remaining satisfied.
The nonlinear push-over analyses, carried out for different load distributions, permitted to
follow the evolution of the seismic behavior for increasing seismic action, which could not
appear in the linear spectrum case. The coupling beams in the shear walls, not designed
according to modern criteria, appear to enter the post-elastic field, for a PGA just below 0.6 g.
The mode of response then changes, with the vertical elements connected only by stiff floor
slabs, yet the very high capacity of the system permits to counteract high acceleration values.
The ultimate value according to the analysis is 0.77 g.
Conclusions
Two high-rise reinforced concrete buildings classified as part of the nation’s cultural heritage
were re-analyzed after over 50 years since their construction. New loading conditions included
seismic action, not defined originally. Both have shown high capacity resources and very
effective behavior characteristics. The two buildings were conceived with a strong initial
collaboration between architects and structural engineers, with neither appearing to prevail, and
both intending to build a significant and durable work for their respective competence. The
original care of details in design and execution was testified by recent inspections. Although
these conditions may be exceptional, they are more likely to occur for important buildings
perceived from the beginning as a possible cultural reference built to last for future generations.
As a conclusion, the seismic safety of existing high-rise buildings in reinforced concrete
necessarily implies their re-analysis and the consideration of many different aspects related to the
time of construction and to the effect of the period elapsed. Yet, from the structural point of
view, heritage reinforced concrete buildings, albeit designed with analysis tools not comparable
to the present ones, may be expected to reflect great wisdom and intuition in the structural
concept and particular care in their details and realization.
Acknowledgments
The precious help of former engineering students Elisa Maggio, Carlo Marini, Soili Munaro,
Andrea Zaffaroni, and Rosa Luisi is gratefully acknowledged.
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