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Steel-Concrete Composite Building Under Seismic Forces,D. R. Panchal (Research)
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Steel-Concrete Composite Building Under
Seismic Forces
Response of A Steel-concrete Composite Building Vis-a-vis An R.C.C. Building Under
Seismic Forces
D. R. Panchal, Lecturer, Applied Mechanics Department, Faculty of Technology and
Engineering, The M. S. University of Baroda, Vadodara, & Dr. S. C. Patodi, Professor, Civil
Engineering Dept., Parul Institute of Engineering & Technology, Limda, Vadodara.
Unlike static forces, amplitude, direction and location of dynamic forces, especially due to
earthquake, vary significantly with time causing considerable inertia effects on building.
Behavior of the building under dynamic forces depends upon the dynamic characteristic of the
building which is controlled by both their mass and stiffness properties. In the present study, for
evaluating the seismic performance of a typical B+G+9 multi-storey building, steel-concrete
composite and R.C.C. options are considered. Two popular methods of seismic analysis i.e.
Equivalent Static Method of Analysis and Linear Dynamic Response Spectrum Method of
Analysis are used. For modeling, the composite and R.C.C. structures STAAD.Pro, V8i software
is used and the results of the analysis are compared; composite structure is found to be more
economical.
Introduction
Steel-concrete composite systems have become quite popular in recent times because of their
advantages against conventional construction. Composite construction combines the better
properties of both i.e. concrete and steel and results in speedy construction with a possibility of
working on parallel front [1]. In case of a composite structure, steel imparts ductility to the
structure which has ability to absorb seismic energy imparted on the structure by the earthquakes
and concrete prevents steel from corrosion and fire. The key feature of this system is composite
action between a concrete slab and a steel beam which is achieved through the shear connection
system which significantly increases the rigidity and the ultimate moment capacity of the
composite beam compared with the properties of a bare-steel or reinforced concrete beam [2-4].
In this way much larger beam spans can be achieved.
Composite frames used for multi-storey buildings usually comprise of a bare-steel frame of H-
section columns supporting I-section beams, laid out in a rectangular grid of primary (shortest
span) and secondary members; supporting an overlaid composite floor deck. The composite deck
system consists of cold-formed profiled steel sheets which act not only as the permanent
formwork for an in situ cast concrete slab but also to some extent as tensile reinforcement.
Composite floors using profiled sheet decking as shown in Fig.1 have become quite popular for
high- rise buildings. In composite floor, the structural behavior is similar to a reinforced concrete
slab with the steel sheeting acting as the tension reinforcement. The main benefit of using
composite floors with profiled steel decking is saving in steel weight up to 30 to 50% over non-
composite construction. [5]
Figure 1: Composite Slab
Figure 2: Beam-Column Connection
The steel decking performs a number of roles, such as:
1. It supports loads during construction and acts as a working platform.
2. It develops adequate composite action with concrete to resist the imposed loading.
3. It transfers in-plane loading by diaphragm action to vertical bracing or shear wall.
4. It stabilizes the compression flange of the beams against lateral buckling.
5. It reduces the volume of concrete in tension zone.
6. It distributes shrinkage strains, thus preventing serious cracking of concrete.
A variety of composite connections are possible. Figure 2 shows a composite connection
between slab, beam and column. Here, beam-to-column connections in a steel frame and the
beams are designed to act compositely with the floor slab.
A number of parameters for composite beam are compared in Table 1 with two different types of
steel beams having I- and H- cross sections with no shear connection to the concrete slab. The
load capacity is nearly the same but the difference in stiffness and construction height is
noticeable.
In order to evaluate the seismic efficiency of a composite structure against an R.C.C. structure, a
B+G+9 multi-storied building is analyzed in the present work using a professional software
STAAD.Pro V8i, while following the guidelines of the IS: 800-2007 [6] and AISC [2] codes.
Results are compared to arrive at suitable recommendations.
Procedures for the Seismic Analysis
Linear Static Procedure
The linear static procedure is a method of estimating the response of the structure to earthquake
induced forces by representing the effects of this response through the application of a series of
static lateral forces applied to an elastic mathematical model of the structure and its stiffness[7].
For this reason, this method is also known as Equivalent Static Method of Analysis (ESMA).
The forces are applied to the structure in a pattern that represents the typical distribution of
inertial forces in a regular structure responding in a linear manner to the ground shaking
excitation, factored to account, in an approximate manner, for the probable inelastic behavior of
the structure. It is assumed that the structure’s response is dominated by the fundamental mode
and that the lateral drifts induced in the elastic structural model by these forces represent a
reasonable estimate of the actual deformation of the building when responding inelastically.
Linear Dynamic Procedure
The main purpose of linear dynamic analysis is to evaluate the time variation of stresses and
deformations in structures caused by arbitrary dynamic loads. The response spectrum provides
the required information for design purposes and at the same time, simplifies the analysis by
reducing the problem to a static problem of the estimated maximum responses. The response
spectrum is defined, on a single degree of freedom system of varying frequency excited by a
specific earthquake, as the maximum response of the system, ignoring the particular time of its
occurrence [7, 8]. The Linear Dynamic Response Spectrum Analysis (LDRSA) is the standard
procedure of modern seismic design codes. It aims to give directly the maximum effects of the
earthquake in the various elements of the structure. It consists of computing the various modes of
vibration of the structure and the magnitude of the maximum response in each mode with
reference to a response spectrum. A rule is then used to combine the responses of the different
modes. For this reason the method is also known as the superposition of modal responses
method. The combination rule will generally be a square root of the sum of squares (SRSS) of
the various modal responses. This combination rule is applied to all computed quantities, i.e.
bending moments, shear forces, normal forces and displacements.
Non Linear Static Procedure
A simplified non linear analysis procedure, in which the forces and deformations induced by a
monotonically increasing lateral loading are evaluated using a series of incremental elastic
analyses of structural models that are sequentially degraded to represent the effects of structural
nonlinearity.
Non Linear Dynamic Procedure
In a non linear dynamic analysis procedure the response of a structure to a suite of ground
motion histories is determined through numerical integration of the equations of motion for the
structure. Structural stiffness is altered during the analysis to conform to nonlinear hysteretic
models of the structural components.
Evaluation of the composite members at the ultimate limit state requires a procedure that takes
into account of the actual redistribution capacity of the structure, which however, depends on
complex phenomena of interaction between the concrete slab and steel profile [4]. These aspects
are vital for structures located in seismic zones. In reality, during earthquakes, buildings are
generally subjected to large inertia forces which cause members of buildings to behave in a
nonlinear manner, i.e. stress does not remain proportional to strain in addition to the nonlinearity
associated with large deformations. Thus, the earthquake shaking of structure is a nonlinear
dynamic problem and structural analysis should be able to incorporate the nonlinear behavior of
members for evaluating the actual response of structure. Nonlinear analysis, however, requires a
lot of input data related to material and section properties and loads, which are generally difficult
to obtain accurately. Therefore, the national codes of most of the countries recommend nonlinear
analysis only for highly irregular and important structures. The linear dynamic analysis is
comparatively simpler. It adequately captures the dynamic behavior in elastic range and,
therefore, it is a reasonably good indicator of the performance. The main purpose of the linear
dynamic analysis is to evaluate the time variation of stresses and deformations in structures
caused by the arbitrary dynamic loads.
Three Dimensional Modeling
A structural model is a diagram which consists of a set of nodes and connections between the
nodes. Analysis process of frames is conducted on a model based on many assumptions
including those for the structural model, the geometric behavior of the structure and its members
and the behavior of the sections and joints. Structures with irregular plans, vertical setbacks or
soft stories will cause no additional problems if a realistic three dimensional computer model is
created. This model should be developed at the early stages of design since it can be used for the
static wind and vertical loads, as well as for the dynamic seismic loads.
Only structural elements with significant stiffness and ductility are modeled. Non-structural
brittle components can be neglected [9]. The beam-to-column connections play an important part
in the overall stability of any frame so one has to model it properly by rigid (continuous
construction), semi-rigid (semi-continuous construction) or pinned (simple construction)
connection. For the purpose of elastic dynamic analysis, gross concrete sections, neglecting the
stiffness of the steel, are normally used.
When bracing is provided, it is normally used to prevent, or at least to restrict, sway in multi-
storey frames. Common bracing systems are trusses or shear walls as shown in Fig. 3. For a
frame to be classified as a braced frame, it must possess a bracing system which is adequately
stiff. The frame without the bracing system can be treated as fully supported laterally and as
having to resist the action of the vertical loads only. Here reinforced concrete building is
considered as unbraced frame and composite building is considered as braced frame. The bracing
system resists all the horizontal loads applied to the frames it braces in addition to any vertical
loads applied to the bracing system and the effects of the initial sway imperfections from the
frames it braces.
Figure 3: Common Bracing Systems
It should be noted that in a frame with a truss type or frame type bracing system, some members
participate in the bracing system in addition to being a part of the frame structure. For frames
without a bracing system, and also for frames with a bracing system but which is not sufficiently
stiff to allow classification of the frame as braced, the structure is classified as unbraced. In all
the cases of the unbraced frames, a single structural system, consisting of the frame and of the
bracing when present, shall be analysed for both the vertical and horizontal loads acting together
as well as for the effects of imperfections. The existence of a bracing system in a structure does
not guarantee that the frame structure is to be classified as braced. Only when the bracing system
reduces the horizontal displacements by at least 80% can the frame be classified as braced.
The following guidelines are considered here for the modeling of the structure:
The members and joints are modeled for global analysis in a way that appropriately
reflects their expected behavior under the relevant loading.
The basic geometry of a frame is represented by the centerlines of the members.
Members are represented as the linear structural elements located at their centerlines,
disregarding the overlapping of the actual widths of the members.
Alternatively, account is taken of the actual width of all or some of the members at the
joints between the members.
Design Example
A "B+G+9 Storied Commercial Building" [10] depicted in Fig. 4 is considered here with the
following parameters:
Figure 4: Three Dimensional Rendered View of an Eleven-Storey Composite Building
Place of construction : Vadodara
Built-up area : 720 sq. m.
Storey height : 4.0 m
Soil Type : Class II – Medium Soil
Grade of concrete : For composite decks, concrete beams and concrete columns - M 25
For concrete columns – M 30 & M 35
Grade of reinforcement : Fe 415
Grade of steel : Fy 250
D.L. at any typical floor level
Moving Partitions = 1.5 kN/m2
Floor Finishes = 1.2 kN/m2
Metal Deck = 0.15 kN/m2
Duct & Plastering = 0.80 kN/m2
D.L. at roof level
Metal Deck = 0.15 kN/m2
False Ceiling, Ducts etc. = 1.00kN/m2
Screened concrete 50 mm thk. = 1.20 kN/m2
Live Load = 4.00 kN/m2 at floor level
Deck slab definition : One way floor load, rib width: 0.18 m, rib height: 0.05 m
Analysis Methods : ESMA and LDRSA
STAAD.Pro, V8i software which has the capabilities of performing analysis and design of both
R.C.C. and composite structures is selected here. For R.C.C. structure, various trial- and- error
procedures are done for getting the optimized section considering its span/depth ratio, grouping
of members, under reinforced criteria etc. To define composite beam in STAAD.Pro, the deck is
defined using graphical tools of the program and for steel members directly the read section
properties from the steel table option is used.
Results obtained for the composite and R.C.C. buildings by the Equivalent Static Method of
Analysis (ESMA) for nodal displacements, support reactions and beam end-actions are compared
in Tables 2 to 6 whereas the results obtained by Linear Dynamic Response Spectrum Analysis
(LDRSA) are reported in Tables 7 to 11. For LDRSA, twelve mode shapes and twentyfive load
cases are considered as per the guidelines of IS: 1893 code [7].
Figure 5: Three Mode Shapes of an Eleven-Storey Composite Building
The main purpose of the linear dynamic analysis is to evaluate the time variation of stresses and
deformations in structure caused by the arbitrary dynamic loads. Structure can vibrate in
different mode shapes, as shown for an eleven-storey composite building in Fig. 5. There can be
as many mode shapes possible as number of dynamic degrees of freedom in the building. The
first three mode shapes of the eleven storeys R.C.C. building are depicted in Fig. 6.
Figure 6: Three Mode Shapes of an Eleven-Storey R.C.C. Building
Conclusion
1. The basic assumption in equivalent static method of analysis is that only the first mode of
vibration of building governs the dynamic behavior and therefore the effects of higher
modes are not considered in the analysis. Thus, ESMA cannot adequately capture the true
behavior of multi-storey building; the design forces for the members may be grossly
underestimated. On the other hand, several uncertainties and approximations are involved
in dynamic analysis in describing the true dynamic loads, estimating the actual material
and sectional properties etc. and therefore, dynamic analysis must be used with great
caution. However, compared to equivalent static method of analysis, it is certainly a
better indicator of the structural performance.
2. From Tables 2 and 7, it is clear that the nodal displacements in a composite structure, by
both the methods of seismic analysis, compared to an R.C.C. structure in all the three
global directions are less which is due to the higher stiffness of members in a composite
structure compared to an RCC structure.
3. As the dead weight of a composite structure is less compared to an R.C.C. structure, it is
subjected to less amount of forces induced due to the earthquake. A comparison of Figs.
5 and 6 clearly indicates that the displacement of composite structural system is less
compared to an R.C.C. structural system so the time period required by the composite
structure for one cycle is also less and the frequency of structure is higher.
4. From Tables 3 and 4, it is evident that the downward reaction (FY) and bending in other
two directions for composite structural system is less. Thus, one can use smaller size
foundation in case of a composite construction compared to an R.C.C. construction.
5. In case of a composite structural system, because of the lesser magnitude of the beam end
forces and moments compared to an RCC system (Tables 5, 6, 10 and 11), one can use
lighter sections in a composite structure. Thus, it reduces the self-weight and cost of the
structural components.
References
Johnson R. P., Composite Structures of Steel and Concrete, Volume 1, Blackwell
Scientific Publications, UK, 2004.
AISC 341-05, "Seismic Provisions for Structural Steel Buildings", An American National
Standard, American Institute of Steel Construction, 2005.
Eurocode 4, "General Rules for Buildings, Design of Composite Steel and Concrete
Structures", Committee Members of the Euro- pean Union, EN 1994-1-1, 2004.
IS: 11384, "Code of Practice for Composite Construction in Structural Steel and
Concrete", Bureau of Indian Standards, New Delhi, 1985.
Narayanan R., and Kalyanaraman V., Teaching Resource of Structural Steel Design,
Volume 2, Institute of Steel Development and Growth, Kolkata, 2001.
IS: 800, "General Construction in Steel – Code of Practice", Bureau of Indian Standards,
New Delhi, 2007.
IS: 1893, Part 1, "Criteria for Earthquake Resistant Design of Structures - General
Provisions for Buildings", Bureau of Indian Standards, New Delhi, 2002.
Scientific Correspondence, "A case for use of Dynamic Analysis in Designing for
Earthquake Forces", Current Science, Vol. 91, No. 7, pp. 874-876, Oct. 2006.
Panchal D. R., Pandya S. N., and Patodi S. C., "Modeling and Parametric Study of a
Typical Building using Steel-Concrete Composite Option", Proceedings of A National
Conference on Current Trends of Research and Development in Civil and Environ-
mental Engineering, CRDCE10, SVIT, Vasad, Jan. 2010.
Hand Book on Composite Construction: Multi-Storey Buildings, Institute of Steel
Development and Growth, Kolkata, 2002.