1 Department of Civil Engineering, N-W.F.P . University of Engineering and Technology PeshawarDepartment of Civil Engineering, N-W.F.P . University of Engineering and Technology PeshawarLecture-7 Concrete Structural Systems Prof. Dr . Qaisar Ali Prof. Dr . Qaisar Ali . Civil Engineering Department NWFP UET Peshawar[email protected]Department of Civil Engineering, N-W.F.P . University of Engineering and Technology PeshawarDepartment of Civil Engineering, N-W.F.P . University of Engineering and Technology PeshawarTopics Addressed Concrete Structural System Introduction General Requirements for Concrete Structural Systems Selection of C oncrete Structural System Liaison between Engineer and A rchitect Prof. Dr . Qaisar Ali Prof. Dr . Qaisar Ali Advantage of Concret e as Construction Material Three major Costs in C oncrete Construction Concrete Fire R esistance
Lecture-7
.
Topics Addressed
Selection of Concrete Structural System
Liaison between Engineer and Architect
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Advantage of Concrete as Construction Material
Three major Costs in Concrete Construction
Concrete Fire Resistance
Topics Addressed
Flat Plate
Flat Slab
One-Way Joist
Two-Way Joist
Banded Beam System
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Concrete Structural
of building, availability of formwork, skill labor and
material
etc., a number of different structural systems such as flat
plate, flat slab, one-way or two way joist system etc. are
possible.
General Requirements
An important part of the total responsibility of the
structural
engineer is to select, from many alternatives, the best
structural system for the given conditions.
The wise choice of structural system is far more important,
in
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General Requirements
Close cooperation with the architect in the early stages of
a
project is essential in developing a structure that not only
meets the functional and aesthetic requirements but exploits
to the fullest the special advantages of reinforced concrete.
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General Requirements
Versatility of form: concrete poured in fluid state can
adopt
any shape.
reinforcement, a reinforced concrete structure provides the
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.
completed in less time than a steel structure.
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General Requirements
Cost: In many cases, the first cost of a structure is less
than
that of a comparable steel structure. In almost every case,
maintenance costs are less.
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.
General Requirements
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General Requirements
Repeat sizes and shapes of concrete members
Strive for simple formwork
General Requirements
used in construction.
steel change significantly at high temperatures.
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Strength, modulus of elasticity are reduced, the
coefficient of thermal expansion increases, and creep
and stress relaxations are considerably high.
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General Requirements
900 oF for some concretes and 1200 oF for others.
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Concrete Fire Resistance
The temperatures stated (in previous slide) are the
internal
temperatures of the concrete and are not to be confused with
the
heat intensity of the exposing fire.
For example, in testing a solid carbonate aggregate slab,
the ASTM
standard fire exposure (ASTM E 119) after 1 hour will be 1700
oF,
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1225 oF at ¼ inch from exposed surface
950 oF at ¾ inch
800 oF at 1 inch
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General Requirements
dealing with the structural behavior of the buildings under
fire
as total multidimensional systems, building codes continue to
specify minimum acceptable levels of fire endurance on a
component by component basis.
General Requirements
Minimum thickness for floors and roof slabs and cast in
place walls (load bearing and non load bearing using normal
weight concrete):
3.5″ 4.5″ 6.0″ 7.0″
General Requirements
concrete).
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The same may be applied to beams as well.
8″ 10″ 12″ 14″
General Requirements
temperature increase from fire exposure.
Protection for reinforcement in concrete is mainly provided
by
concrete cover.
concrete will generally equal or exceed the minimum
cover
requirements.
General Requirements
Fire resistance rating
3/4″ 1″ 1 ¼ ″
General Requirements
Beam width,
. . .
≥ 10 3/4″ 3/4″ 1 ″ 1 ¾ ″
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General Requirements
Fire resistance rating
″ ″ ″ ″
Types of Concrete
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Flat Plate
A flat plate is a slab floor system in which the slab of
uniform
thickness is supported directly on columns.
Prof. Dr. Qaisar Ali 22
Flat Plate
Short and medium spans
and
o era e ve oa s
Punching shear is a typical
problem in flat plates.
Flat Plate
moderate and loads relatively light.
Renders low story heights.
Architectural finish can be applied directly to underside
of
slab.
Flat Plate
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Flat Plate
Shear stresses near the columns may be very high,
requiring
the use of special forms of slab reinforcement there.
At exterior columns, where shear and moment transfer
may
cause particular difficulty, the design is much improved by
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.
Flat Plate
DDM or any other FEA software.
Design:
Flexural reinforcement: Since there are no beams, the amount
of
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Shear reinforcing: Flat plates or strengthened against
punching shear
using shear head, integral beam with stirrups etc. .
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Flat Plate
Minimum thickness = ln/30
Flat Plate
Design consideration
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Introduction:
Beamless systems with drop panels or column capitals or both
are termed as flat slab systems.
Drop Panel: Thick part of slab in the vicinity of
columns
Column Capital: Column head of increased size
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Flat Slab
Prof. Dr. Qaisar Ali 31
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Flat Slab
Widely used for storage warehouses, parking garages,
and
below grade structures carrying heavy earth-fill loads.
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Drop panel and column capital increases the shear
strength
of the flat slabs in the critical region around the column
and
provide increased effective depth in the region of high
negative bending moment over the support .
Characteristics
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Characteristics
Design considerations
Flat Slab
DDM or any other FEA software.
Design:
Flexural reinforcement: Since there are no beams, the amount
of
flexural reinforcement is much higher than slabs supported on
beams
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but less than flat plates
Shear reinforcing: Capital and/or drop panel resolves the
problem of
punching shear up to a great extent
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Design considerations
Flat Slab
Minimum thickness = ln/33
Design considerations
Flat Slab
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Introduction
Joist construction consists of a monolithic combination of
regularly
spaced ribs and a top slab arranged to span in one direction or
two
orthogonal directions.
T-beam called joist are formed by creating void spaces in
what otherwise
would be a solid slab
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One-Way Joist
Prof. Dr. Qaisar Ali 39
Wide-Module
Introduction
One-Way Joist
A structural system will be called as joist system if the
pan
width (clear spacing between ribs) is less than or equal to
30
inches (ACI 8.11.3).
When the pan width exceeds 30″, the system is designed
as
regular slab beam system.
Characteristics
Electrical/mechanical equipment can be placed between
joists.
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ess e ec o v ra ons ue o s er s a .
Standard forms for the void spaces between ribs are
either
20 or 30 inches wide, and 8, 10, 12, 16, or 20 inches deep
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Characteristics
Economical for buildings such as apartment houses,
hotels,
and hospitals, where the live loads are fairly small and the
spans comparatively long.
Forms are tapered in cross section generally at a slope of
1
to 12 to facilitate removal.
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Design considerations
One-Way Joist
and beams or FEA software
Design: Slabs are only provided with nominal
shrinkage
reinforcement in both directions. Beams are designed in the
usual manner.
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If the system does not fulfill the requirements of joist
system then it
shall be designed as regular slab beam system
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Design considerations
One-Way Joist
Reinforcement placement
Reinforcement for the joists usually consists of two bars in
the positive
bending region, with one bar discontinued where no longer needed
or
bend up to provide a part of negative steel requirement over
the
supporting girder.
One way joists are generally proportioned with the concrete
providing
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all of the shear strength, with no stirrups used.
Desi n considerations
Minimum thickness = l/18.5
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Design considerations
One-Way Joist
Based on 4-1/2 in. slab
Desi n considerations
Controlled by either structural or fire-resistance
requirements
Slab thickness > one-twelfth the clear distance between
ribs or 2 in
(ACI 8.11.6).
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Two-Way Joist
both directions and a reinforced concrete slab cast
integrally with the joists.
Two-Way Joist
Prof. Dr. Qaisar Ali 49
Heavy loads
Two-Way Joist
Electrical fixtures can be placed in the voids
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Prof. Dr. Qaisar Ali 51
Department of Civil Engineering, N-W.F.P. University of Engineering
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Minimum Slab Thickness
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Design Consideration
Two-Way Joist
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Minimum Joist Depth
slabs
Minimum Joist Depth
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Characteristics
composite action and for improved fire resistance.
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Beam-Supported Slab
Long spans
Parking structures
eavy oa s on spans up o
about 30 ft.
Beam-Supported Slab
Banded-Beam System
which the beams are omitted in one direction, the one-
way slab being carried directly by column line beams
that are very broad and shallow.
These beams, supported directly by the columns,
become little more that a thickened portion of the slab.
This type of construction is known as banded slab
construction.
Banded-Beam System
Banded-Beam System
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Banded-Beam System
Characteristics
Flexibility in locating columns
Formwork is simplified because of the reduction in number
of
framing members.
Composite Construction
buildings for which the columns, beams, and girders consist
of
structural steel.
Characteristics
The slab is normally designed for full continuity over
the
supporting beams, and the usual methods followed. The
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Characteristics
To provide composite action, shear connectors are
welded
to the top of the steel beam and are embedded in the
concrete slab.
Characteristics
By preventing the longitudinal slip between the slab and
the
steel beam in the direction of the beam axis, the combined
member is both stronger and stiffer than if composite action
were not developed.
,
Characteristics
Composite floors may also use encased beams, offering
the
advantage of full fire proofing of the steel, but at the cost
of
more complicated formwork and possible difficulty in placing
the concrete around and under the steel member.
as a rule.
Steel Deck Reinforced
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Characteristics
The steel deck serves as a stay-in-place form and with
suitable detailing the slab becomes composite with the steel
deck, serving as the main tensile flexural steel.
Suitable for relatively light floor loading and short
spans,
and beams in the perpendicular direction subdividing panels
into spans up to about 12 ft.
Characteristics
Concrete Slab
Temporary shoring may be used at the mid span or third
point of the panels to avoid excessive stresses and
deflections while the concrete is placed, when the steel deck
panel alone must carry the load.
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Concrete Floor Systems as a
function of Span Length and Load
50
100
20 25 30 35 40 45 50
Square bay size (ft)
Relative costs of reinforced concrete
floor systems for 50 psf live load
Prof. Dr. Qaisar Ali 73
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The End
One Way Slabs
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Topics Addressed
Sizes
Loads
Analysis
Design
Behavior
One Way Slab
A slab when loaded in flexure may bend in one or both
directions
A slab having bending in one direction is called
one-way
slab.
A slab having bending in both directions is called
two-way
slab.
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Strip Method of Analysis
One Way Slab
For purposes of analysis and design, a unit strip of one way
slab,
cut out at right angles to the supporting beams, may be
considered as a rectangular beam of unit width, with a depth
h
and a span la as shown.
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Applicability of Strip Method
The strip method of analysis and design of slabs having
bending in one direction is applicable only when:
Slab is supported on only two sides on stiff beams or
walls,
Slab is supported on all sides on stiff beams or walls
with
ratio of larger to smaller side greater than 2.
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o e: o app ca e o a p a es e c., even en ng s pr mar y n
one
direction.
79
Basic Steps for Structural Design
Ste No. 01: Sizes:-Sizes of all structural and non
structural
One Way Slab
elements are decided.
based on occupational characteristics and functionality
(refer
Appendix C of class notes)
structural elements
the respective load effects following code provisions.
80
Sizes:
ACI table 9.5 a ives the minimum one wa slab
One Way Slab
l = Span length
Sizes (Definition of Span Length, l )
Slab
c c stance c c stance
2) l = Minimum of [(ln +h) or c/c distance] ; --- for non-integral
supports (walls)
& for integral supports (beams and columns) with ln >
10′
1) l = ln ; --- for integral supports (beams and columns) with
ln ≤ 10′
82
Loads:
According to ACI 8.2.2 — Service loads shall be in
accordance with the general building code of which this
code forms a part, with such live load reductions as are
permitted in the general building code.
BCP SP-2007 is General Building Code of Pakistan and it
refers
to ASCE 7 for minimum design loads for buildings and
other
structures.
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ne way s a s are usua y es gne or grav y oa ng
(U = 1.2D + 1.6L).
Visit ASCE 7
Analysis:
Chapter 8 of the ACI addresses provisions for the
analysis
and design of concrete members.
According to ACI 8.3.3, as an alternate to frame
analysis,
ACI approximate moments shall be permitted for design
of
one-way slabs with certain restrictions, which are as
follows.
84
Analysis: ACI 8.3.3
One Way Slab
Prismatic
. n n
wu
Positive
Moment
x
Moment
* 1/12 (for all spans w ith l n < 10 ft)
Column
support 1/16
Note: For simply supported slab, M = wul 2/8, where l = span
length (ACI 8.7).
86
Design:
(φ) × Nominal Strength
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Bar spacing (in inches) = Ab/As × 12
(Ab = area of bar in in2, As = Design steel in
in2/ft)
87
Design:
Reinforcement is placed perpendicular to main steel to
control
shrinkage and temperature cracking.
Design:
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Design:
One Way Slab
Shrinkage Reinforcement
Minimum reinforcement Requirement for main
reinforcement
Same as shrinkage reinforcement requirement (ACI
7.12.2.1)
One Way Slab
Long term Deflection
One Way Slab
s = (540/f s) – 2.5cc
OR 12(36/f s)
Crack width Calculation:
w = 0.076βf s(dc A)1/3
92
Live Load Effects
Two-Way Joist
An increase in LL does not have a proportional impact
on
cost
LL of 100 psf increases the total cost ≅ 4% over
the cost for a
LL of 50 psf