lectures-08 concrete structural systems

Lecture-7
.
Topics Addressed
Selection of Concrete Structural System
Liaison between Engineer and Architect
Prof. Dr. Qaisar AliProf. Dr. Qaisar Ali
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
Department of Civil Engineering, N-W.F.P. University of Engineering and Technology Peshawar Department of Civil Engineering, N-W.F.P. University of Engineering and Technology Peshawar
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
,

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.

Versatility of form: concrete poured in fluid state can adopt
any shape.
reinforcement, a reinforced concrete structure provides the
.
completed in less time than a steel structure.

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.
.

Repeat sizes and shapes of concrete members
Strive for simple formwork

used in construction.
steel change significantly at high temperatures.
Strength, modulus of elasticity are reduced, the
coefficient of thermal expansion increases, and creep
and stress relaxations are considerably high.
900 oF for some concretes and 1200 oF for others.

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,
1225 oF at ¼ inch from exposed surface
950 oF at ¾ inch
800 oF at 1 inch
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.

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″
concrete).
The same may be applied to beams as well.
8″ 10″ 12″ 14″
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.
Fire resistance rating
3/4″ 1″ 1 ¼ ″

Beam width,
. . .
≥ 10 3/4″ 3/4″ 1 ″ 1 ¾ ″
Fire resistance rating
″ ″ ″ ″

Types of Concrete
Department of Civil Engineering, N-W.F.P. University of Engineering and Technology Peshawar
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
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
.

Flat Plate
DDM or any other FEA software.
Design:
Flexural reinforcement: Since there are no beams, the amount of
Shear reinforcing: Flat plates or strengthened against punching shear
using shear head, integral beam with stirrups etc. .
Flat Plate
Minimum thickness = ln/30

Flat Plate
Design consideration
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

Flat Slab
Flat Slab
Widely used for storage warehouses, parking garages, and
below grade structures carrying heavy earth-fill loads.
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
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
but less than flat plates
Shear reinforcing: Capital and/or drop panel resolves the problem of
punching shear up to a great extent
Flat Slab
Minimum thickness = ln/33

Flat Slab
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
Prof. Dr. Qaisar AliProf. Dr. Qaisar Ali Rib

One-Way Joist
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.
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
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.

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.
If the system does not fulfill the requirements of joist system then it
shall be designed as regular slab beam system
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
all of the shear strength, with no stirrups used.

Desi n considerations
Minimum thickness = l/18.5
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).
Two-Way Joist
both directions and a reinforced concrete slab cast
integrally with the joists.

Two-Way Joist
Heavy loads
Two-Way Joist
Electrical fixtures can be placed in the voids

Minimum Slab Thickness

Design Consideration
Two-Way Joist
Minimum Joist Depth
slabs

Minimum Joist Depth
Characteristics
composite action and for improved fire resistance.

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
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

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
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.

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
The End

One Way Slabs
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.
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.
Prof. Dr. Qaisar AliProf. Dr. Qaisar Ali 78

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.
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.
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
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:
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

lectures-08 concrete structural systems

Documents