Design principles of post-tensioned slabs

frontdotardUrban and Civil

Nov 15, 2013 (3 years and 7 months ago)

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Design principles of post
-
tensioned slabs


In this paragraph we will have a brief view of design related issues:


a) Steps of design



Given geometry & loading

Determine actions by structural analysis (moments & shears)

• Design of PT reinforcement & ordinary reinforcement

• Detailing


b) Structural system model for slabs


• One
-
way system:

-

Carry the load primarily in one direction

-

Treated as one dimensional beam or plane frame

-

One
-
way slabs are viewed as a beam strip

• Two
-
way system:

-

Have the ability to sustain the applied loads in two directions


c) Common geometries for post
-
tensioned slabs : Slab, Beam & Slab, Waffle Slab



d) Methods of floor slab analysis and desig
n


The foundation of an effective floor system design is a clear understanding of
the concepts of plate theory and behavior. In particular, when dealing with post
-
tensioned floors, an awareness of the relative strengths and weaknesses
of the
current analysis techniques is essential.


There are several methods of design, the most common is the equivalent

frame method, but with the adequate development of computer software the
finite element method is placing itself as the latest and ve
ry efficient design
method with real time understanding of the slab behavior.


1. Equivalent Frame Method


The EFM involves modeling the three
-
dimensional slab system as a two
-

dimensional frames which are independently analyzed for loads assumed to
act in the plane of each frame.


Computer view of

a Design Strip






2. Finite Element Method

In the finite element method (FEM) the plate is subdivided into a number of

small parts, referred to as elements. The elements are connected at reference

points called nodes. All forces between the elements are transferred through

the nodes.


Typical example of Finite Element post
-
tensioning design




Structure perspective















Element mesh plan














Tendons part plan

















Service deflection plan







One of the main advantages of the FEM can be applied
to structures having irregular geometries, support
conditions, and loadings with the same ease a it
can be
applied to regular structures.


The FEM is seen by many engineers as the wave of the
future

in
floor system

design.



Post
-
tensioning benefits

(
* These benefits are general, but are mo
re specific in suspended slabs)



Long Span


Post
-
tensioning permits longer spans than conventional
reinforced concrete. Large column
-
free floor area
provide

Increased flexibility for internal planning
and
space utilization.



Flat Soffit

Two
-
way post tensioned flat slabs show no down
strands, providing best
flexibility for space
partitioning and for the placing of services network
(HVAC, electrical, sanitary, etc…).



Flexibility of Layout

The design of post
-
tensioned slab can cope with
irregular grids and tendons can easily be deflected
horizontally to suit any layout’s geometry or to allow
for openings in slabs.



Thinner Slab


The additional stiffness achieved by Prestressing
enables reduced slab

thickness. Maximum ceiling zone is available for
horizontal services.



Flexibility of Services

The simple concept of post tensioned slabs, the
reduced amount of ordinary

reinforcement and the flexibility in the positioning of
openings and inserts for

services.

Little coordination is required between designers for
this purpose, and late modificati
ons can be
implemented without prejudice to the initial structural
and architectural design.



Deflection Control

By varying the amount of post
-
tensioning, the
designer is able to control
deflection under service
loads.






Future Flexibility


The
myth about the possible demolition of post
-
tensioned slabs are now dispelled. Knockout zones
can be easily identified for future service

penetrations. Tried methods are available to enable
large openings to be
formed subsequently in future
alterations.




Reduced Storey Height


Due to the minimal need for deep down strands and the reduced slab

thickness, minimum storey height is achieved;
leading therefore to a lower
overall building height.

In situations where the height of the building is
limited, the reduced storey
height allows additional
storeys to be constructed within the building

envelope. Alternatively, this can reduced the façade
area, as well as the
vertical runs of mechanical and
electrical systems.



Crack Control

Post
-
tensioned floors in compression allow for easier
control of cracking. If

appropriate design criteria is applied, crack
-
free
construction can be

achieved. This is often exploited in car parks with
concrete surfa
ces exposed

to an aggressive environment. With reduced cracking,
better water

tightness is achieved and the durability of the structure
is improved.



Lighter Structure

Post
-
tensioning enhances the efficiency of concrete
sections and hence, saves cons
iderably on the concrete
volume. Slender slabs produce lighter overall
structures
which reduce on foundation size and cost.



Fast Construction


Easy and simple shuttering, reduced reinforcement.
easy

casting and early stripping and redeployment of
the formwork obviously result in very fast

construction.



Cost Saving


All the above advantages imply significant savings in
the overall cost of the
construction.