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Unformatted text preview: hemical reaction of hydration.
Therefore, only a small portion of the water in a typical concrete mix is consumed in the
chemical reaction and most of the water needs to evaporate from the hardened
concrete. When the excess mix water evaporates from a particular concrete member, it
loses volume and therefore tends to shrink. Reinforcing steel and surrounding
construction can minimize concrete shrinkage to some extent, but nonetheless
shrinkage stresses are developed. Factors that influence concrete shrinkage include
the volume to surface ratio of the member, the timing of the application of prestressing
force after concrete curing, and the relative humidity surrounding the member. www.SunCam.com Copyright 2010 John P. Miller Page 29 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course Tendon Relaxation As a prestressed concrete member shortens, the tendons shorten by the same
amount, thus relaxing some of their tension. The concrete member shortens due to the
above three sources – elastic strain, creep, and shrinkage. Thus, the total tendon
relaxation is a summation of these three concrete shortening sources. There may also
be some relaxation in the prestressing steel over time, similar to concrete creep. This
steel relaxation is a function of the type of steel used and on the ratio of actual steel
stress to the specified steel stress.
As mentioned earlier, most design offices only show the required effective prestress
force and the location of the center of gravity of the tendons on the construction
documents. Remember that the effective prestress force is the force in the tendons
after all losses have been accounted for. Calculating prestress losses for the design
office can be very tedious and may not be exact, and therefore it is customary for the
tendon supplier to calculate all prestress losses based on their experience and the
specific tendon layout. Then, the number of tendons are determined to satisfy the given
effective prestress force.
ACI 31808 Requirements
We will now review some of the requirements contained in the 2008 edition of the
Building Code Requirements for Structural Concrete ACI 318. We will be focusing on
Chapter 18, Prestressed Concrete. The requirements in Chapter 18 have changed
very little over the last several editions of the ACI 318.
ACI 318 places limits on the allowable extreme fiber tension stress at service loads
according to the classification of a structure. Class U members are assumed to behave
as uncracked sections and therefore gross section properties may be used in service
load analysis and deflection calculations. Class C members are assumed to be cracked
and therefore cracked section properties must be used in service load analysis, and
deflection calculations must be based on an effective moment of inertia or on a bilinear
momentdeflection relationship. Class T members are assumed to be in a transition
state between cracked and uncracked, and the Code specifies that gross section
properties may be used for analysis at service loads, but deflection calculations must be
based on an effective moment of inertia or on a bilinear momentdeflection relationship.
Prestressed twoway slabs must be designed as Class U and the extreme fiber tension
stress must not exceed 6
www.SunCam.com .
Copyright 2010 John P. Miller Page 30 of 49 Fundamentals of Post‐Tensioned Concrete Design for Buildings – Part One A SunCam online continuing education course The allowable extreme fiber tension stresses in flexural members at service loads are
as follows: : 7.5 : 7.5 12 : 12 ACI 318 stipulates two cases of serviceability checks. The first is a check of the
concrete tension and compression stresses immediately after transfer of prestress.
The concrete stresses at this stage are caused by the prestress force after all short
term losses, not including long term losses such as concrete creep and shrinkage, and
due to the dead load of the member. These limits are placed on the design to ensure
that no significant cracks occur at the very beginning of the life of the structure. The
initial concrete compressive strength, , is used in this case. is normally taken as
75% of the specified 28day concrete compressive strength, but can be any specified
minimum as long at it is greater than 3000 psi. The maximum permissible concrete
stresses at force transfer are as follows:
Extreme fiber stress in compression at force transfer: : : 0.70 0.60 Extreme fiber stress in tension at force transfer: : : 3 6 If the above stresses are exceeded at force transfer, then additional bonded
reinforcement shall be provided in the tensile zone to resist the total tensile force.
The second serviceability check is a check of the concrete tension and compression
stresses at sustained service loads (sustained live load, dead load, superimposed dead
load, and prestress) and a check at total service loads (live load, dead load,
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