Post-Tensioned Concrete Fundamentals

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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 pre-stressing 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 pre-stressed 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 pre-stressing 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 pre-stress force and the location of the center of gravity of the tendons on the construction documents. Remember that the effective pre-stress force is the force in the tendons after all losses have been accounted for. Calculating pre-stress 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 pre-stress losses based on their experience and the specific tendon layout. Then, the number of tendons are determined to satisfy the given effective pre-stress force. ACI 318-08 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, Pre-stressed 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 moment-deflection 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 moment-deflection relationship. Pre-stressed two-way 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 pre-stress. The concrete stresses at this stage are caused by the pre-stress 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 28-day 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 pre-stress) and a check at total service loads (live load, dead load, www.SunCam.com Copyright 2010 John P. Miller Page 31 of 49...
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This document was uploaded on 01/28/2014.

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