Rules for Average Prestress

Prestressed concrete is commonly used in many building and civil structure projects for its improved concrete performance. Average prestress is defined as P/A, where P is the final effective prestress force after losses and A is the cross sectional area of the member (or design strip for two-way slabs). It is important to maintain P/A within certain defined limits.  Some limits are code imposed and some are based on design experience. Minimum P/A for two-way slabs is 125 psi (0/875 MPA) as mandated by ACI 318-14. Maximum P/A is not code mandated but seen as a function of what is achievable from a practical standpoint.

 

In my opinion, the biggest challenge with too much P/A is increased shortening of the slab, which can lead to other challenges such as cracking due to restraint to shortening (RTS). Too much prestress can also lead to congestion and challenges fitting anchorages and anchorage zone reinforcement in the member in addition to increasing long term prestress losses. I have developed the set of guidelines for P/A listed in Table 1.

 

Table 1: Pragmatic Guidelines for Average Prestress As a Function of Structure Type

Structure type Minimum P/A, psi Maximum P/A, psi
Parking garage slab
(Zone II and III)
175
300
Parking garage slab
(Zone I)
125
300
Parking garage beam
(Zone II and III)†
200
500
Parking garage beam
(Zone I)†
125
500
Building slab
125
300
Building beam†
125
500
Building transfer girder†
200
700

Note: 1 psi = 0.007 MPa

 

†When calculating P/A for beams and girders, the cross-sectional area is based on the effective flange width chosen by the designer.

 

The maximum P/A values in Table 1 should be considered general guidelines for typical building structures that will provide reasonable results. They are not meant as absolute maximum values, as there will be occasions when it is necessary and desirable to go above these values. If you have any questions as you work with average prestressed materials, consult an expert!

 

Reference for definitions of zones I, II, and III:

ACI Committee 362, “Guide for the Design and Construction of Durable Concrete Parking Structures (ACI 362.1R-12),” American Concrete Institute, Farmington Hills, MI, 2012, pp. 13-14.