2. Number of cables

 

As experience for internal prestressing cables, a compressive stress of 4-5MPa is needed over the entire concrete

cross section Area incl. the two wings for bridge span less than 20m. By expression ncab´Pmax/Area @ 4-5MPa,

one can find the number of cables ncab, where the prestressed force Pmax is calculated in example below.

 

The case the maximum compression needed, we need to find the minimum distance between the cables anchor as

example below with tendon 13C15. Here the area (0.760*0.300) is the associated area to the anchorage (0.250*0.210)

of tendon 13C15. We get a compressive stress of 11.9MPa which is less the 60% of the concrete f_ck=20MPa at the time

of tensioning, cf. /3/ Eurocode 2 for Concrete bridges (J.101).

Figure: Example of Freyssinet cable 13C15 in minimum distance in 760mm depth deck.

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cf. /3/ Eurocode 2 Design of concrete structure - Concrete bridges BS EN 1992-2:2005, p. 61 

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cf. /2/ Eurocode 2 Design of concrete structures DS/EN 1992-1-1+AC:2008, p. 76

Remarks for Lusas model:

›  Refer /1/ for the 5-Steps input in Lusas > Bridge > Prestress Wizard > Multiple Tendon…

›  Graphical check (X, Y, Z) for location of cables in bridge deck. Check (X, Y) of the cable fictive beam on

   the mesh is also the (X, Y) of the Tendon profile entered in Multiple Tendon step 2. Note that all points

   distribute along the tendon curve in step 2 do not need to coincide with the points created the cable fictive

   beams on the mesh. The vertical distance dzBot from bottom of deck to center of cable can be checked as

dz_Bot = Z_Cab - Z_Mesh  ,   where

        Z_Mesh = Z-Coordinate at bottom of deck at a selected point below the cable in Lusas

        Z_Cab  = Z-Coordinate of the cable entered in Step2 Tendon Profile at that point

The vertical distance dzTop from top of deck to center of cable can be checked as

dz_Top = Thick - dz_{Bot    ,}   where 

       Thick = thickness of deck box at the edge defined by the cable fictive beam on the mesh.

Remarks for Revit model:

›  Positions of the cables are relative to the deck when loaded the cable family into the bridge project.

   The absolute coordinate for the cable obtained automatically when assign the true coordinate and the

   true angle to the north at Project Base Point in Floor Plans Site view.

›  The cable setting-out can both be in relative to the deck, or as absolute coordinate (X,Y,Z).

Minimum radius of tendon curvature and tangent length for internal tendons

cf. /11/ VSL Post Tensioning Solutions, p. 25

Prestressing tendon Modulus of Elasticity & Density

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cf. /2/ Eurocode 2 Design of concrete structures DS/EN 1992-1-1+AC:2008, p. 44