This article describes notional size scaling when inputting user data for creep and shrinkage

Notional size

Notional size scaling is used to specify how the user-defined curve will be modified by the notional size of a member. Each section property (frame, shell) has a notional size that is calculated automatically or specified by the user. Notional size h is essentially the twice the ratio of volume to the surface area around the perimeter of a frame or shell section. This works out, for example, to be equal to the thickness of a shell if you ignore the edges. For a circular frame section, the notional size is the radius.

For larger notional size, moisture takes longer to migrate from the interior of the member to the surface where it can evaporate, so the rate of creep and shrinkage is slower in that case.

Accounting for notional size in CEB and ACI codes

Each code has its own way of relating notional size to creep and shrinkage. CEB is very complicated. The ACI code is relatively simple, and uses the following formulas:

• Creep: C = C0 * 2⁄3 [1+1.13 exp(-0.54 h⁄2)] for notional size h in inches, where C is the creep coefficient considering notional size, and C0 is the specified creep coefficient taking into account all other factors.
• Shrinkage: S = S0 * 1.2 exp(-0.12 h⁄2) for notional size h in inches, where S is the shrinkage strain considering notional size, and S0 is the specified shrinkage strain taking into account all other factors.

Note that the ACI formulas specify v/s = volume/surface = h/2,

So for user-defined creep curves that are going to use the ACI formula for notional-size effect, specify a = 0.67,  b = 0.75, and h0 = 3.70 in = 94 mm.

So for user-defined shrinkage curves that are going to use the ACI formula for notional-size effect, specify a = 0,  b = 1.2, and h0 = 16.7 in = 423 mm.