Serviceability assessments are carried out for stress limitation, crack width, and deflection limits. Stresses are checked in concrete and reinforcement elements according to AASHTO LRFD in a similar manner to that specified for the Strength.
Stress limitation
The concrete stress in compression is evaluated only for prestressed members (when the Prestressing load case is present in the model) as the ratio between the maximum principal compressive stress fc = σc2 obtained from FE analysis for serviceability and the limit values, which are set based on AASHTO LRFD Table 5.9.2.3.2a-1.
\[ \textsf{\textit{\footnotesize{Fig. 54\qquad Concrete compressive stress limits at service loads}}}\]
In the application, Prestress plus permanent load is treated as a Sustain load, and Prestress, permanent, and transient load as a Total load.
\[ \textsf{\textit{\footnotesize{Fig. 54\qquad Serviceability combination types}}}\]
In addition, it is always possible to perform an analysis for both short-term and long-term effects, using material models that either account for or do not account for the creep factor—see the "Material models (AASHTO)" section.
\[ \textsf{\textit{\footnotesize{Fig. 54\qquad Serviceability material models}}}\]
Deflection
Based on the selected combination type (long-term or short-term), either long-term or short-term deflection is evaluated. The maximum allowable deflection value shall be determined by the user and shall be considered in accordance with ACI 138-19 24.2.
\[ \textsf{\textit{\footnotesize{Fig. 55\qquad Maximum allowable deflection value}}}\]
In the application, it is possible to display the deflections from dead load ΔDL and live load ΔLL separately as well as the total deflection ΔTot (deal+live), all while displaying the deformed shape.
Deflections at trimmed ends cannot be checked.
Crack width
Crack widths and crack orientations are calculated for serviceability short-term or long-term combinations. Since ACI does not directly prescribe limiting crack widths, the user must specify a limiting crack width wlim.
The verifications are presented as follows:
\[\frac{w}{w_{lim}}\]
where:
w short- or long-term crack width calculated by FE analysis,
wlim limit value of the crack width defined by the user.
The method of calculating crack widths used in the application, also described in more detail in this document, is in accordance with ACI 224R-01. It is, therefore, possible to use ACI 224R-01 Table 4.1 to determine the limiting value of crack widths.
\[ \textsf{\textit{\footnotesize{Fig. 56\qquad Reasonable crack widths for reinforced concrete under service load}}}\]
There are two ways of computing crack widths (stabilized and non-stabilized cracking). In the general case (stabilized cracking), the crack width is calculated by integrating the strains on 1D elements of reinforcing bars. The crack direction is then calculated from the three closest (from the center of the given 1D finite element of reinforcement) integration points of 2D concrete elements. While this approach to calculating the crack directions does not correspond to the real position of the cracks, it still provides representative values that lead to crack width results that can be compared to code-required crack width values at the position of the reinforcing bar.