| == 6.7.Linear elastic second-order analysis | |
| == 6.7.1 General | |
| === 6.7.1.1 A linear elastic second-order analysis shall consider the influence of axial loads, presence of cracked regions along the length of the member, and effects of load duration. These considerations are satisfied using the cross-sectional properties defined in 6.7.2. | |
| == R6.7.Linear elastic second-order analysis | |
| == R6.7.1 General | |
| In linear elastic second-order analyses, the deformed geometry of the structure is included in the equations of equilibrium so that PDELTA effects are determined. The structure is assumed to remain elastic, but the effects of cracking and creep are considered by using an effective stiffness EI. In contrast, linear elastic first-order analysis satisfies the equations of equilibrium using the original undeformed geometry of the structure and estimates PDELTA effects by magnifying the column-end sway moments using Eq. ( 6.6.4.6.2 a) or ( 6.6.4.6.2 b). | |
| === R6.7.1.1 The stiffnesses EI used in an analysis for strength design should represent the stiffnesses of the members immediately prior to failure. This is particularly true for a second-order analysis that should predict the lateral deflections at loads approaching ultimate. The EI values should not be based solely on the moment-curvature relationship for the most highly loaded section along the length of each member. Instead, they should correspond to the moment-end rotation relationship for a complete member. | |
| To allow for variability in the actual member properties in the analysis, the member properties used in analysis should be multiplied by a stiffness reduction factor @phiK less than 1. The cross-sectional properties defined in 6.7.2 already include this stiffness reduction factor. The stiffness reduction factor @phiK may be taken as 0.875. Note that the overall stiffness is further reduced considering that the modulus of elasticity of the concrete, Ec, is based on the specified concrete compressive strength, while the sway deflections are a function of the average concrete strength, which is typically higher. | |
| American Concrete Institute – Copyrighted © Material – www.concrete.org | |
| 84 ACI 318-19: BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE | |
| No further reproduction or distribution is permitted. | |
| === 6.7.1.2 Slenderness effects along the length of a column shall be considered. It shall be permitted to calculate these effects using 6.6.4.5. | |
| === 6.7.1.3 The cross-sectional dimensions of each member used in an analysis to calculate slenderness effects shall be within 10 percent of the specified member dimensions in construction documents or the analysis shall be repeated. | |
| === 6.7.1.4 Redistribution of moments calculated by an elastic second-order analysis shall be permitted in accordance with 6.6.5. | |
| === R6.7.1.2 The maximum moment in a compression member may occur between its ends. In computer analysis programs, columns may be subdivided using nodes along their length to evaluate slenderness effects between the ends. If the column is not subdivided along its length, slenderness effects may be evaluated using the nonsway moment magnifier method specified in 6.6.4.5 with member-end moments from the second-order elastic analysis as input. Second-order analysis already accounts for the relative displacement of member ends. | |
| == 6.7.2 Section properties | |
| === 6.7.2.1 Factored load analysis | |
| ==== 6.7.2.1.1 It shall be permitted to use section properties calculated in accordance with 6.6.3.1. | |
| === 6.7.2.2 Service load analysis | |
| ==== 6.7.2.2.1 Immediate and time-dependent deflections due to gravity loads shall be calculated in accordance with 24.2. | |
| ==== 6.7.2.2.2 Alternatively, it shall be permitted to calculate immediate deflections using a moment of inertia of 1.4 times I given in 6.6.3.1, or calculated using a more detailed analysis, but the value shall not exceed Ig. | |
| == R6.7.2 Section properties | |
| === R6.7.2.2 Service load analysis | |
| ==== R6.7.2.2.2 Refer to R6.6.3.2.2. | |
| American Concrete Institute – Copyrighted © Material – www.concrete.org | |
| PART 2: LOADS & ANALYSIS 85 | |
| [ Lanjut Ke 6.8.Inelastic analysis ... ] | |
| | |
| | |