Aci coefficients for continuous beams and slabs

Civil Engineering

Design Project: Structural Analysis Assumptions 6.2.2 All members and structural systems shall be analyzed to determine the maximum load effects including the arrangements of live load in accordance with 6.4. 6.2.3 Methods of analysis permitted by this chapter shall be (a) through (e): (a) The simplified method for analysis of continuous beams and one-way slabs for gravity loads in 6.5 (b) Linear elastic first-order analysis in 6.6 (c) Linear elastic second-order analysis in 6.7 (d) Inelastic analysis in 6.8 (e) Finite element analysis in 6.9 6.3.1.2 To calculate moments and shears caused by gravity loads in columns, beams, and slabs, it shall be permitted to use a model limited to the members in the level being considered and the columns above and below that level. It shall be permitted to assume far ends of columns built integrally with the structure to be fixed. For a factored load analysis, it is permitted to use A = Ag, and I = 0.70Ig for columns, I = 0.35Ig for beams.

6.4.2 For one-way slabs and beams, it shall be permitted to assume (a) and (b): (a) Maximum positive Mu near midspan occurs with factored L on the span and on alternate spans (b) Maximum negative Mu at a support occurs with factored L on adjacent spans only 6.5.1 As an alternative to a structural analysis using requirements of 6.3.1.2 and 6.4.2 load patterns, it shall be permitted to calculate Mu and Vu due to gravity loads in accordance with the ACI shear and moment coefficients for continuous beams and one- way slabs satisfying (a) through (e): (a) Members are prismatic (b) Loads are uniformly distributed (c) L ≤ 3D (d) There are at least two spans (e) The longer of two adjacent spans does not exceed the shorter by more than 20% 6.6.2.3 It shall be permitted to simplify the analysis model by the assumptions of (a), (b), or both: (a) Solid slabs or one-way joist systems built integrally with supports, with clear spans not more than 10 ft, shall be permitted to be analyzed as continuous members on knife-edge supports with spans equal to the clear spans of the member and width of support beams otherwise neglected. (b) For frames or continuous construction, it shall be permitted to assume the intersecting member regions are rigid.

Live Load pattern for maximum positive and negative moments in slab for Risa analysis (otherwise analyze using ACI shear and moment coefficients):

Frame model for analysis of Rectangular Girders:

36 ft typical between columns. Concentrated forces = 2R come from the T Beam reactions and are 12 ft apart. Distributed load is girder self-weight.

2R 2R 2R 2R 2R 2R 2R 2R

20 ft

20 ft

Fixed support at bottom of lower columns (typical)

Fixed support at top of upper columns (typical)

20 ft

Section Properties:

Assume columns are 24” by 24”

Use A = Ag for both beams and columns

Use I = 0.70Ig for columns

Use I = 0.35Ig for girders