6.6 First-Order Analysis
(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.
|Member and condition||Moment of Inertia||Cross-sectional area|
|Flat plates and flat slabs||0.25Ig|
|Member||Alternative value of I for elastic analysis|
|Columns and walls||0.35Ig||0.875Ig|
|Beams, flat plates, and flat slabs||0.25Ig||0.5Ig|
Notes: For continuous flexural members, I shall be permitted to be taken as the average of values obtained for the critical positive and negative moment sections. Pu and Mu shall be calculated from the load combination under consideration, or the combination of Pu and Mu that produces the least value of I.
where ∑Pu and Vus are the total factored vertical load and horizontal story shear, respectively, in the story being evaluated, and Δo is the first-order relative lateral deflection between the top and the bottom of that story due to Vus.
where βdns shall be the ratio of maximum factored sustained axial load to maximum factored axial load associated with the same load combination and I in Eq. (220.127.116.11.4c) is calculated according to Table 18.104.22.168.1(b) for columns and walls.
where M1/M2 is negative if the column is bent in single curvature, and positive if bent in double curvature. M1 corresponds to the end moment with the lesser absolute value.
|Cm = 1.0||(22.214.171.124.3b)|
|(a) M1 = M1ns+ δsM1s||(126.96.36.199.1a)|
|(b) M2 = M2ns+ δsM2s||(188.8.131.52.1b)|
|(c) Second-order elastic analysis|
where ∑Pu is the summation of all the factored vertical loads in a story and ∑Pc is the summation for all sway-resisting columns in a story. Pc is calculated using Eq. (184.108.40.206.2) with k determined for sway members from 220.127.116.11.3 and (EI)eff from 18.104.22.168.4 or 22.214.171.124.5 as appropriate with βds substituted for βdns.
(a) Flexural members are continuous
(b) εt ≥ 0.0075 at the section at which moment is reduced