Effects of global imperfections in plane of wall A

Figure 6.7 Global imperfections

* For medium rise shear walls there are a number of methods of design. Cl. 9.6.1 suggests strut-and-

tie (see Section xx). Another method [ref to Concrete Buildings Design manual] is to determine elastic tensile and compression stresses from NEJbi +/- 6MEd/bi2 and determine reinforcement requirements based on those maxima. The method used here assumes a couple, consisting of 1.0 m of wall either end of the wall. The reinforcement in tension is assumed to act at the centre of one end and the concrete in compression (with a rectangular stress distribution) acts at the centre of the other end. The forces generated by the couple add or subtract from the axial load in the 1 m ends of the walls. The method is useful for typical straight shear walls of say 2.5 to 5.0 m in length.

Global imperfections can be represented by forces H at floor level where

= 0.67 < 2 / 14.705 < 1.0 = 0.67 < 0.52 < 1.0 = 0.67 am = [0.5(1 + 1 / m)]05 where m = no. of members contributing to the total effect = 25 vertical elements on 4 floors = 100 am = 0.71

= axial forces in members below and above = axial load from each level

At 3rd floor

At 2nd floor

At 1st floor

Area

= 30.4 x 14.5 - 1.3 x 2.5 - 3.6 x 4.8 = 2 x (30.4 + 14.5) = axial load from roof level =4203 x (705+0.6) +898 x 09 x 4.0

HiR = 0.0024 x (3286.4 + 252.2) = 7.9 + 0.6 = 8.5 kN Hi3 = Hi2 = Hi1 = 0.0024 x (3615.7 + 1050.8) = 8.7 + 2.5 = 11.2 kN Characteristic design moment at ground floor

Mk = 8.5 x 13.2 + 11.2 x (9.90 + 6.60 + 3.30) = 112.2 + 221.8 = 334.0 kNm As before, wall A resists 51% of this moment. Resolving into couple using 1 m either end of wall,

ยง As H derives mainly from permanent actions its resulting effects are considered as being a permanent action too.

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