•For•Determine moment in the x-direction edge strips →They are half middle strip values45Dr. Girma Z. & Dr. Adil Z.366.00.2=⇒=αxfxsmmChapter 2-Strip Method for Slabs•Determine moments in the y-direction middle strip•It is reasonable to choose the same ratio b/n support and span moments in the y-direction as in the x-direction.•To achieve this, choosethe distance from the right support to maximum moment section as αb46Dr. Girma Z. & Dr. Adil Z.
CEng 5506 Structural DesignChapter 2 -Strip method for slabsCompiled by AZ & GZ24Chapter 2-Strip Method for Slabs•The moment at fixed support•So the ratio of negative to positive moment is as before →47Dr. Girma Z. & Dr. Adil Z.=-=22)(22wbbbwbbwmyfααααα-=---=2)21(22)1()1(222wbwbbbwmysαααα221αα-=yfysmmChapter 2-Strip Method for Slabs•Determine moment in the y-direction edge strips•Cantilever moment =•→1/8 of y-direction middle strip48Dr. Girma Z. & Dr. Adil Z.16)(2bwmyfα=16)()21(2wbmysα-=4)1(2)1(2bbwαα-⋅-
CEng 5506 Structural DesignChapter 2 -Strip method for slabsCompiled by AZ & GZ25Chapter 2-Strip Method for Slabs•With the above expressions, all the design moments for the slab can be found once a suitable value for αis chosen•0.35≤α≤0.39 give corresponding ratios of Negative to positive moments from 2.45 to 1.4549Dr. Girma Z. & Dr. Adil Z.Chapter 2-Strip Method for Slabs2.6 Unsupported Edges•The real power of the strip method becomes evident when dealing with nonstandard problems, such as with unsupported edge, slabs with holes, or slabs with reentrant corners (L-shaped)50Dr. Girma Z. & Dr. Adil Z.
CEng 5506 Structural DesignChapter 2 -Strip method for slabsCompiled by AZ & GZ26Chapter 2-Strip Method for Slabs•For a slab with one edge unsupported, a reasonable basis for analysis by the simple strip method is that a strip along the unsupported edge takes a greater load per unit area than the actual load acting, i.e., that the strip along the unsupported edge acts as a support for the strips at right angles.51Dr. Girma Z. & Dr. Adil Z.Chapter 2-Strip Method for Slabs•Such strips have been referred to as “strong bands”.•A strong band is, in effect, an integral beam, usually having the same total depth as the remainder of the slab but containing a concentration of reinforcement.•The strip may be made deeper than the rest of the slab to increase its carrying capacity, but this will not usually be necessary 52Dr. Girma Z. & Dr. Adil Z.
CEng 5506 Structural DesignChapter 2 -Strip method for slabsCompiled by AZ & GZ27Chapter 2-Strip Method for Slabs•Consider the rectangular slab carrying a uniformly distributed ultimate load wwith fixed edges along three side and no support along one short side, shown in Figure 8.