The 1971 ACI Building Co d e(Ref. 1), for the first time, al-l owed the use of shear re i n-f o rcement in concrete slabs

less than 10 inches thick. Althoughthe code permitted it, engineersand contractors have nt enthusias-tically endorsed the concept. Ty p i-cal slab shear re i n f o rcement hasbeen costly, hard to place, and onlypartially effective.

Why use shear reinforcement?The thickness of most long-span

flat slabs, two-way slabs withoutb e a m s, is controlled by the shears t rength of the concrete slab sur-rounding the supporting column.The shear resistance of these slabscan be increased by providing dro pp a n e l s, column capitals, or bymeans of shear re i n f o rcement. If theshear resistance is increased, slabthickness can be decreased re s u l t-ing in less concrete materials andl ower stru c t u re deadweight, re d u c-ing costs.

Drop panels and column capitals,h owe ve r, are often not feasible fora rc h i t e c t u ral re a s o n s. They also arecostly to build because they donttake advantage of flying form s.

Using shear re i n f o rcement re-duces the slab thickness and stillmaintains the flat bottom slab sur-face for economical forming. But in-stalling typical shear re i n f o rc e m e n t ,shear heads, and stirrup cages is dif-ficult and costly.

The once widely used shear headsa re expensive to install and arera rely used anymore. A stirrup cagewith longitudinal rebar in both di-

rections is difficult to place and in-t e rf e res with the column re i n f o rc e-ment (Fi g u re 1).

To take advantage of the re d u c e dslab thickness and decreased costthat shear re i n f o rcement could pro-v i d e, a more efficient shear re i n-f o rcement has been deve l o p e d .

Studded steel stripsprovide full anchorage

This shear re i n f o rcement consistsof large-head studs welded to steels t rips (see photo). The steel strip ispositioned with bar chairs and fas-tened to the form by nails dri ve n

t h rough holes in the steel stri p. Thechairs provide the re q u i red concre t ec over and the nails anchor the stri pto pre vent movement during con-s t ru c t i o n .

These studded steel strips havebeen designed to be more effectivethan conventional shear re i n f o rc e-ment. Co n ventional shear re i n-f o rcement is not fully effective be-cause the stirrups cant beadequately anchored into the con-c re t e. In thin slabs this ineffectivea n c h o rage increases the shear cra c kwidth and the shear re i n f o rc e m e n tn e ver yields. The ACI Building Co d e,

Practical shear reinforcementfor concrete flat slabs

BY WALTER H. DILGERPROFESSOR OF CIVIL ENGINEERINGTHE UNIVERSITY OF CALGARYCALGARY, ALBERTA, CANADA

Figure 1.Conventionalstirrup cagesrequire largediameterlongitudinal barsas anchors.These usuallyinterfere with thecolumnreinforcementmaking the cageshard to install.Using studdedsteel stripsinstead of thisconventionalstirrup cage, inone case,decreased theamount of steelrequired from 82pounds to 20pounds percolumn.

recognizing these limitations, re-q u i res a conserva t i ve design.

The dimensions of the studdedsteel strips (Fi g u re 2) have been setto provide full anchorage and to en-s u re that at ultimate load the steelstuds yield. To achieve complete an-c h o rage and yield at ultimate load,the yield strength of the stud mater-ial is specified between 40,000 and60,000 psi.

Testing proves studded steel strips work well

Tests on re i n f o rced and pre-s t ressed concrete panels (Ref. 2 and3) we re conducted to study the per-

f o rmance of the studded steel stri p s.For re i n f o rced concrete test panelswith identical flexural re i n f o rc e-ment, a panel with no shear re i n-f o rcement failed at an axial load of84,300 pounds. For a re i n f o rc e dpanel with studded steel stri p s, theultimate load was 133,100 pounds.The pre s t ressed panel tests withstudded steel strips showed a 35%i n c rease in moment at failure asc o m p a red with the pre s t ressed testpanel with no shear re i n f o rc e m e n t .

How easy to design?Sometimes innovations are

s l owed by difficult or cumbersome

design pro c e d u re s. For studdedsteel strip design, a micro c o m p u t e rp ro g ram is available from the man-u f a c t u rer that does all the work forthe designer. When studded steels t rips are used, engineers dont haveto design concrete-column slabconnections that look like birdc a g e s. And contractors can take ad-vantage of the flat bottom slabf o rming.

References1. Building Code Requirements forReinforced Concrete (ACI 318-71),American Concrete Institute, P.O. Box19150, Detroit, Michigan 48219.2. A. F. van der Voet, W. H. Dilger, andA. Ghali, Concrete Flat Plates WithWell-anchored Shear ReinforcementElements, Canadian Journal of CivilEngineering, Vol. 9, No. 1, 1982, pp.107-114.3. W. H. Dilger and M. Shatila, ShearStrength of Prestressed ConcreteEdge Slab-column Connections Withand Without Shear Stud Reinforce-ment, Canadian Journal of Civil Engi-neering, Vol. 16, No. 4, 1989.

Figure 2. The dimensions of the studded steel strips have been set to provide fullanchorage and to ensure that at ultimate load the steel studs yield. The steelstrip is positioned with bar chairs and fastened to the form by nails driventhrough holes in the steel strip. The chairs provide the required concrete coverand the nails anchor the stirrup to prevent movement during construction andconcrete placing.

P U B L I C AT I O N# C 9 0 0 2 2 3Copyright 1990, The Aberdeen Gro u pAll rights re s e r v e d