rciii 04 beams ledges

7/24/2019 RCIII 04 Beams Ledges

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Beam s Ledges

BAU-RCIII-Lecture 3

Dr. Zaher Abou SalehBAU-Dept. of CivilEngineering-Debbieh Campus-RCIII-Lecture 3 1

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INTRODUCTION

Beam ledges have to be designed for overall member actions(flexure, shear, axial forces, and torsion ). This section addresses onlylocal failure modes and reinforcement requirements to prevent thesefailure modes.Design of beam ledges is somewhat similar to that of a bracket orcorbel with respect to loading conditions .Additional design considerations and reinforcement details need tobe considered in beam ledges .

Some failure modes discussed in lecture 2 for brackets and corbelsare also shown for beam ledges in Fig. 3-1.In addition to the four failure modes for brackets and corbels, twoadditional failure modes need to be considered for the design of beam ledges:


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(5) separation between ledge and beam web near the top of theledge in the vicinity of the ledge load, and(6) punching shear .


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Reinforcement for the different failure modes is determined based on theeffective widths or critical sections discussed below. In all cases, the requiredstrengths (V u, Mu, or N u) should be less than or equal to the design strengthsVn, Mn, or Nn).

Assuming0.2fc is thesmallest


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Assuming0.2fc is thesmallest


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Fig. 3-1, flexure and direct tension

Fig. 3-4

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12SI unit


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e- Reinforcement details

Hanger reinforcements, A vPrimary reinforcements, A s

Minimum reinforcements, A h

Beam stirrups

Figure 3-7 Reinforcement Details


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f-Development and Anchorage of Reinforcement

Within the bracket or corbel, the distance between load andsupport face is usually too short , so that special anchorage mustbe provided at the outer ends of both primary reinforcement Asand shear reinforcement Ah. Anchorage of As is normallyprovided by welding an anchor bar of equal size across the endsof As ( Fig. 3-8a) or welding to an armor angle . In the formercase, the anchor bar must be located beyond the edge of theloaded area. Where anchorage is provided by a hook or a loop inAs, the load must not project beyond the straight portion of thehook or loop (Fig. 3-8b)

In beam ledges, anchorage may be provided by a hook or loop,with the same limitation on the load location (Fig. 3-9). Where acorbel or beam ledge is designed to resist specific horizontalforces, the bearing plate should be welded to As


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Figure 3-8 Anchorage Details Using (a) Cross-Bar Weld and (b) Loop Bar Detail

Figure 3-9 Bar Details for Beam Ledge


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ApplicationThe L-beam shown is to support a double-tee parking deck spanning 20 m. Maximum service loads perstem are: DL = 50 kN; LL =30 kN; total load = 80 kN.

The stems of the double-tees rest on 110 mm 110 mm 6 mm neoprenebearing pads ( 7 Mpa maximum service load).Concrete compressive strength: 35 MpaSteel yielding strength: 420 MpaDesign in accordance with the code provisions for brackets and corbels may require a wider ledge thanthe 150 mm as shown. To maintain the 150 mm width, one of the following may be necessary:(1) Use of a higher strength bearing pad (up to 14 Mpa ); or(2) Anchoring primary ledge reinforcement A

sto an armor angle.

600 mm

300 mm

180 mm 150 mm

25 mm

270 mm

Effective width

Stems spaced at

1.2m o.c


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1-Check for bearing size (service laod is 7Mpa)Plate size is 110 mm 110 mmCapacity=(110)(110)(7)= 84.7 kN> 80kN O.K

2- Determine shear spans and effective widths for both shear and flexure- For shear friction

av=(2/3)L+ 25= 100 mmShear effective width=b ws=w+4a v=110+4(100)= 510 mm- For flexure, critical section is measured from the center of hanger reinf.

a f =100+25+12.9= 138 mm

Flexural effective width=b wf =w+5a f =110+5(138)= 800 mm

3. Check concrete bearing strengthVu=1.2DL+ 1.6LL=1.2(50)+1.6(30)= 108 kNBearing strength=(0.85) = 0.65 0.85 (35) 110 110 = 34 kN>Vu4. Check effective ledge section for maximum nominal shear-transfer strength V n

0.2 =7Mpa3.3+0.08 =6.1Mpacontrols V n =(0.75)(6.1)(510)(270)= 630 kN>Vu0r 11Mpa

Assume No. 13 size for hanger reinf.Clear cover

O.K

O.K


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5. Determine shear-friction reinforcementAvf =Vu/(f y)=245 mm 2, this reinforcement is distributed over b ws, Avf =245/510= 0.48 mm 2/mm

6. Determine reinforcement to resist direct tensionAn=Nu/(f y)=69 mm 2 where N u=0.2Vu=21.6 kNthis reinforcement is distributed over b wf , An=69/800= 0.086 mm 2/mm

7. Determine flexural reinforcementAf =Mu/(j uf yd)=229 mm 2 where M u=Vua f +Nu (h-d)= 15.5 kNthis reinforcement is also distributed over b wf , Af =229/800= 0.286 mm 2/mm

8. Determine the primary tension reinforcement

As=(2/3)A vf +An=0.406 mm 2/mm. controlsor As =Af +An=0.372 mm 2/mmCheck for min. reinf.

(As )min=0.04 bd=0.04( )(1)(270)= 0.9 mm 2/mm..As=0.9 mm 2/mm and no need to check forductility use No. 16 at 200mm o.c

9. Minimum horizontal reinforcement.Ah=0.5(A s-An)=0.5(0.905-.086)= 0.409 mm 2/mm use No. 13 at 200mm o.cFor ease of constructability, provide reinforcement at same spacing as As


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Dr. Zaher Abou SalehBAU-Dept. of Civil

Engineering-Debbieh Campus16

10 . Check required area of hanger reinforcement.

Assume s=200mmS=1.2m

Then hanger reinforcement, A v=58mm 2

W+3a=110+3(100)=410mmAssume s=200mm

Check for serviceability

Then, A v=186mm 2 controlsuse No. 16 at 200mm o.c

11 . Check for punching shear

Vu= (W+2L+2d f )d f where W=L=110mm and d f =300-50=250mm

Vu=306.9 kN>108kN..OK

12 . Reinforcement Details

150 mm

No. 13 @200 mm

No. 10

No. 16 @200 mm

Assume cover toreinforcement 50mm

rciii 04 beams ledges

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