56 2.5 RC FRAME ELEMENTS DESIGN SR EN 1992-1-1-2004 2.5.1 RC GIRDER DESIGN 2.5.1.1 General rules : RC Girder design computation for Bending Moment and Shear Force The monolith girder’s cross-section have commonly T shape but also can have rectangular, trapezoidal shape or other shapes. The minimum height of girders 1/15 of the span. The ratio between the height and width of the transversal cross-section will be less than 4, but it is recommended to have values h/b = 1.5 – 3 for the rectangular cross-section girders and h/b = 2 – 3 for the T shape cross-sections. The width of the transversal cross-section b must have the minimum value 200 mm for the safety in case of fire. For monolith girders the dimensions will be multipliers of 50 mm. Monolith girders commonly have a constant cross-section on span. Function of the ratio between the span of the beam (l) and the height of the cross-section (h), the following classification is made : - Long beams (slender), if 8 h l ; - Short beams, if 8 h l 2 ; - Deep beams (walls) if 2 h l Constructive aspects regarding longitudinal reinforcement : The minimum and maximum reinforcing percentages will be chosen in function of the following aspects : - The minimum cross-section of the reinforcement will be determined with the following formula: d b f f 0,5 A t yk ctm smin in seismic zones, no matter the ductility class; b t - medium width of the beam
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2.5 RC FRAME ELEMENTS DESIGN
SR EN 1992-1-1-2004
2.5.1 RC GIRDER DESIGN
2.5.1.1 General rules :
RC Girder design computation for Bending Moment and Shear Force
The monolith girder’s cross-section have commonly T shape but also can haverectangular, trapezoidal shape or other shapes.
The minimum height of girders 1/15 of the span.
The ratio between the height and width of the transversal cross-section will be lessthan 4, but it is recommended to have values h/b = 1.5 – 3 for the rectangular cross-sectiongirders and h/b = 2 – 3 for the T shape cross-sections.
The width of the transversal cross-section b must have the minimum value 200 mm forthe safety in case of fire. For monolith girders the dimensions will be multipliers of 50 mm.
Monolith girders commonly have a constant cross-section on span. Function of theratio between the span of the beam (l) and the height of the cross-section (h), the followingclassification is made :
The minimum and maximum reinforcing percentages will be chosen in function of thefollowing aspects :
- The minimum cross-section of the reinforcement will be determined with thefollowing formula:
dbff0,5A t
yk
ctmsmin in seismic zones, no matter the ductility class;
b t - medium width of the beam
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- The effective surface of the reinforcement cross-section must be greater than thenecessary surface in order to resist cracking;
- The concrete cross-sections with reinforcement areas less than the area obtained bythe formula mentioned above will be considered unreinforced cross-sections;
- Maximum cross-section for the tensioned or compressed reinforcement, out of theoverlaying zones, corresponds to the maximum height of the compressed zone
dx where:o 0,25 for girder’s plastic zones;o lim , for other cases.
But it will be less than 4 % of the concrete cross-section area
csmax A0,04A
The medium reinforcing percent will be between 0.8 and 2 % .
The reinforcement mounting :
The cost of the steel reinforcement is influenced by the trimming and mountingprocesses:
- A very low variation of diameters used;- The use of straight bars;- The adequate jointing systems;- Use of mechanized tools for trimming and assembling.
The longitudinal resistance reinforcement is usually composed of flexible barsassociated with constructive and transversal reinforcements. Bounded by spatial enclosures(welded or connected by wires).
The stirrups will be mounted at a maximum 200 mm distance.
The materials used:
PC 52 : 30015,1
345γ
ff
s
ykyd N/mm²;
Concrete C 20/25 : 33,135,1
20γ
ffb
ckcd N/mm²;
2,2fctm N/mm².
fctm – the medium axial strength of concrete;
fyd – yielding design limit strength of reinforcement for RC;
fyk – characteristic yielding limit strength of reinforcement for RC;
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fck – characteristic compression strength of the concrete, determined on cylinders at 28 days;
The maximum efforts evaluation was made with Robot, with automatic combinationsaccording SR – EN 1990 : 2004 / NA 2006.
The girder from most loaded frame was studied. (see part 2.3 Static and modalanalysis)
My diagram
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Vz diagram
Design Efforts – max values Support Span
My,Ed 176 kNm 94 kNm
Vz,Ed 119 kN 119 kN
The cross-section will have rectangular shape ( see part 2. Computation Summary)
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2.5.1.2 Longitudinal reinforcement computation in the support (superior part):