D1.F.4.6.2 Design of I-shaped beams per ACI-318
From the standpoint of the analysis – determining member forces, nodal displacements and support reactions – the same set of facilities and rules which are applicable for any normal reinforced concrete frames or other structures can be used when I-sections or tapered concrete members are specified. In other words, there isn’t anything unique or special to account for in the analysis model simply because I-shaped concrete beams are part of it.
From the standpoint of design, the following rules are applicable:
The member can be designed as a beam using the general principles explained in D1.F.4 Beam Design . It currently cannot be designed as a column. Design as a beam is done for flexure (MZ), shear (FY) and torsion (MX) just like that for rectangular, tee or trapezoidal beams. Axial forces (FX) are used during the capacity computations in shear and torsion. At each section along the length that the member is designed at, the depth at that section location is used for effective depth computation.
The program performs the following tests on the section dimensions before starting the design:
- If the thickness of the web is the same as the width of the top and bottom flanges, the member is designed as a rectangular section.
- If the thickness of the web is the same as the width of one of the flanges but not the other, the member is designed as a T-section or a rectangular section, depending on which side the compression due to bending is at.
- If the web thickness does not match the width of either flange, design is done using the rules applicable for T-beams – one flange is in compression, the other in tension, and tensile capacity of concrete on the tensile side of the neutral axis is ignored.
- The program is also able to design the beam as a doubly reinforced section if it is unable to design it as a single-reinforced section.
The parameters for designing these members are as shown in D1.F.3 Design Parameters of this manual. Detailed output on design at individual section locations along the member length may be obtained by setting the TRACK parameter to 3.0.
An example for I-beam design is shown below.
STAAD PLANE I BEAM CONCRETE DESIGN PER ACI-318 UNIT FEE KIP JOINT COORDINATES 1 0 0 0; 2 10 0 0 MEMBER INCIDENCES 1 1 2 UNIT INCHES KIP MEMBER PROPERTY 1 TAPERED 18 10 18 15 2.5 CONSTANTS E 3300 ALL DENSITY CONCRETE ALL POISSON CONCRETE ALL SUPPORTS 1 2 PINNED UNIT FEET KIP LOAD 1 DEAD LOAD MEMBER LOAD 1 UNI GY -5.76 LOAD 2 LIVE LOAD 1 UNI GY -7.04 LOAD COMB 3 ACI 318-02 1 1.4 2 1.7 PERFORM ANALYSIS LOAD LIST 3 START CONCRETE DESIGN CODE ACI 2002 UNIT INCHES KIP MINMAIN 9 ALL FC 4 ALL FYMAIN 60 ALL TRACK 2.0 ALL DESIGN BEAM ALL END CONCRETE DESIGN FINISH