AECT250-Lecture 4

# AECT250-Lecture 4 - Lecture 4 Beam Design for Moment Beams...

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Lecture 4 - Page 1 of 12 Lecture 4 – Beam Design for Moment Beams, or sometimes referred to as “flexure” members, are designed on the basis of moment. In ASD and LRFD, the design strength of a beam in flexure is called the “nominal flexural moment”, M n . This M n must be greater than the maximum applied factored (LRFD ) or service (ASD) moment. Beams are designed on the basis of the following LRFD references: AISC Part 3 AISC Spec Chapter F p. 16.1-44 1. Beam Design Considering Yielding : Assuming a beam is adequately laterally braced , it will fail by yielding on the compression flange. Most beams are laterally braced by the metal decking that is attached to the compression flange as shown below: 1) LRFD Beam Design : Design Flexural Strength = φ b M n Where: φ b = 0.90 M n = nominal flexural moment = M p = Plastic moment = F y Z x Z x = plastic section modulus = from properties Concrete slab over metal decking Metal decking puddle-welded to top flange of beam

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Lecture 4 - Page 2 of 12 2) ASD Beam Design : Allowable Flexural Strength = b n M Where: b = 1.67 M n = nominal flexural moment = M p = Plastic moment = F y Z x Z x = plastic section modulus = from properties GIVEN : A W16x26 steel beam using A992 steel is continuously laterally braced, and experiences a FACTORED moment = 104 KIP-FT. REQUIRED : 1) Determine the design flexural moment, φ b M n for the beam. 2) Determine if the beam is adequate. Step 1 – Determine φ b M n for the beam : φ b M n = 0.90(F y Z x ) since it is continuously laterally braced = 0.90(50 KSI)(44.2 in 3 ) = 1989 KIP-IN φ b M n = 165.8 KIP-FT Step 2 – Determine if the beam is adequate : Since φ b M n = 165.8 KIP-FT > 104 KIP-FT beam is adequate GIVEN : The W16x26 beam from Example 1. REQUIRED : Determine the design flexural moment, φ b M n for the beam using the LRFD “Z x ” Table 3-2 (see AISC p. 3-11 thru 3-19) Step 1 – Refer to AISC p. 3-18 for W16x26 : Look in the LRFD column φ b M px = 166 KIP-FT From properties, AISC p. 1-21
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AECT250-Lecture 4 - Lecture 4 Beam Design for Moment Beams...

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