Design of steel steel beam or Channel with AISC 13th Ed.

Design code: AISC 13th edition, 2005.

Design requirements:

LRFD design:

1. M_u £ fM_n.
2. V_u £ fV_n.
3. Deflection should not exceed allowable limit.

Where M_u and V_u are factored moment and shear calculated based on strength design provision of building code.  M_n and V_n are  normal moment and shear strength calculated based on AISC 13th edition, f is resistance factor.

ASD design:

1. M_a £ M_n/W.
2. V_a £ V_n/W.
3. Deflection should not exceed allowable limit.

Where M_a and V_a are moment and shear calculated based on allowable stress design provision of building code, M_n and V_n are  normal moment and shear strength calculated based on AISC 13th edition, W is safety factor.

Design procedure:

LRFD

1. Calculate factored load based on strength design provision of building code
2. Calculate moment, M_u.
3. Select a trial beam size and calculate normal moment strength, M_n from Chapter F of AISC 13th edition, and multiply it by f.
4. Calculate factored shear force, V_u.
5. Calculate normal shear strength, V_n from Chapter F of AISC 13th edition, and multiply it by f.
6. Calculate deflection and check with allowable deflection ratio from building code.

ASD design:

1. Calculate apply load based on allowable stress design provision of building code
2. Calculate moment, M_a.
3. Select a trial beam size and calculate normal moment strength, M_n from Chapter F of AISC 13th edition, and divide it by W.
4. Calculate shear force, V_a.
5. Calculate shear strength, V_n from Chapter F of AISC 13th edition, and divide it by W.
6. Calculate deflection and check with allowable deflection ratio from building code.

Determine factor moment M_u and factored shear, V_u

Factored moment and shear shall be determined by multiply moment and shear calculated from structural analysis by load factored based on load combination listed in building codes.

Loads and load combinations:

AISC 13th edition Section B2 states that loads and load combinations shall be based on applicable building code or use SEI/ASCE 7  in the absence of a building code.

Determine flexural strength , M_n and shear strength, V_n

Nominal flexural strength, M_n

Chapter F of AISC 13th edition cover flexural design of beams.  The chapter is divided to 13 sections from F1 to F13. 

The nominal flexural strength based on compact or non-compact section as well as their unsupported length.  Compact or non-compact section is determined based on section porperties (b/t and h/t_w).  Unbraced length if depends how lateral supported is provided.

Hot rolled I shape or channel

Sections cover hot rolled hot rolled I shape sections are as follows

Section F2 cover compacted double symmetric compact I-shaped members and channels bend about their major axis.

Section F3 cover non-compacted double symmetric compact I-shaped members and channels bend about their major axis.

Section F6 cover I-shaped members and channels bend about their minor axis.

Moment and shear strength of I shape and Channel with Fy £ 50 kai in AISC 13th edition

Deflection

AISC 13th edition section L.3 states that deflection limit shall not impair the serviceability of the structures.

Acceptable deflection in most building codes

Deflection limits listed in most Building Code are

For cantilever beam, L is 2 time the length of cantilever.

Example 1