Steel Design Tutoring

Steel design is the first course where you choose a member instead of just analyzing one. The AISC specification is the rulebook, and the course teaches you to work through it. Every member has several limit states, and the capacity is the smallest one. A tension member can yield or rupture. A column can buckle in several ways. A beam can reach its plastic moment or fail earlier by lateral-torsional buckling. The skill is knowing which checks apply and where to find the equations quickly.

Sessions follow your textbook, usually Segui, McCormac, or Salmon and Johnson, along with the AISC Steel Construction Manual tables you are allowed to use. We practice both LRFD and ASD, since many courses teach LRFD first and then show ASD as a variation. Sizing problems get a repeatable routine: assume, check every limit state, adjust, and verify. We also build a one-page map of the specification so you know where each check lives.

A typical steel design problem, worked

A bolted tension member has a gross area $A_g = 4.5\ \text{in}^{2}$ and a net area $A_n = 3.8\ \text{in}^{2}$. The shear lag factor is $U = 0.85$. Steel is $F_y = 50\ \text{ksi}$ and $F_u = 65\ \text{ksi}$. Find the LRFD design strength.

Yielding on the gross section: $$\phi P_n = 0.90 \times F_y \times A_g = 0.90 \times 50 \times 4.5 = 202.5\ \text{kips}$$ The resistance factor for yielding is 0.90.

Rupture on the net section: first the effective net area, $$A_e = U \times A_n = 0.85 \times 3.8 = 3.23\ \text{in}^{2}$$ Then $$\phi P_n = 0.75 \times F_u \times A_e = 0.75 \times 65 \times 3.23 = 157.5\ \text{kips}$$ The resistance factor for rupture is 0.75.

The design strength is the smaller value, so $\phi P_n = 157.5\ \text{kips}$ and rupture governs. For ASD, the allowable strength is $P_n / \Omega$, with $\Omega = 2.00$ for rupture: $$\dfrac{65 \times 3.23}{2.00} = 105.0\ \text{kips}$$ A full solution would also check block shear at the connection.

Where students usually get stuck

• Stopping at the first limit state that passes. Every applicable limit state needs a check.
• Effective length and the $K$ factor, especially for frames with sway
• Choosing $L_b$ and $C_b$ in lateral-torsional buckling problems
• Mixing LRFD factors with ASD safety factors in the same calculation
• Getting lost in the specification. We practice finding the right chapter and table quickly.