Steel Design Reference

Q: What steel grades does this reference cover?

The reference covers general structural steels SS400 (Fy=235 MPa), SM490 (Fy=315 MPa), SM520 (Fy=355 MPa), SM570 (Fy=420 MPa), and seismic-grade SHN steels (SHN275, SHN355, SHN460) which have controlled yield ratio (Fy/Fu <= 0.80) and guaranteed elongation, required for Special Moment Frames.

Q: How do I check a compression member for buckling?

Calculate the slenderness ratio KL/r, then the Euler stress Fe = pi^2*E/(KL/r)^2. If KL/r <= 4.71*sqrt(E/Fy), use inelastic buckling: Fcr = 0.658^(Fy/Fe)*Fy. Otherwise, use elastic buckling: Fcr = 0.877*Fe. The design strength is phi*Pn = 0.90*Fcr*Ag.

Q: What are the lateral-torsional buckling limits for beams?

For compact sections: if Lb Lr, use elastic LTB with Fcr*Sx. The Cb factor adjusts for moment gradient (1.0 for uniform moment, 1.14 for UDL, 1.32 for midpoint load).

Q: What are the key requirements for Special Moment Frames (SMF)?

Beams must be compact sections with Lb = 1.0*sum(Mpb). Connections must achieve 0.04 radian interstory drift and use prequalified or tested details. Panel zone shear strength must also be verified.

Free reference guide: Steel Design Reference

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About Steel Design Reference

The Steel Design Reference is a comprehensive, searchable guide to structural steel design per KDS 14 31 and AISC 360. It covers material specifications (SS400 Fy=235 MPa through SM570 Fy=420 MPa, SHN seismic-grade steels), H-beam standard sections (KS D 3502), and strength reduction factors (phi = 0.90 for tension yielding, compression, and flexure; phi = 0.75 for fracture and bolts). Every design formula includes the applicable resistance factor for LRFD methodology.

The member design section provides complete formulas for tension members (gross section yielding, net section rupture, block shear), compression members (Euler buckling stress Fe, inelastic/elastic critical stress Fcr, effective length factor K), flexural members (plastic moment Mp, lateral-torsional buckling with Lp/Lr limits, moment modification factor Cb), shear strength (web Cv coefficient), combined axial-plus-bending interaction equations (H1-1a/H1-1b), and section classification (compact/noncompact/slender width-to-thickness limits).

Beyond individual member design, the reference covers connection design (high-strength F10T bolts in slip-critical and bearing, fillet and CJP welds, moment connections, shear tabs, brace gusset plates, and column base plates), seismic design requirements (Special Moment Frames with strong-column-weak-beam, Buckling-Restrained Braces with LYP225 core, Special Concentrically Braced Frames), composite beam design with shear studs, and serviceability limits (deflection L/360 for floors, drift H/300 for wind, slenderness L/r limits).

Key Features

Structural steel material specifications: SS400, SM490, SM520, SM570 with yield/ultimate strengths, and SHN seismic-grade steels (yield ratio <= 0.80)
Tension member design: gross section yielding (phi=0.90*Fy*Ag), net section rupture (phi=0.75*Fu*Ae), shear lag factor U, and block shear formulas
Compression member design: Euler buckling (Fe), critical stress (Fcr) for elastic and inelastic regimes, effective length factor K for braced and unbraced frames
Flexural member design: plastic moment (Mp=Fy*Zx), lateral-torsional buckling (LTB) with Lp/Lr limits, moment modification factor Cb for common load cases
Connection design: F10T high-strength bolt slip-critical/bearing capacity, fillet weld strength (E70XX), moment connections, shear tabs, and gusset plates with Whitmore section
Combined force interaction equations (H1-1a/H1-1b) for axial-plus-biaxial bending with the 0.2 threshold switching criterion
Seismic design: SMF (strong-column-weak-beam, 0.04 rad rotation), BRB (LYP225 core, omega*beta adjustment), SCBF (KL/r <= 200, 2t hinge zone)
Composite beam design (effective width, full/partial composite, shear stud capacity) and serviceability limits (L/360 floor deflection, H/300 wind drift, L/r slenderness)

Frequently Asked Questions

What steel grades does this reference cover?

The reference covers general structural steels SS400 (Fy=235 MPa), SM490 (Fy=315 MPa), SM520 (Fy=355 MPa), SM570 (Fy=420 MPa), and seismic-grade SHN steels (SHN275, SHN355, SHN460) which have controlled yield ratio (Fy/Fu <= 0.80) and guaranteed elongation, required for Special Moment Frames.

How do I check a compression member for buckling?

Calculate the slenderness ratio KL/r, then the Euler stress Fe = pi^2*E/(KL/r)^2. If KL/r <= 4.71*sqrt(E/Fy), use inelastic buckling: Fcr = 0.658^(Fy/Fe)*Fy. Otherwise, use elastic buckling: Fcr = 0.877*Fe. The design strength is phi*Pn = 0.90*Fcr*Ag.

What are the lateral-torsional buckling limits for beams?

For compact sections: if Lb <= Lp (plastic limit), use full plastic moment phi*Mn = 0.90*Fy*Zx. For Lp < Lb <= Lr, use inelastic LTB with linear interpolation modified by Cb. For Lb > Lr, use elastic LTB with Fcr*Sx. The Cb factor adjusts for moment gradient (1.0 for uniform moment, 1.14 for UDL, 1.32 for midpoint load).

How do I design a high-strength bolt connection?

For F10T M22 slip-critical (single shear): phi*Rn = phi*mu*Du*hf*Tb*ns = 1.0*0.45*1.13*1.0*205*1 = 104 kN/bolt, where mu is the friction coefficient (0.45 for Class A surfaces), Tb is the bolt pretension (205 kN), and ns is the number of slip planes. For bearing, use phi*Rn = 0.75*2.4*d*t*Fu.

What is the combined force interaction equation?

When Pu/(phi*Pn) >= 0.2: use Pu/(phi*Pn) + (8/9)*(Mux/(phi*Mnx) + Muy/(phi*Mny)) <= 1.0. When Pu/(phi*Pn) < 0.2: use Pu/(2*phi*Pn) + Mux/(phi*Mnx) + Muy/(phi*Mny) <= 1.0. This is the AISC H1-1 interaction formula for combined axial force and biaxial bending.

What are the key requirements for Special Moment Frames (SMF)?

Beams must be compact sections with Lb <= 0.086*ry*E/Fy. Columns must satisfy the strong-column-weak-beam criterion: sum(Mpc) >= 1.0*sum(Mpb). Connections must achieve 0.04 radian interstory drift and use prequalified or tested details. Panel zone shear strength must also be verified.

How does Buckling-Restrained Brace (BRB) design work?

BRB uses a low-yield-point steel core (LYP225) confined by a steel tube filled with mortar to prevent buckling. The core yields equally in tension and compression. Design force is Pysc = Ry*Fy*Asc, with adjustment factors omega=1.1 and beta=1.1 for column design forces: Pu = omega*beta*Pysc.

What are the deflection and drift limits for steel structures?

Live load deflection limits are L/360 for floor beams and L/240 for roof beams. Total load (D+L) limits are L/240 for floors and L/180 for roofs. Cantilevers use L/180. Story drift limits are H/300 for wind and H/200 for seismic. Member slenderness limits are KL/r <= 200 for compression and L/r <= 300 for tension.