How do steel plate shear walls (SPSW) work?

Steel plate shear walls (SPSW) per AISC 341-22 Section F5 consist of thin steel infill plates bounded by vertical boundary elements (VBE, columns) and horizontal boundary elements (HBE, beams). Under lateral load, the thin plate buckles in shear and develops a diagonal tension field — conceptually similar to a Pratt truss where the plate acts as diagonal tension members and the boundary elements act as chords. The nominal shear strength Vn = 0.42 × Fy × tw × Lcf × sin(2α) where Fy is the plate yield strength (typically A36, 36 ksi), tw is plate thickness, Lcf is the clear span between VBE flanges, and α is the tension field angle from vertical (approximately 40-45° for typical panel aspect ratios). Example: a 20 ft bay SPSW with 3/16" A36 plate at a 3-story building with story shear Vu = 480 kips and α = 42°. φVn = 0.90 × 0.42 × 36 × 0.1875 × 228 × sin(84°) = 0.90 × 0.42 × 36 × 0.1875 × 228 × 0.995 = 0.90 × 516 = 464 kips. Since 464 kips < 480 kips, increase plate to 1/4": φVn = 0.90 × 0.42 × 36 × 0.25 × 228 × 0.995 = 0.90 × 688 = 619 kips — acceptable. SPSW system factors per ASCE 7 Table 12.2-1: R = 7, Ω₀ = 2, Cd = 5½. The infill plate must be A36 (not higher strength) to ensure yielding occurs before boundary element failure per capacity design. SPSWs provide 2-3× the stiffness of equivalent braced frames for the same bay width, making them very efficient in high-seismic applications.

How are SPSW boundary elements designed?

SPSW boundary element design follows capacity design principles per AISC 341-22 Section F5. The boundary columns (VBE) and beams (HBE) must resist forces corresponding to the fully yielded and strain-hardened infill plate: (1) VBE axial force Pu_VBE = Σ(0.5 × Ry × Fy × tw × H × sin²α) from each adjacent panel plus gravity loads, where Ry = 1.3 for A36 plate per AISC 341 Table A3.1. (2) VBE must also satisfy compactness requirements (λ ≤ λps per AISC 341 Table D1.1) to sustain plastic rotations at the panel corners. (3) HBE must resist the vertical component of the tension field: Mu_HBE = (1/8) × Ry × Fy × tw × Lcf² × ω where ω accounts for the vertical component of the tension field. (4) The VBE moment demand from tension field anchorage: Mu_VBE = (1/12) × Ry × Fy × tw × H² × cos²α, added to axial-flexural interaction. (5) VBE splices must develop the full expected strength, not just the demand, per AISC 341 Section D2.5. (6) HBE-to-VBE connections are typically full-penetration welds developing the HBE flange forces. The infill plate is fillet-welded to the fish plates (connection plates welded to the VBE and HBE). Fish plate thickness should be at least 1.5 × tw to ensure the plate yields before the connection. Minimum plate thickness for handling and welding: 1/4" for plates up to 10 ft panel height, 3/8" for 10-15 ft.

What are the drift limits for steel framed wall systems?

Drift limits per ASCE 7-22 Table 12.12-1 and IBC Table 1604.3 apply to steel framed wall systems: (1) Seismic drift: story drift Δ ≤ 0.020hsx for Risk Category I-II (SPSW, SCBF, SMF), 0.015hsx for Risk Category III, and 0.010hsx for Risk Category IV — where hsx is the story height below the level under consideration. For a 13 ft story, allowable story drift = 0.020 × 13 ft = 3.12" for standard occupancies. The design story drift Δ = Cd × δe / Ie where Cd = 5½ for SPSW systems and δe is the elastic displacement from reduced seismic forces. (2) Wind drift: typically H/400 for buildings over 10 stories (common industry practice, not a code mandate) to control cladding damage, and H/500 for hotels and residential to control partition cracking. (3) SPSW stiffness: an SPSW panel with 1/4" A36 plate, 20 ft bay, 13 ft story has initial stiffness K_initial ≈ 2,000-3,000 kips/inch before plate buckling. After buckling, the post-buckled stiffness is approximately 10-15% of the initial — the tension field action provides reliable post-buckling capacity with 0.02+ rad drift capacity. (4) P-delta effects: ASCE 7-22 Section 12.8.7 requires that the stability coefficient θ = Px × Δ × Ie / (Vx × hsx × Cd) ≤ 0.10 (or θmax = 0.5/(β × Cd) ≤ 0.25). If θ > 0.10, member forces and drifts must be amplified by 1/(1 − θ).

How do cold-formed steel (CFS) stud walls differ from structural steel walls?

Cold-formed steel (CFS) stud walls per AISI S213 and AISI S100 differ fundamentally from hot-rolled structural steel walls: (1) Material — CFS uses thin sheet steel (33-97 ksi yield, typically Grade 50) formed at room temperature into C-shape studs and U-shape tracks, typically 0.018" to 0.071" thick (25-14 gauge). (2) Load path — CFS studs are primarily vertical load-bearing (axial) or non-structural partition walls. Lateral resistance is typically provided by steel strap X-bracing or sheathing (plywood, OSB, gypsum, or steel sheet). (3) Design code — AISI S100 governs member design with provisions for local buckling, distortional buckling, and global buckling. The effective width method accounts for post-buckling strength in thin elements. (4) Typical stud sizes: 2-1/2", 3-5/8", 4", 6", 8", 10", 12" web depths × 1-3/8" or 1-5/8" flanges. Stud spacing 12", 16", or 24" o.c. (5) Load capacity — a 6" × 1-5/8" × 54 mil (16 gauge, Grade 50) stud at 8 ft height carries approximately 3-8 kips axial depending on sheathing bracing. (6) CFS is NOT used for lateral force-resisting systems in mid-to-high-rise buildings — that role goes to structural steel SPSW, SCBF, or SMF. CFS lateral systems are limited to low-rise (≤ 3 stories for bearing wall systems in SDC D per ASCE 7 Table 12.2-1). The key advantage: CFS walls are 40-60% lighter than equivalent concrete/masonry walls and can be erected without heavy equipment.

What are the practical considerations for SPSW construction?

Practical SPSW construction considerations: (1) Plate delivery and handling — SPSW infill plates are typically 3/16" to 3/8" thick, 8-15 ft tall, and 15-25 ft wide, weighing 500-2,500 lb each. They are shipped flat and must be handled without excessive bending that could kink the plate. (2) Welding sequence — fish plates are shop-welded to the VBE and HBE. The infill plate is field-welded to the fish plates using fillet welds. Welding should proceed from the center outward to minimize distortion. (3) Plate camber — due to the thin plate dimensions, SPSW plates may exhibit slight out-of-plane camber from rolling, handling, and welding. AISC 341 permits up to H/100 out-of-plane deviation without remediation (approximately 1.5" for a 13 ft panel). (4) Fish plate detailing — fish plates provide a backing bar for the infill plate fillet weld. Minimum fish plate thickness = 3/8" or 1.5 times the infill plate thickness. Fish plates are continuously fillet-welded to the boundary elements. (5) Openings — SPSW panels can accommodate small openings (up to 15% of panel area) for doors, windows, or MEP penetrations by adding local boundary elements around the opening. Larger openings require a redesign to transfer the tension field around the opening. (6) Cost — SPSW construction is competitive with SCBF for buildings above 12-15 stories where the stiffness advantage reduces drift-control member sizes. For low-rise, braced frames are typically more economical. (7) Fire protection — the infill plate must be fireproofed similarly to columns; the thin plate requires stiffened SFRM attachment or intumescent coating.

Steel Framed Walls -- Steel Plate Shear Walls and Cold-Formed Steel Stud Walls

Q: How are SPSW boundary elements designed?

SPSW boundary element design follows capacity design principles per AISC 341-22 Section F5. The boundary columns (VBE) and beams (HBE) must resist forces corresponding to the fully yielded and strain-hardened infill plate: (1) VBE axial force Pu_VBE = Σ(0.5 × Ry × Fy × tw × H × sin²α) from each adjacent panel plus gravity loads, where Ry = 1.3 for A36 plate per AISC 341 Table A3.1. (2) VBE must also satisfy compactness requirements (λ ≤ λps per AISC 341 Table D1.1) to sustain plastic rotations at the panel corners. (3) HBE must resist the vertical component of the tension field: Mu_HBE = (1/8) × Ry × Fy × tw × Lcf² × ω where ω accounts for the vertical component of the tension field. (4) The VBE moment demand from tension field anchorage: Mu_VBE = (1/12) × Ry × Fy × tw × H² × cos²α, added to axial-flexural interaction. (5) VBE splices must develop the full expected strength, not just the demand, per AISC 341 Section D2.5. (6) HBE-to-VBE connections are typically full-penetration welds developing the HBE flange forces. The infill plate is fillet-welded to the fish plates (connection plates welded to the VBE and HBE). Fish plate thickness should be at least 1.5 × tw to ensure the plate yields before the connection. Minimum plate thickness for handling and welding: 1/4" for plates up to 10 ft panel height, 3/8" for 10-15 ft.

Q: What are the drift limits for steel framed wall systems?

Drift limits per ASCE 7-22 Table 12.12-1 and IBC Table 1604.3 apply to steel framed wall systems: (1) Seismic drift: story drift Δ ≤ 0.020hsx for Risk Category I-II (SPSW, SCBF, SMF), 0.015hsx for Risk Category III, and 0.010hsx for Risk Category IV — where hsx is the story height below the level under consideration. For a 13 ft story, allowable story drift = 0.020 × 13 ft = 3.12" for standard occupancies. The design story drift Δ = Cd × δe / Ie where Cd = 5½ for SPSW systems and δe is the elastic displacement from reduced seismic forces. (2) Wind drift: typically H/400 for buildings over 10 stories (common industry practice, not a code mandate) to control cladding damage, and H/500 for hotels and residential to control partition cracking. (3) SPSW stiffness: an SPSW panel with 1/4" A36 plate, 20 ft bay, 13 ft story has initial stiffness K_initial ≈ 2,000-3,000 kips/inch before plate buckling. After buckling, the post-buckled stiffness is approximately 10-15% of the initial — the tension field action provides reliable post-buckling capacity with 0.02+ rad drift capacity. (4) P-delta effects: ASCE 7-22 Section 12.8.7 requires that the stability coefficient θ = Px × Δ × Ie / (Vx × hsx × Cd) ≤ 0.10 (or θmax = 0.5/(β × Cd) ≤ 0.25). If θ > 0.10, member forces and drifts must be amplified by 1/(1 − θ).

Steel framed walls serve two distinct structural roles: as lateral force-resisting elements (steel plate shear walls, SPSW) and as gravity-load-bearing or infill partitions (cold-formed steel stud walls). This reference covers both systems, their design provisions, and the critical engineering checks required for each.

Steel Plate Shear Walls (SPSW) per AISC 341

A steel plate shear wall consists of a thin steel infill plate (typically 1/4 in. to 3/8 in. thick) shop-welded to horizontal and vertical boundary elements (HBEs and VBEs) that form a moment-resisting frame around the plate. The plate resists shear through diagonal tension field action -- after buckling, the plate forms a series of diagonal tension bands anchored by the boundary elements.

SPSWs provide lateral resistance in high-seismic applications comparable to reinforced concrete shear walls at roughly half the weight and with significantly faster erection. They are used in hospitals, data centers, and high-rise buildings in regions of high seismicity (Seismic Design Category D and above).

Tension Field Action

Under lateral load, the thin steel plate buckles in shear at very low load (typically at less than 10% of the ultimate capacity). Post-buckling, the plate develops a diagonal tension field -- the plate acts like a series of diagonal tension ties, with the VBEs resisting the vertical component of the tension field and the HBEs resisting the horizontal component.

The ultimate shear strength of an SPSW panel per AISC 341 Section F5:

Vn = 0.42 Fy t_w L_cf sin(2 alpha) + 0.5 Fy t_w L_cf sin(2 alpha) (if VBEs resist the full tension field)

where t_w = infill plate thickness, L_cf = clear distance between VBE flanges, alpha = tension field inclination angle (typically 40-45 degrees from vertical, determined iteratively per Canadian CAN/CSA S16 Annex M or the AISC 341 Commentary procedure).

The first term is the buckling strength. The second term is the post-buckling tension field strength. The total capacity is 2-4 times the buckling capacity, which is why SPSWs are designed to buckle early and rely on the post-buckling reserve.

Boundary Element Design

The VBEs (vertical boundary elements, typically W14 or W18 heavy columns) and HBEs (horizontal boundary elements, typically W18 to W24 beams) must resist the tension field anchor forces without yielding or buckling. The VBE design is the most demanding aspect of SPSW engineering:

VBE axial force: Accumulated tension field vertical component from all panels above. The VBE at the base carries the weight of the building plus the vertical component of the tension field from every panel -- comparable to the column in a braced frame.
VBE flexural stiffness: Must limit out-of-plane deformation of the infill plate to prevent loss of tension field anchorage. The VBE must have a minimum moment of inertia per AISC 341 Section F5.4: I_VBE >= 0.00307 t_w h^4 / L.
HBE design: The HBE must resist the horizontal component of the tension field plus the beam moment from the frame action. For a multi-story SPSW, the HBE at each floor is also the collector element that drags the diaphragm shear into the wall.

SPSW Worked Example -- Single Panel Shear Strength

Given: Single-story SPSW panel: L = 20 ft (clear horizontal distance between VBE flanges), h = 15 ft (centerline of HBE to HBE). Infill plate: A36, t_w = 1/4 in. VBE: W14x132 (Fy = 50 ksi). HBE: W18x76 (Fy = 50 ksi). Tension field angle alpha = 42 degrees (from iterative solution).

Shear strength: Vn = 0.42 x 36 x 0.25 x (20 x 12) x sin(2 x 42) Vn = 0.42 x 36 x 0.25 x 240 x sin(84 degrees) Vn = 0.42 x 36 x 0.25 x 240 x 0.9945 Vn = 902 kip

phi Vn = 0.90 x 902 = 812 kip (LRFD)

This panel resists 812 kip of lateral shear -- comparable to a 20 ft long, 12 in. thick reinforced concrete shear wall.

VBE stiffness check: I_VBE_min = 0.00307 x 0.25 x (15 x 12)^4 / (20 x 12) I_VBE_min = 0.00307 x 0.25 x (180^4) / 240 I_VBE_min = 0.00307 x 0.25 x 1.050 x 10^9 / 240 I_VBE_min = 0.00307 x 0.25 x 4.374 x 10^6 I_VBE_min = 3,358 in^4

W14x132 Ix = 1,530 in^4 < 3,358 in^4. VBE is too flexible. Either increase VBE size (W14x233, Ix = 3,010 in^4), add intermediate stiffeners to the infill plate, or reduce panel aspect ratio. This is the most common SPSW design challenge -- the VBE stiffness requirement drives the column size.

Perforated SPSW Panels

To accommodate architectural openings (doors, windows, MEP penetrations) or to reduce the panel strength to match the demand, SPSW panels may be perforated with a regular pattern of circular holes. AISC 341 Section F5.5b permits perforated panels with a maximum perforation ratio of 40% (hole area / gross plate area). The reduced panel strength accounts for the reduced cross-sectional area and the modified tension field angle around the perforations.

Cold-Formed Steel (CFS) Stud Walls per AISI S213

Cold-formed steel stud walls are the standard interior partition and exterior non-load-bearing wall system in steel-framed buildings. CFS studs (C-shaped or track sections, typically 3-5/8 in., 6 in., or 8 in. deep, 33 mil to 68 mil thickness) span vertically between the floor slab and the underside of the deck above.

Axial Capacity of CFS Studs

The axial capacity of a CFS stud is governed by AISI S100 Section C4 (compression) or D (flexural-torsional buckling for singly-symmetric sections). Unlike hot-rolled steel design, CFS design accounts for:

Local buckling of the individual plate elements (flange, web, lip) at stresses below Fy.
Distortional buckling of the lip-flange assembly -- a mode unique to CFS that involves rotation of the flange about the web-flange junction.
Global buckling (flexural, torsional, or flexural-torsional) of the member as a whole.

The effective width method (AISI S100 Section B2) accounts for local buckling by reducing the width of the compression elements to an effective width b_e that carries Fy at the ultimate load:

b_e = w (when lambda <= 0.673)
b_e = rho x w (when lambda > 0.673)
rho = (1 - 0.22/lambda) / lambda
lambda = sqrt(Fy / Fcr)
Fcr = k x pi^2 x E / (12(1-mu^2)) x (t / w)^2

where k is the plate buckling coefficient (4.0 for a stiffened element like a stud web, 0.43 for an unstiffened element like a stud flange), t is the base steel thickness, and w is the flat width of the element.

Worked example -- CFS stud axial capacity: 600S162-54 stud (6 in. deep, 1-5/8 in. flange, 54 mil base thickness = 0.0566 in. design thickness). Fy = 50 ksi (AISI S100, for G60 galvanized sheet). Stud height = 10 ft. Sheathing on one side (gypsum board) provides weak-axis bracing.

The effective width calculation reduces the web width from approximately 5.75 in. (flat) to an effective width of approximately 3.2 in. at Fy = 50 ksi. The resulting effective area A_e is approximately 0.270 in^2 (compared to gross area A_g = 0.535 in^2 -- roughly half the gross area is effective after local buckling).

Global buckling (flexural about the weak axis due to sheathing bracing on one side): Pn = A_e x Fn where Fn from AISI S100 Section C4.1 with KL/r about the effective axis.

For a 10 ft stud with sheathing on the flange: KL/r approximately 70-90 depending on the bracing condition. The resulting axial capacity Pn is approximately 9.5 kip per stud. At 16 in. on center, the wall provides approximately 7.1 kip per linear foot of axial capacity -- sufficient for 3-4 stories of typical floor loading above.

CFS Wall Deflection and Drift

CFS stud walls that are not part of the lateral force-resisting system (the typical case -- lateral resistance is provided by the steel moment frame, braced frame, or SPSW) must accommodate the interstory drift of the primary lateral system without damage. AISI S213 Section C5 requires:

Slip track connection at the top: The top track is a deeper section than the stud, and the stud is NOT fastened to the top track through the flanges. The stud can slide vertically within the track as the floor above deflects. This prevents the stud from being loaded axially by the interstory drift.
Drift gap at the top: A gap of at least the design story drift plus 1/2 in. is provided between the top of the stud and the underside of the track web. For a 10 ft story with 2% design drift, the drift gap is 2.4 + 0.5 = 2.9 in. The stud engages the track in bending only through friction clips at the flanges.
Bypass framing at the exterior: Exterior CFS stud walls that bypass the floor edge must be detailed with a deflection track at each floor that allows the slab edge to deflect independently of the stud.

Girt Design for Wall Cladding

Horizontal girts (CFS C-sections spanning between vertical columns) support metal panel or insulated metal panel cladding on industrial buildings and PEMBs. The girt spans horizontally, resisting wind pressure (inward) and suction (outward). The governing load case is typically wind suction on the leeward or side wall, producing bending about the strong axis plus torsion from the eccentric attachment of the cladding to the outer flange.

For a 6 in. deep, 16 ga CFS girt spanning 20 ft between columns, with wind suction of 30 psf (ASCE 7-22 components and cladding, Zone 5, corner zone): Mu = w x L^2 / 8 = (30 x 5 ft trib) x 20^2 / 8 = 150 plf x 400 / 8 = 7,500 lb-ft. Check AISI S100 Section C3.1 for the girt bending capacity including the effect of the unbraced compression flange on the suction side.

Related Tools and References

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. SPSW and CFS wall designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) for the specific project. The site operator disclaims liability for any loss arising from the use of this information. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.