Engineering Assumptions
Every assumption, on one page.
OmniCFS applies the assumptions below to every calculation. Items marked Conservative are deliberately conservative — the tool errs on the safe side of the standard. Items marked Assumed are defaults a PE may override in the designer.
Structural framing
Conservative
Simply supported studs
Wall studs are modeled as pin-pin members with effective length factor K = 1.0 for weak-axis buckling.
Ref: AISI S100-16 §E2
Conservative
Uniform wind loading
Wind pressure is applied as a uniform lateral load across the full wall height. Actual pressure may vary per ASCE 7 provisions.
Ref: ASCE 7-22 Ch. 30
Assumed
Platform framing
Floor-to-floor construction with joists bearing on top track. Eccentric moment from platform offset is included when enabled.
Ref: AISI S240 §B5
Conservative
Bracing at 48 in o.c. (default)
Studs are assumed braced against weak-axis buckling at 48-inch intervals by horizontal bridging or structural sheathing.
Ref: AISI S100-16 §C2
Conservative
No composite action with sheathing
Sheathing is not credited for axial or bending capacity. Only framing members resist structural loads.
Conservative
Back-to-back bending
B2B (paired) studs use 2× single-section Mn for bending. No composite bending action is credited between paired members.
Ref: AISI S100-16 §D1.1
Assumed
Blocking reduces unbraced length
Mid-height blocking (0, 1, or 2 rows) reduces unbraced length for compression. The engine tries blocking before upsizing a section.
Ref: AISI S100-16 §C2
Material properties
Assumed
Yield strength by gauge
Fy is derived from steel gauge: 33 ksi (20/18 ga), 50 ksi (16/14 ga), 50 ksi (12 ga). Matches common CFS steel grades.
Ref: SSMA Product Technical Guide
Assumed
Modulus of elasticity
E = 29,500 ksi for all CFS members per AISI specification.
Ref: AISI S100-16 §A2
Assumed
Poisson's ratio
ν = 0.30 for steel.
Ref: AISI S100-16 §A2
Conservative
No strain hardening credit
Design uses nominal yield strength without accounting for strain hardening in cold-formed corners.
Assumed
Steel headers — A992 Fy = 50 ksi
W-shape steel headers use ASTM A992 with Fy = 50 ksi. Selected automatically when CFS headers are exhausted.
Ref: AISC 360-22
Assumed
Steel posts — A500 Grade C Fy = 46 ksi
HSS tube columns use ASTM A500 Grade C with Fy = 46 ksi. Selected automatically when quadrupled CFS studs fail.
Ref: AISC 360-22
Loading
Assumed
LRFD load combinations
All design checks use LRFD load combinations per ASCE 7-22 §2.3. ASD is available as an alternative.
Ref: ASCE 7-22 §2.3
Conservative
Wind exposure B default
Default Exposure B (urban/suburban, typical for residential CFS). Configurable to C or D in the Building Designer. Height-variable Kz per ASCE 7-22 Table 26.10-1 with linear interpolation. Topographic factor Kzt defaults to 1.0 (flat terrain).
Ref: ASCE 7-22 §26.7 / §26.8
Assumed
Risk Category default
Default Risk Category II with importance factor Ie = 1.0. Configurable to I, III, or IV — Ie and the Table 12.12-1 drift limit adjust accordingly.
Ref: ASCE 7-22 Table 1.5-1
Conservative
Snow load = ground snow
Roof snow load uses ground snow without drift or sliding reductions (conservative for simple roofs).
Ref: ASCE 7-22 Ch. 7
Assumed
Wall self-weight default
Default wall self-weight is 8 psf (gypsum both sides). Adjustable per story in multi-story mode.
Ref: Typical practice
Assumed
Soil bearing capacity default
Presumptive soil bearing defaults to 1,500 psf (sandy gravel). Adjustable from 1,000 to 4,000 psf. Uses unfactored service loads.
Ref: IBC Table 1806.2
Assumed
Site Class D default
Seismic site class defaults to D (stiff soil). Configurable to A/B/C/E per ASCE 7-22 Tables 11.4-1 + 11.4-2. Site Class F triggers a site-specific referral per §11.4.8.
Ref: ASCE 7-22 Ch. 20
Connections
Assumed
Two screws per end
Standard stud-to-track connection uses two #8 self-drilling screws per end.
Ref: AISI S240 §D1
Assumed
Screw shear capacity
0.45 kips per #8 screw (ASD) with bearing and tilting checks per AISI S100.
Ref: AISI S100-16 §J4
Assumed
Hold-down from Simpson catalog
Shear-wall hold-downs use Simpson Strong-Tie HDU-series. Capacity from manufacturer catalog.
Ref: Simpson HTT / HDU catalog
Fire rating & seismic detailing
Assumed
UL assemblies
Fire-rated assemblies reference UL Design Numbers. Minimum stud depth and spacing per assembly listing.
Ref: UL Fire Resistance Directory
Assumed
Seismic detailing scope
Seismic Design Category D and above requires special detailing per AISI S400. SDC A–C uses standard framing.
Ref: AISI S400-20
Assumed
Cd for ordinary CFS
Seismic drift amplification uses Cd = 2.5 per ASCE 7-22 Table 12.2-1 for ordinary CFS bearing-wall systems (R = 2.5).
Ref: ASCE 7-22 Table 12.2-1
Lateral systems & drift
Conservative
Shear-wall stiffness — panel systems
Wall stiffness uses the prescriptive AISI S400-20 model for OSB/wood structural panels and the §E2.4.1.4 steel-sheet model for steel sheet sheathing. Gypsum/fiberboard §E6 stiffness is flagged for PE rational analysis.
Ref: AISI S400-20 §E1.4.1.4 / §E2.4.1.4
Conservative
Hold-down deformation da = 0.125 in
Conservative default hold-down deformation for the fourth stiffness term. Typical of Simpson HTT5 at design load.
Ref: AISI S400-20 §E1.3.1
Assumed
Rigid diaphragm stiffness aggregation
Shear walls in parallel are summed directly (k_total = Σk_i). Left + Right walls resist N-S loads, Front + Back resist E-W loads.
Conservative
Drift is advisory
Story-drift exceedance is reported for PE review but does not set StoryPasses = false by itself. Conservative stiffness assumptions systematically over-predict drift.
Assumed
Story drift vs stud deflection
Lateral story displacement (H/400) is distinct from wall stud bending deflection (L/240). Both are computed and reported independently.
Ref: IBC §1604.3 / AISI S100
Assumed
Flexible diaphragm
Diaphragm chord forces use simple-beam analogy. Aspect ratio L/W must be ≤ 4:1 per AISI S240 §E1.2.
Ref: AISI S240-20 §E1.2
Conservative
Chord stud carries gravity + chord force
Perimeter studs resist combined gravity axial and diaphragm chord tension / compression. Checked via AISI S100 §H1.2 interaction.
Ref: AISI S100-16 §H1.2
Conservative
Drag strut = top track
Drag-strut capacity assumes the top-track section as the drag-strut member. Tension capacity φPt = 0.90 × Ag × Fy.
Ref: AISI S100-16 §C2
Assumed
P-Delta amplification
When the stability coefficient 0.10 < θ ≤ 0.25, story shear is amplified by 1/(1 − θ) and shear walls are re-verified against the amplified demand.
Ref: ASCE 7-22 §12.8.7
Foundations
Assumed
Strip footing design
Continuous strip footings designed for gravity loads using ACI 318-19 flexure and one-way shear provisions.
Ref: ACI 318-19 §9.5, §22.5
Assumed
3 in concrete cover
Concrete cover = 3.0 in per ACI 318-19 §20.6.1.3 for concrete cast directly against earth.
Ref: ACI 318-19 §20.6.1.3
Assumed
Effective depth
Effective depth d = h − 3 in − bar/2, recalculated for each (footing depth, bar size) combination during iteration.
Ref: ACI 318-19
Assumed
Minimum rebar
Minimum steel uses the structural-beam provision As,min = max(0.25 √f'c / fy × bw × d, 200 / fy × bw × d). Uses effective depth d, not total h.
Ref: ACI 318-19 §9.6.1.2(a)
Assumed
Rebar candidates
Footing design iterates through #3, #4, #5 bars at 6 in / 8 in / 10 in / 12 in spacing and selects the smallest adequate combination.
Conservative
SDC C/D seismic footing scope
Bounded residential strip footings in Seismic Design Category C/D include an automated ACI 318 §18.13 detailing package when the checks pass. SDC E/F and outside-bounds cases require a licensed structural engineer.
Ref: ACI 318-19 §18.13
Assumed
Default f'c = 3,000 psi
Concrete compressive strength defaults to 3,000 psi (common for residential strip footings). Configurable up to 5,000 psi.
Ref: ACI 318-19
Section properties
Conservative
Sharp-corner model
Gross section properties calculated using centerline dimensions with the sharp-corner model. Conservative vs. actual rounded corners.
Ref: AISI S100-16 §A1.3
Assumed
Effective width per DSM
Effective section properties use Direct Strength Method provisions for local and distortional buckling.
Ref: AISI S100-16 §1.1
Assumed
Standard lip = 0.5 in
Default lip depth = 0.5 in when not specified. Matches common retail-grade studs.
Ref: SSMA
Manual override available
Enterprise users can override many of these defaults directly in the Building Designer. Overridden values are flagged in the calculation trace and the permit packet so any PE reviewing the output sees the override explicitly.