By AEDO Engineering · Updated 2 November 2026 · Code basis: NSCP 2015
How NSCP decides whether a covered court or shed is a cantilevered column system (R = 2.2), when the roof counts as a diaphragm, and why that brings in accidental torsion. Worked on a real AEDO design, a single-bay steel covered shed in Seismic Zone 4, with the numbers from its own analysis model.
Covered courts: usually yesTrusses that sit on a bolted seat at the column top pass shear but almost no moment. The columns are true cantilevers and R 2.2 is right.
The worked shed, first scheme: noThe rafter and tie joints were moment end plates. The model showed the column top carrying about as much moment as the base, so it behaved as a portal frame. The issued design seats the truss on the columns instead (section 6).
Pinned-base portal: special frames onlyPin the base and the knee carries all the moment. NSCP puts that under moment frames, where Zone 4 allows only the special moment frame (R 8). Small footings, but full AISC 341 detailing.
The roof is a diaphragmPurlins, rods, rafters and long beams tie the frames together. The roof bends far less than twice the drift, so NSCP treats it as not flexible and accidental torsion applies.
1Two ways a frame resists sway
Push the roof sideways (wind or earthquake) and the columns must stop it. What decides the behaviour is the joint at the top of the column.
Pinned top (court). The truss rests on a seat plate and is bolted down. It can slide the column top sideways but cannot twist it, so the column bends in one curve like a flagpole. The moment is zero at the top and largest at the footing: Mbase = V·h.
Rigid top (portal). A moment end plate (or knee brace, or a column that runs into the rafter) locks the column top's rotation. The column bends in an S shape with moment at both ends, roughly Mbase ≈ Mtop ≈ V·h/2. It is stiffer, but the joint now carries moment and must be detailed for it.
Pinned base, rigid knee (classic portal shed). Turn it around: let the base rotate (a pin, or a semi-pinned plate with the anchors close to the column centre) and keep the knee rigid. Now the base passes only axial load and shear, and all the sway moment goes to the knee: Mbase = 0, Mknee = V·h. Footings and anchors become small, but the column and rafter need more stiffness at the knee (a deeper section, a haunch or a tapered column), and the frame sways more.
Deflection exaggerated. Orange = bending moment in the columns.
Left: truss on pinned seats, columns cantilever from the footing, moment only at the base. Middle: the shed as first drawn, rafter and tie fixed to the column with moment plates, fixed base, moment at base and top. Right: pinned base with a rigid, haunched knee, moment only at the knee. Same lateral force on all three.
2What NSCP says
The definition (Sec. 202, p. 2-5)
CANTILEVERED COLUMN ELEMENT is a column element in a lateral-force-resisting system that 2cantilevers from a fixed base and has 1minimal moment capacity at the top, with lateral forces applied essentially at the top.
NSCP 2015 Vol. I, Sec. 202 Definitions, p. 2-5 (retyped)
"minimal moment capacity at the top" is the test. A seated, bolted truss passes this; a 4-bolt extended end plate does not, because it is designed to carry moment.
"cantilevers from a fixed base": the footing, pedestal and anchors must hold the full column moment. That is why the base plate, anchors and pedestal of a court are the heavy part.
The system table (Table 208-11B, row F, p. 2-225)
Basic seismic-force resisting system
R
Ω0
System limitation and building height limitation by seismic zone, m
Zone 2
Zone 4
F. Cantilevered Column Building Systems
2Special steel moment frames
2.2
2.0
10
10
Intermediate steel moment frames
1.2
2.0
10
3NP
Ordinary steel moment frames
1.0
2.0
10
NP
1Cantilevered column elements
2.2
2.0
NL
10
NSCP 2015 Vol. I, Table 208-11B, section F, p. 2-225 (excerpt, retyped). NL = no limit, NP = not permitted.
COURTSCantilevered column elements: R 2.2, Ω0 2.0, no height limit in Zone 2, 10 m in Zone 4. This is the row for seated trusses.
RIGID TOPSpecial steel moment frames used as cantilever columns: the same R 2.2 but the frame must meet AISC 341 special moment frame detailing.
NOT ALLOWED Intermediate (R 1.2) and ordinary (R 1.0) moment frames are NP (not permitted) in Zone 4. A portal with ordinary end plates has no row of its own here.
V = 2.5 Ca I W / R (upper limit, governs a short-period shed)
With Ca = , I = 1.0, R = : V/W = 2.5 × / = . R 2.2 is the lowest R in the table, so it gives the largest seismic force. Choosing it is safe for force, but the label must still match how the joints are built.
The pinned-base portal (Table 208-11B, section C, p. 2-224)
A portal with pinned or semi-pinned bases has no cantilever action at all; it stands only because the knee is rigid. NSCP therefore files it under moment-resisting frame systems, not under row F.
Basic seismic-force resisting system
R
Ω0
System limitation and building height limitation by seismic zone, m
Zone 2
Zone 4
C. Moment-Resisting Frame Systems
1Special moment-resisting frame (SMRF)
38.0
3.0
NL
NL
Intermediate steel moment frames (IMF)
4.5
3.0
NL
2NP
Ordinary moment frames (OMF)
3.5
3.0
NL
NP
Special truss moment frames (STMF)
6.5
3.0
NL
NP
NSCP 2015 Vol. I, Table 208-11B, section C, p. 2-224 (excerpt, steel rows, retyped). NL = no limit, NP = not permitted.
ALLOWEDSpecial moment-resisting frame (SMRF): no height limit in Zone 4. This is the only steel moment frame a pinned-base portal can be in Zone 4.
NOT ALLOWED Intermediate, ordinary and special truss moment frames are NP in Zone 4, with no footnote or single-storey exception on this page.
R 8.0 and Ω0 3.0: the seismic force is far smaller than with R 2.2, but every member and knee joint must follow the AISC 341 special moment frame rules.
V = 2.5 Ca I W / R with R = 8.0
V/W = 2.5 × / 8.0 = , against with R 2.2: about times less seismic force. Seismic drift is amplified more (ΔM = 0.7 R ΔS = 5.6 ΔS), and a pinned base is far more flexible, so drift and wind usually govern the frame.
What changes
Cantilever columns (courts)
Fixed base + moment joints (first scheme)
Pinned base + rigid knee
Where the sway moment goes
base only
base and column top
knee only
NSCP system (Zone 4)
row F, cantilevered column elements
row F, special moment frame cantilever row
section C, special moment-resisting frame
R / Ω0 / height
2.2 / 2.0 / 10 m
2.2 / 2.0 / 10 m
8.0 / 3.0 / no limit
Seismic V / W
0.50 (R 2.2)
0.14 (R 8)
Footing, pedestal, anchors
heaviest (full base moment)
heavy (part of the moment)
light (axial, shear, uplift)
Columns and rafters
column sized at the base
shared
deeper at the knee: haunch or tapered column
Joint detailing
simple bolted seat
AISC 341 special moment frame
AISC 341 special moment frame + AISC 358 prequalified knee
Drift
moderate
stiffest
most flexible
Qualitative comparison of the three systems for a single-bay covered shed or court in Zone 4. Section 6 compares the options on the same shed.
3Evidence from the first-scheme model
If the first scheme had been a true cantilever system, the column moment would fall to about zero at the top. The STAAD model of that scheme says otherwise. The plots show the bending moment up a middle column (frame 3) on each side, for a seismic combination and for gravity alone. Positive moment is drawn to the right.
Orange: LC 252 = 1.42D + 0.5L + E (+X, with torsion factor). Blue dashed: LC 201 = 1.4D. Values in kN·m read from the STAAD member end forces (internal moment = −M at the start node, +M at the end node). The jump at the tie level is the moment the tie beam delivers into the column.
4Is the roof a diaphragm?
A diaphragm is the horizontal element that collects the lateral load and shares it between the frames. A concrete slab is the obvious case, but a steel roof with purlins, X-rod bracing, rafters and long beams also does it, just more softly. NSCP does not ask whether the roof is "solid"; it asks how much it bends compared with how much the columns sway.
208.5.1.3 Horizontal Distribution of Shear
[…] 1Where diaphragms are not flexible, the mass at each level shall be assumed to be displaced from the calculated center of mass in each direction a distance equal to 5 percent of the building dimension at that level perpendicular to the direction of the force under consideration. The effect of this displacement on the storey shear distribution shall be considered.
2Diaphragms shall be considered flexible for the purposes of distribution of storey shear and torsional moment when the maximum lateral deformation of the diaphragm is more than two times the average storey drift of the associated storey. This may be determined by comparing the computed midpoint in-plane deflection of the diaphragm itself under lateral load with the storey drift of adjoining vertical-resisting elements under equivalent tributary lateral load.
NSCP 2015 Vol. I, Sec. 208.5.1.3, p. 2-213 (excerpt, retyped)
The 5 percent rule. If the diaphragm is not flexible, move the mass 5% of the building length sideways and design for the twist this causes (accidental torsion).
The flexibility test. Flexible only if the roof's own in-plane bending exceeds 2 × the average drift of the frames below it.
Flexible ⇔ δdiaphragm > 2 Δavgδdiaphragm = how far a middle frame moves beyond the straight line joining the two end frames. Δavg = average sway of the frames. Both from the STAAD column-top displacements under E (+X).
3D view of the six frames under E (+X), drift exaggerated. Numbers are the column-top drifts in mm from STAAD, grid A and grid D. The dashed line joins the two end frames; the orange ticks are how far each frame strays from it, which is the roof's own bending δ. Rigid roof puts every frame on the line. Flexible roof is an illustration, not this shed: only the end bays are braced, so they are the lateral system and barely move, while the roof spans between them like a beam and carries the middle frames out. Now δ passes 2 Δavg (the end frames' drift) and NSCP calls the roof flexible. With all six frames resisting load, as here, bulging the middle also raises the average drift, which is why a frame shed like this almost never comes out flexible.Each dot is the average column-top sway of one frame (grid A and D). The dashed line joins the end frames; a rigid roof would keep every frame on it. The orange bars are the diaphragm's own bending.
Column-top drift, every frame
From the STAAD joint displacements, mm. Frame 6 grid A stands at x = m because that roof corner is chamfered.
208.5.1.4 Horizontal Torsional Moments
1Provisions shall be made for the increased shears resulting from horizontal torsion where diaphragms are not flexible. The most severe load combination for each element shall be considered for design.
The torsional design moment at a given storey shall be the moment resulting from eccentricities between applied design lateral forces at levels above that storey and the vertical-resisting elements in that storey plus an accidental torsion.
2The accidental torsional moment shall be determined by assuming the mass is displaced as required by Section 208.5.1.3.
NSCP 2015 Vol. I, Sec. 208.5.1.4, p. 2-213 (retyped)
Required where the diaphragm is not flexible, which section 4 just showed.
Accidental torsion = mass displaced per 208.5.1.3 (the 5% offset).
5Accidental torsion, as applied
STAAD's seismic generator ignores the accidental-torsion setting unless the model has a rigid floor diaphragm, and the warning only appears in the output file. So the torsion was applied by hand: shift the seismic force by e = 5% of the length, turn it into a torque T = V·e, and share the torque between the frames in proportion to their distance c from the centre.
extra share of frame i = V · e · ci / Σc2 ⇒ amplification = 1 + e · cmax · n / Σc2
The animation runs in four steps: (1) V acts at the centre of mass and every frame sways the same; (2) the mass shifts 5% of the length; (3) the off-centre force makes a torque T = V·e and the roof twists; (4) each frame's share of V, from the formula above. Frames past the centre take more, frames before it less. The end-frame amplification was applied to every frame in the design, which is conservative. Sway is schematic, not to scale.
6The options, and what we chose
Adopted
A. Make it a true cantilever system
Change the rafter-to-column and tie-to-column joints to seated, shear-only joints, as on the covered courts. The rafter, tie and posts act as a truss resting on the columns.
R 2.2 becomes genuinely correct, no special moment frame detailing.
Simpler joints to fabricate and erect.
Column base moments rise without portal action: the columns, anchors and footings were re-checked in the model and resized.
Alternative
B. Keep the moment joints, detail as special moment frame
Keep the four-bolt extended end plates and classify as Table 208-11B row F "special steel moment frames" (R 2.2, 10 m).
Heavier connections and more checks; the current W10X33 rafters and W10X12 eave beams do not qualify as highly ductile (see the note under the table).
Tested, see table
C. Pinned-base portal, rigid haunched knee
Pin (or semi-pin) the column bases and keep a rigid knee, as in a classic portal shed. Section C special moment-resisting frame, R 8, with braced bays along the shed (R 6).
Seismic force about 3.6 times smaller; in the model, wind governed its members.
Footings, pedestals and anchors shrink: the grid D edge pads lose most of their overturning.
Knee needs a haunch or deeper sections; full AISC 341 / AISC 358 special moment frame detailing.
7Sources
NSCP 2015 Vol. I: Sec. 202 definition "Cantilevered column element" (p. 2-5); Table 208-11B sections C and F (pp. 2-224, 2-225); Sec. 208.5.1.3 and 208.5.1.4 (p. 2-213); Eq. 208-9 upper-limit base shear.
AEDO STAAD.Pro model of the shed, first scheme: column-top joint displacements (cases E+X, E+Z) and member end forces (LC 201, 252).
Geometry, seismic coefficients and torsion factors from the same design.
Want this checked on your own building?
The AEDO team runs these checks on every structure we design and seal: the system label against the code table, the
diaphragm test, accidental torsion, and every footing against overturning. Send us your plans before you file for the permit.
An explanation of how a real AEDO design was checked, for learning. Code text is quoted in short excerpts
for commentary, each with its NSCP 2015 section and page; read the full clause in your own copy of the code. It is not a
design for your building: every structure needs its own analysis by a licensed engineer.
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