By AEDO Engineering · Updated 2 November 2026 · Code basis: NSCP 2015
On a steel covered shed AEDO designed, most of the edge footings on grid D ended up with their long side along grid D, where an earlier revision (R8) had them almost square with the long side across the frame. The program did not flip them by accident: for a footing pressed against the property line, the efficient direction to grow is along the line. Here is why, with the project's own numbers. R8, R9 and R10 below are the design revisions; F4, the corner pad at D1, stays long across the frame in all three because it also has the north fence to clear.
Edge pads F3 and F5: long side along the lineThey cannot cross the property line, so a wider pad only grows inward. That moves its centre away from the column and adds moment. Lengthening along grid D adds weight with no extra moment.
What governs them: overturning under 0.6D + 0.6WWind lifts the roof; the D columns pull kN up. Only the footing weight holds them down, and the resultant must stay within 0.45 B of the centre.
Concentric pads (F1, F2): long side follows the bigger momentR9: the across-frame moment was much larger, so F1 was long across. R10 pinned the long beams; the along-building moment rose to nearly the same, so F1 became almost square.
1What changed, R8 to R10
Bx is measured across the frame (along the numbered grid lines, perpendicular to grid D); Bz is along the building (along grid D).
R8 sized each support alone and then took the largest Bx and the largest Bz of the group as one type. That envelope made the D pads look square (2.80 × 2.75). From R9 the program picks the least common size that passes at every support of a type, which shows the efficient direction: long along grid D.
2The edge pad on the property line
Drag the sizes. The column stays where it is; the pad's east edge stays m from the column (50 mm inside the line). Every number below is recomputed with the same formula the footing program uses, for support D4 under its governing uplift case, .
Overturning across the frame e / (0.45 Bx)
Footing centre from the column, dx
Weight W (footing + soil + pedestal)
Net down force P = Fy + 0.6 W
Moment at the base M
e = M / P
Limit 0.45 Bx
Concrete in the pad
Plan, north up, the property line on the right. The orange dot is the column, the cross is the pad centre, the red arrow shows where the resultant lands (e from the centre). The dashed band is the 0.45 Bx limit. The black tick on the meter is the limit (ratio 1.0).
3The arithmetic
P = Fy + 0.6 W(Bx, Bz) M = |Mz| + |Fx H| + |dx (Fy + 0.6 Wped)| e = M / P ≤ 0.45 Bxdx = Bx/2 − a (the pad centre moves inward as the pad widens), H = Df + column base height (the shear arm to the footing underside), W = footing + soil over it + pedestal. Every moment term is added as an absolute (AEDO rule: P·e is never allowed to cancel the base moment).
Widen across by 1 m
Lengthen along grid D, same added concrete
4Code basis
203.4.2 Alternate Basic Load Combinations
In lieu of the basic load combinations specified in Section 203.4.1, structures and portions thereof shall be permitted to be designed for the most critical effects resulting from the following load combinations. 3When using these alternate basic load combinations, a one-third increase shall be permitted in allowable stresses for all combinations, including W or E.
10.6D + 0.6W + H(203-14)
20.6D + E/1.4 + H(203-15)
NSCP 2015 Vol. I, Sec. 203.4.2, Eq. 203-14 and 203-15, p. 2-11 (retyped; Eq. 203-13 and 203-16 omitted)
Eq. 203-14, 0.6D + 0.6W: only 60 % of the dead load is counted while the full service wind acts. This is the uplift and overturning combination; it governs the grid D pads (LC 101-200 are the ASD set, LC 114 here).
Eq. 203-15, 0.6D + E/1.4: the seismic twin. It governs the bearing of most pads (LC 168, 171, 183, 186) but not the overturning of the edge pads. The other pads are governed by Eq. 203-13 with E, D + 0.75L + 0.75Lr + 0.536E, which keeps the full dead load.
203.4.2 permits a one-third increase in allowable stresses with these combinations. AEDO does not take it: they are checked as basic combinations, as the parent IBC / ASCE 7 basic ASD set does.
The 0.45 B limit (resultant inside the middle 90 % of the footing, some soil contact kept) is the AEDO footing rule; bearing is checked separately against qa = kPa. There is no soil report: that is a presumptive value, which NSCP Table 304-1 reaches only for sand or gravel through its depth increase at 1.5 m, and 304.2 needs a site inspection before it is used.
5Concentric pads: the long side follows the bigger moment
A concentric pad has no property line in the way, so it can grow either way at the same cost. It grows in the direction of the larger moment, because that is where the pressure is worst. What changed at grid A between R9 and R10 is the moment along the building.
6Rules to reuse on the next job
Edge pad against a lot line: keep the short side across the line and grow along it. A corner pad has a second line to clear (F4 at D1 sits against the north fence and is shifted along grid D to clear it), so it cannot grow freely along either; size it on its own governing case. A wider pad moves away from the column and raises P·e; check overturning under 0.6D + 0.6W (203-14) and 0.6D + E/1.4 (203-15) first, bearing second.
Concentric pad: the long side goes in the direction of the larger base moment (M + V·H). If the two moments are close, a square pad is the economical answer.
Anything that releases moment along the building (pinned long beams, cantilever columns both ways) raises the along-building base moment and squares the concentric pads up. Expect it when changing the system.
Do not size a type by the envelope of separately sized pads (largest Bx with largest Bz). Use the least common size that passes at every support of the type.
If an edge pad still fails overturning when lengthened, the next steps are a deeper pad (more soil weight), a strap / combined footing to the next column, or a grade beam taking the moment back to an interior pad.
Sources
NSCP 2015 Vol. I, Sec. 203.4, Eq. 203-14 and 203-15 (p. 2-11); Table 304-1 (allowable bearing).
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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