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Strap Footing for a Column on the Property Line: Why an Ordinary Tie Beam Is Not Enough

By 8 min readUpdated

On tight lots, the outer columns often stand on the property line, against the neighbour's fence. Their footings cannot be centred, or half would sit on the neighbour's lot.

So the footing is moved inward, with the column at its edge, and a tie beam is run to the next column to "hold" it. On a small drawing, that looks like a strap footing. But an ordinary tie beam is not automatically a strap. Here is why, with one real sample checked to NSCP 2015. (A combined footing is the other common answer.)

What happens when the column sits at the edge

A footing works best when the load is at its centre. Put the column at the edge and the footing wants to turn: the side under the column presses down hard, the far side lifts off, and the footing tilts (tumatagilid) toward the property line.

The test case

This is the shipped sample of our Strap Footing Designer (NSCP 2015), footing SF-1 of a warehouse:

  • Column 1: 400 × 400 mm, PD 500 + PL 300 = 800 kN, centre 0.2 m from the property line
  • Footing F1: 2.2 m along the strap × 3.2 m × 0.6 m, starting at the property line, base 1.5 m below ground
  • Column 2 (interior): 450 × 450 mm, PD 900 + PL 600 = 1,500 kN, 5.0 m away, on a centred footing F2 of 3.2 × 3.2 × 0.7 m
  • Soil: qa = 180 kPa, 18 kN/m³. Concrete 23.5 kN/m³, f'c = 28 MPa, fy = 420 MPa
Plan of the sample strap footing SF-1 (to scale) Plan drawn to scale. The property line is at the left, with the neighbour's lot beyond it. Footing F1, 2.20 m along the strap by 3.20 m, starts at the property line. Column 1, 400 by 400 mm with 800 kN, is centred 0.20 m from the line, which is 0.90 m from the centre of F1. A strap 450 mm wide by 850 mm deep runs to column 2, 450 by 450 mm with 1,500 kN, 5.00 m away, on a centred footing F2 of 3.20 by 3.20 m. Sample SF-1: plan, to scale neighbour's lot property line F1 2.20 × 3.20 m F2 3.20 × 3.20 m strap 450 × 850 mm column 1, 400 × 400 800 kN column 2, 450 × 450 1,500 kN 0.90 m + centre of F1 2.20 m 3.20 m 0.20 m 5.00 m between the columns 3.20 m
Figure 1. Plan of the sample SF-1, to scale. F1 (2.20 × 3.20 m) starts at the property line, column 1 is centred 0.20 m from the line and 0.90 m from the centre of F1, and a 450 × 850 mm strap runs 5.00 m to column 2 on F2 (3.20 × 3.20 m).

First, the footing alone

The sheet always includes a strap, so I checked F1 alone by hand:

  • Footing and soil on it: 2.2 × 3.2 × (0.6 × 23.5 + 0.9 × 18) = 213 kN, so N = 800 + 213 = 1,013 kN
  • The column is 1.1 − 0.2 = 0.9 m from the footing centre: M = 800 × 0.9 = 720 kN·m
  • Eccentricity of N: eN = 720 / 1,013 = 0.711 m, more than L/6 = 0.37 m, so part of the footing lifts off
  • L/2 − eN = 1.1 − 0.711 = 0.389 m, so the contact length is 3 × 0.389 = 1.17 m of the 2.2 m
  • q,max = 2N / [3B (L/2 − eN)] = 2 × 1,013 / (3 × 3.2 × 0.389) = 542 kPa

That is 3.0 times the 180 kPa allowed. In this sample, only about half the footing touches the soil, and that half is overloaded.

Footing F1 alone, without a strap: section and soil pressure (to scale) Section drawn to scale of footing F1, 2.20 m long and 0.6 m thick with 0.9 m of soil on it, starting at the property line. Column 1 carries 800 kN at the edge, 0.90 m from the footing centre. The soil pressure under the footing is a triangle: 542 kPa at the property-line edge, falling to zero 1.17 m from it. The rest of the 2.20 m footing lifts off the soil. The allowable pressure is 180 kPa, so the edge pressure is 3.0 times the allowed. Hand calculation. Footing F1 alone, no strap (section, to scale) 180 kPa allowed property line neighbour's lot 800 kN column 1, 400 × 400 ground 0.90 m centre of F1 0.9 m of soil on the footing F1, 0.6 m thick 542 kPa 1.17 m on the soil rest lifts off F1 = 2.20 m EDGE PRESSURE 542 kPa allowed 180 FAIL 3.0 times the allowed. N = 800 + 213 = 1,013 kN acts 0.711 m from the footing centre: more than L/6 = 0.37 m, so part of the footing lifts off. Hand calculation.
Figure 2. F1 alone, to scale. With 800 kN at the edge, only 1.17 m of the 2.2 m footing presses on the soil, and the pressure at the property line reaches 542 kPa against 180 kPa allowed.

What a strap does

A strap is a stiff, heavy beam from the edge footing to the next interior footing. It does not rest on the soil. It works like a lever: column 1 pushes down at one end, column 2 holds the other end down, and the soil under F1 pushes up in between. Two effects are easy to miss.

1. The edge footing carries more than its column. Taking moments about column 2:

R1 = (P1 · s + Ws · s/2) / (s − e)

Here s = 5.0 m between the columns, e = 0.9 m is the distance from column 1 to the centre of F1, and Ws = 44.9 kN is the strap's weight (23.5 × 0.45 × 0.85 = 8.99 kN/m, times 5.0 m):

R1 = (800 × 5.0 + 44.9 × 2.5) / (5.0 − 0.9) = 4,112 / 4.1 = 1,003 kN

F1 is sized for 1,003 kN, not 800.

2. The strap bends with tension at the top. The soil pushes it up while both ends are held down, like a plank pushed up in the middle. So the main bars belong at the top.

Case A: an ordinary tie beam as the strap

Same footings and loads, with a tie beam 250 × 400 mm, 3-Ø16 top and bottom, 2-leg Ø10 stirrups @ 200 mm as the strap.

Check (NSCP 2015) Demand Capacity Result
Bending, top bars (φMn ≥ Mu−)755.0 kN·m67.7 kN·mFAIL (ratio 11.1)
Shear at the face of column 1839.4 kN132.4 kNFAIL (ratio 6.3)
Stirrup spacing200 mm provided159 mm allowed (d/2)FAIL

Overall: REVISE FOOTING.

The sheet's required-steel line reads "too thin": more bars alone will not fix it. Bigger ties also failed in bending: 300 × 500 with 4-Ø20 top had a demand 4.2 times its capacity, and 350 × 600 with 4-Ø25 top 2.2 times.

The soil check under F1 still passes (139.6 kPa against 149.7) only because the sheet assumes the strap does its job. The strap lines say it cannot.

Case B: the sample strap (gravity loads)

The sheet's own sample, unchanged: strap 450 × 850 mm, top 6-Ø28, bottom 4-Ø20, 4-leg Ø12 stirrups @ 150 mm.

An ordinary tie beam and the sample strap: sections at the same scale Two beam cross-sections drawn at the same scale. Case A, the ordinary tie beam: 250 by 400 mm, 3 bars of 16 mm at the top and 3 at the bottom, 2-leg 10 mm stirrups at 200 mm. Its top bars can carry 67.7 kN m against 755.0 kN m needed: FAIL, ratio 11.1. Case B, the sample strap: 450 by 850 mm, 6 bars of 28 mm at the top in two layers (4 + 2), 4 bars of 20 mm at the bottom, 4-leg 12 mm stirrups at 150 mm. Its top bars can carry 955.1 kN m against 746.1 kN m needed: PASS, ratio 0.78. Tie beam and strap, drawn at the same scale Case A: ordinary tie beam 250 × 400 mm 3-Ø16 top 2-leg Ø10 @ 200 3-Ø16 bottom 400 250 Case B: sample strap 450 × 850 mm top 6-Ø28 two layers, 4 + 2 4-leg Ø12 @ 150 mm bottom 4-Ø20 850 450 Bending at the top bars, kN·m: what the strap must carry (Mu−) and what the section can carry (φMn) Mu− 755.0 φMn 67.7 FAIL · ratio 11.1 Mu− 746.1 φMn 955.1 PASS · ratio 0.78
Figure 3. The two sections at the same scale. The 250 × 400 tie beam with 3-Ø16 top bars can carry 67.7 kN·m against 755.0 needed. The 450 × 850 strap with 6-Ø28 top bars in two layers (4 + 2) can carry 955.1 kN·m against 746.1.
Check Demand Capacity Result
Bending, top bars746.1 kN·m955.1 kN·mPASS (0.78)
Shear at the face of column 1836.1 kN948.2 kNPASS (0.88)
Top bars, straight anchorage at the property-line endld = 1,700 mm1,735 mm availablePASS (0.98), tight
Soil under F1142.5 kPaq,net 149.7 kPaPASS (0.95)
F2 stays down with the live load off column 2R2 > 0741.9 kNPASS

Overall: FOOTING ADEQUATE.

The anchorage row assumes 50 mm end cover. If that face is cast against earth (75 mm), the room is 1,710 mm: still enough, but only just. So I would hook the bars there.

A short hand check:

q,net = 180 − 23.5 × 0.6 − 18 × 0.9 = 149.7 kPa
q under F1 = 1,003 / (2.2 × 3.2) = 142.5 kPa ≤ 149.7, OK

For the strap, Pu1 = 1.2 × 500 + 1.6 × 300 = 1,080 kN (405.3.1). The same lever gives a factored soil force of 1,355.4 kN under F1, or 616.1 kN per metre of its 2.2 m. The strap weight is taken × 1.4 = 12.58 kN/m (the sheet's own conservative assumption; 1.2D + 1.6L would use 1.2). The largest moment, inside F1, is where the shear is zero, x0 = 1.785 m from the property line, which is 1.585 m from the centre of column 1:

Mu− = 1,080 × 1.585 + 12.58 × 1.585² / 2 − 616.1 × 1.785² / 2 ≈ 746 kN·m

Strap of the sample (case B): loads, shear and moment along the strap Three diagrams on the same horizontal scale, x measured from the property line. Loads: column 1 pushes down with Pu1 = 1,080 kN at x = 0.2 m, the soil under F1 pushes up with 616.1 kN per metre from x = 0 to 2.2 m, the strap weight of 12.58 kN per metre acts down, and column 2 holds the far end down at x = 5.2 m. Shear: 123 kN just left of column 1, dropping to minus 957 kN just right of it, minus 836.1 kN at the face of column 1, zero at x0 = 1.785 m, 250.3 kN at the inner edge of F1 and 212.5 kN at column 2. Moment: 12.3 kN m with tension at the bottom under column 1, then tension at the top all the way, largest at x0 with Mu minus = 746.1 kN m, and zero at column 2. The sample strap: loads, shear and moment (factored) All three share one scale along the strap. x is measured from the property line. Pu1 = 1,080 kN column 2 holds this end down soil under F1 pushes up: 616.1 kN per metre strap weight, down: 12.58 kN per metre F1, 2.2 m Shear V, kN 123 −957 −836.1 at the face of column 1 V = 0 at x0 = 1.785 m 250.3 212.5 Moment M, kN·m (drawn on the tension side) Mu− = 746.1 tension at the TOP of the strap 12.3 under column 1 (tension at the bottom) M = 0 0 0.2 1.785 2.2 5.2 m 0 property line · 0.2 column 1 · 1.785 zero shear · 2.2 inner edge of F1 · 5.2 column 2
Figure 4. Loads, shear and moment along the sample strap. The shear is zero 1.785 m from the property line, inside F1, and that is where the moment is largest: 746.1 kN·m with tension at the top. Under column 1 the bottom moment is only 12.3 kN·m.

For the top bars, a = 3,695 × 420 / (0.85 × 28 × 450) = 144.9 mm, so φMn = 0.9 × 3,695 × 420 × (756.3 − 144.9/2) ≈ 955 kN·m.

The lesson

In both cases the strap has to carry about 750 kN·m. The tie beam can carry 67.7. Two more things stand out:

  • The main bars are at the top. The bottom moment is only 12.3 kN·m.
  • Column 2's load holds the strap down, so the sheet checks it with the live load off (R2 = 741.9 kN, still down).
Crops from the sheet for the two runs, with the columns in between left out. Top, the tie beam: strap flexure 755.0 against 67.7, FAIL, REVISE FOOTING. Bottom, the sample strap: 746.1 against 955.1, ratio 0.78, PASS, FOOTING ADEQUATE.
Figure 5. Crops from the sheet for the two runs, with the columns in between left out. Top, the tie beam: strap flexure 755.0 against 67.7, FAIL, REVISE FOOTING. Bottom, the sample strap: 746.1 against 955.1, ratio 0.78, PASS, FOOTING ADEQUATE.

The method assumes a rigid strap, so the soil pressure under F1 stays even. Here the sheet flags the strap-to-footing stiffness ratio as a note for the engineer. A flexible strap lets F1 tilt and moves pressure toward the property line.

This is also a gravity-only check, with no column moments. In the Philippines the engineer of record must also check the strap and footings for the earthquake load combinations of NSCP 2015 Chapter 2, and for settlement.

What to check on site

  1. Before digging, check the neighbour's side (fence, wall, footing). A 1.5 m pit beside it may need shoring or underpinning, as the engineer of record directs. Do not leave it open long.
  2. F1 starts at the property line and does not cross it, with the column at its edge.
  3. The strap has the size on the plans (here 450 × 850), not a standard tie beam size.
  4. Main bars at the top, in the layers shown (here 4 + 2, with spacer bars between), hooked at the property-line end, with no laps near column 1 unless the plans allow.
  5. Closed stirrups with the legs and spacing on the plans (here 4 legs of 12 mm @ 150 mm).
  6. The strap does not bear on the soil. Leave the void or compressible fill under it as detailed.
  7. In F1, the main bottom bars run perpendicular to the strap (along the 3.2 m side): F1 bends as two cantilevers off it.
  8. If the site does not match the plans, stop and ask the engineer of record.

A note on blame

This is not about any one trade. On a small plan, a strap and a tie beam look the same: a line between two footings. A clear strap section on the plans fixes that.

General information only. Use the soil data from your own geotechnical investigation and follow the approved plans and the engineer of record.