Garage Layout With Unequal Diagonals
A crew is setting out a detached 8.0 m × 6.0 m concrete-block garage on level ground. The permit requires the structure to be at least 1.0 m from the side fence and 2.0 m from the front property line. The mason establishes the front-left corner using a 3-4-5 triangle and a 30 m steel tape. He measures 8.0 m along the front and 6.0 m along the left side. He then sets the other two corners by measuring 8.0 m and 6.0 m from those points with the same tape and driving stakes at the tape ends. No diagonal check is made. Overnight, a wheelbarrow hits one batter board and it leans outward. The next morning the strings are restrung, and the diagonals read 10.12 m and 9.92 m. The owner wants the slab poured the next day and asks if the layout is acceptable.
1. Calculate the diagonal that a true 8.0 m × 6.0 m rectangle should have, and interpret the two field readings against normal layout tolerance.
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For a true rectangle, diagonal = √(8.0² + 6.0²) = √100 = 10.00 m. Both diagonals should be equal to 10.00 m. The field readings are 10.12 m and 9.92 m: one is 120 mm too long, the other 80 mm too short, and they differ by 200 mm. That is not a small tape error; it means the corners are not 90°. The shape is a parallelogram or trapezoid. For a garage slab and CHB walls, a typical tolerance is ±10 mm over this size, so the layout is well outside tolerance. The leaning batter board also means the reference may have moved, but even before that, the absence of a diagonal check allowed the error. Do not pour.
2. Give the step-by-step procedure to re-establish the four corners and verify squareness before excavation.
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Re-establish from the property line. Clear the area and fix two offset reference lines (front and side) from the permit setbacks of 2.0 m and 1.0 m. Set batter boards beyond the excavation and mark a nail on each. Start from one corner and use a 3-4-5 (or 6-8-10) triangle to establish a right angle, using a tape and string line. Measure 8.0 m along the front and 6.0 m along the side from the same nail, then set the fourth corner. Adjust the stakes until both diagonals equal 10.00 m and the sides read 8.0 m and 6.0 m. Check all four corners with the 3-4-5 method, not just one. Keep the strings taut and at the same elevation; protect the batter boards. Mark the corners with durable stakes or nails and record the readings. Recheck after any disturbance and before excavation or concrete.
3. What are the consequences of pouring the slab on the current layout, and what should the mason tell the owner?
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If the slab is poured, the walls will follow the out-of-square layout: rooms will not be rectangular, precast lintels and roof framing may not fit, and the 1.0 m side clearance may be reduced on one side. Correcting after the slab is poured means chipping or demolishing concrete, which is far more expensive than resetting stakes. The mason should tell the owner that the diagonal difference is 200 mm, that the layout fails tolerance, and that the slab cannot be poured until the corners are re-established and checked. He should not hide the error with thicker plaster or wider footings. A short delay is the cheaper choice.