Chair Spacing Under a Heavily Reinforced Beam
You are fixing the bottom mat of a 600 mm deep by 400 mm wide transfer beam. The bottom mat has two layers of N32 bars at 100 mm centres, giving a heavy, stiff mat. The site has 50 mm mesh chairs rated for light slab work. The beam spans 9 m between supports and carries a column above mid-span. The concrete will be placed by pump at a high slump, and the vibrator operator is known to work quickly. The inspector has flagged that the top mat must also be held down. You have one day to fix and tie before the pour.
1. Assess whether the 50 mm mesh chairs are suitable and describe the failure mode if they are used.
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The 50 mm mesh chairs are not suitable. They are rated for light slab mesh, not for a double layer of N32 bars at 100 mm centres. The mat is heavy and stiff, and the chairs will crush, tilt, or punch into the formwork under the load.
The likely failure mode is progressive collapse of the bottom mat during the pour. High-slump concrete and aggressive vibration will liquefy the mix around the chairs, and the vibrator can dislodge individual chairs. Once a few chairs fail, the mat sags, cover is lost, and the effective depth reduces. In a 9 m transfer beam carrying a column, that is a structural issue, not a cosmetic one.
The correct action is to use heavy-duty bar chairs or continuous bar supports rated for the load, spaced to the engineer's or supplier's recommendation, and to tie the mat down so it cannot lift or roll. Document the chair type and spacing before the pour.
2. Explain how you would hold down the top mat and why this matters for a beam of this span.
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The top mat must be held down so it does not float or lift during the pour. High-slump concrete exerts buoyancy on the top bars, and vibration can displace them upward. Use tie wire to connect the top mat to the bottom mat or to the form through purpose-made hold-downs, and use top chairs or spacers sized for the bar weight.
On a 9 m transfer beam, the top mat resists negative moment over the supports and the concentrated load from the column above mid-span. If the top bars lift, the effective depth reduces and the beam's capacity drops exactly where demand is highest. The inspector's flag is therefore a structural concern, not just a tidiness issue.
Check the hold-down spacing against the supplier's data, tie every intersection in the hold-down zone, and re-check after the pour starts. If the mat moves, stop the pour and reset before continuing.