A shoring layout built on guesswork is a risk no project should take. Adjustable shoring props carry the weight of freshly poured concrete, the formwork above it, and the workers and equipment moving across the slab—until the concrete cures and can support itself. If the load is underestimated, props can buckle or settle; if it is grossly overestimated, you waste money on unnecessary material. This guide walks through how to calculate the load on adjustable shoring props in clear, practical steps, so you can spec the right prop for the job.
What Are Adjustable Shoring Props?
Adjustable shoring props, also called shoring props, acrow props or slab props, are height-adjustable steel supports used in temporary shoring systems. Each prop is built from an inner tube that slides inside an outer tube, a top plate that connects to the formwork, a base plate that stands on the floor, a prop nut for fine height adjustment, and lock pins that hold the prop at the required height. The top plate is often fitted with a U-head jack or fork head so it can seat against a formwork beam cleanly.
Because their height can be changed quickly, they suit projects with several slab or beam elevations on the same site. They are typically rated as light, medium or heavy duty, and they are often used together with modular systems such as ringlock scaffolding to build a complete, stable shoring tower.
What Actually Affects the Load a Prop Can Carry?
Before you calculate the load, you need to understand the three things that decide how much an adjustable shoring prop can safely carry.
1. Working Height and Extension
The most important factor is the unsupported, or effective, length of the prop. As a prop is extended, its resistance to buckling drops quickly. The same prop that handles a heavy load near its minimum height can carry far less at full extension. This is why a single “maximum load” on a datasheet is misleading—you need the safe working load at your actual working height, not just the best-case number.
2. Steel Grade and Tube Thickness
The material and the wall thickness of the tubes set the strength of the prop. Common structural steels such as Q235 and Q355 are used for construction props, and a small difference in tube wall thickness can make a noticeable difference to both buckling resistance and service life. A reputable supplier should be able to tell you the exact steel grade and wall thickness of the prop you are buying.
3. Manufacturing Quality and Accessories
Strength is not only about the main tube. Poorly welded base plates, rough adjustment threads, weak lock pins, misaligned holes or rust all reduce the real capacity of a prop under load. Buy from a manufacturer that can show consistent welding quality and proper surface treatment, such as painted, electro-galvanized or hot-dip galvanized finishes, so the prop lasts on site.
How to Calculate the Load: Step by Step
The calculation follows a simple logic: find the force acting on one square metre of slab, decide the prop spacing, then check each prop has enough safe working load with a safety margin. Here is the process.
Step 1: Calculate the Dead Load
The dead load is the permanent weight pressing down on the shoring. It includes the self-weight of the concrete slab and the weight of the formwork itself.
- Slab self-weight = unit weight of concrete × slab thickness
- Formwork weight = the weight of panels, beams and bearers per square metre
- Dead load (GK) = slab self-weight + formwork weight
Step 2: Calculate the Live Load
The live load covers temporary loads: workers, tools, small equipment and the impact of placing concrete. A typical allowance for workers and light equipment is around 1.5 kN/m².
Step 3: Combine into the Ultimate Load
Dead and live loads are combined using load factors set by the design code you are following. A widely used combination (as in BS 8110) is 1.4 × dead load plus 1.6 × live load:
Ultimate load per m² = (1.4 × GK) + (1.6 × QK)
Step 4: Find the Load on Each Prop
Multiply the ultimate load per square metre by the area each prop supports. The supported area depends on your prop spacing. For example, at a 1.2 m × 1.2 m grid, each prop supports 1.44 m².
Step 5: Check the Safe Working Load and Apply a Safety Margin
The design load on each prop must be less than the safe working load of the prop at that working height, with a margin for safety and variations on site. A common rule is to use no more than about 70% of the rated safe working load of the prop. If the required load is higher, either reduce the spacing or move to a heavier duty prop.
Worked Example
Let us put the steps together with a concrete example for a suspended slab.
- Slab size: 6 m × 5 m, thickness 0.15 m
- Unit weight of concrete: 24 kN/m³
- Formwork weight: 0.5 kN/m²
- Live load: 1.5 kN/m²
- Target prop spacing: 1.2 m × 1.2 m
Dead load: slab self-weight = 24 × 0.15 = 3.6 kN/m²; plus formwork 0.5 kN/m², so GK = 4.1 kN/m².
Live load: QK = 1.5 kN/m².
Ultimate load: (1.4 × 4.1) + (1.6 × 1.5) = 5.74 + 2.4 = 8.14 kN/m².
Load per prop at 1.2 m × 1.2 m: 8.14 × 1.44 = 11.7 kN.
Safety check: if the prop’s safe working load at the working height is 20 kN, then 70% of 20 = 14 kN, which is greater than the 11.7 kN required. The layout is safe.
Number of props: slab area = 6 × 5 = 30 m²; 30 ÷ 1.44 = about 21 props, plus a small contingency for edges and openings.
Practical Tips for Selecting Shoring Props
- Choose the prop size and duty class that matches the required height and load, from light duty for small slabs to heavy duty for deep slabs and beams.
- Use a U-head jack or fork head on top so the prop seats squarely against the formwork beam and stays stable under load.
- Pair your props with a complete shoring system—such as ringlock scaffolding—when the structure is tall, so bracing handles lateral forces.
- Inspect props before use: check the threads, lock pins, base plate welds and any rust or damage.
- Work with a supplier that can provide clear specifications, steel grades and load data, not just a price.
Conclusion
Calculating the load on adjustable shoring props comes down to knowing your dead and live loads, combining them with the right factors, and checking every prop against its safe working load at the actual working height—with a sensible safety margin. Done correctly, the calculation keeps your shoring safe, your slab true, and your budget under control.
If you are planning a project and need reliable shoring, Bythai Scaffolding is a steel shoring props manufacturer serving construction, oil & gas and distribution clients worldwide. Our props are manufactured from Q235 and Q355 steel, comply with international EN and BS standards, and are available with painted, electro-galvanized or hot-dip galvanized finishes. Contact us for load data, specifications and a quote for your project.