10.3 Shoring and Reshoring

The Mechanics of Shoring and Reshoring

In multi-story cast-in-place concrete construction, the cycle of pouring, stripping, and supporting slabs is a complex, high-risk sequence. Managing these operations dictates both the pace of construction and the structural safety of the project. As columns and walls are cast, horizontal floor slabs are poured on temporary formwork.

To maintain the schedule, formwork must be removed and moved to the next level as quickly as possible. However, young concrete takes 28 days to reach its full design strength ($f'_c$) and cannot support the weight of subsequent pours immediately. To solve this, construction engineers design systems of shoring and reshoring:

• Shoring: The initial vertical support system (wood or steel posts) erected to support the formwork and the weight of the freshly poured concrete slab.
• Reshoring: The process of stripping the formwork and shores from under a partially cured slab, allowing it to deflect and carry its own self-weight, and then inserting snug-fitting vertical supports (reshores) back under the slab.

The primary structural purpose of reshoring is to link multiple floor slabs together. By interconnecting several levels, the massive load of a freshly poured slab and its associated construction live loads is shared among several lower slabs that have cured longer and possess greater load-bearing capacity.

Shoring/Reshoring Load Redistribution Principles

Calculating construction loads on interconnected slabs is essential to prevent cracking, serviceability issues, or structural failure. The standard analysis method, developed by Grundy and Kabaila (1963), relies on several key simplifying assumptions:

1. Infinite Shore Stiffness: Shores and reshores are considered infinitely stiff in axial compression compared to the bending stiffness of the concrete floor slabs.
2. Equal Deflection: Because the shores are rigid, all interconnected slabs deflect by the same amount under any newly applied construction load.
3. Stiffness-Proportional Sharing: Interconnected slabs share any new load in direct proportion to their relative bending stiffnesses. For slabs of identical thickness, span, and boundary conditions, this means the applied load is distributed equally among all supporting slabs in the system.
4. Self-Weight Release: When formwork and shores are stripped from a slab, that slab is forced to support its own self-weight. Any loads previously transferred to the shores are released and must be carried by the slab or shared through the remaining reshores.

Construction Sequence Notation

Engineers specify shoring systems using the notation $N_S + N_R$, where $N_S$ is the number of shoring levels and $N_R$ is the number of reshoring levels. For example, a $2S+1R$ system utilizes two levels of active shores and one level of reshores.

Influence of Early-Age Concrete Properties

While the Grundy and Kabaila method assumes equal stiffness for all slabs, young slabs have a lower Modulus of Elasticity ($E_c$) than older slabs. The concrete elastic modulus at time $t$ can be estimated using ACI guidelines: $E_c(t) = w_c^{1.5} 33 \sqrt{f'_c(t)}$ Since a younger slab is more flexible, it actually carries slightly less load than the older, stiffer slabs in the system. However, the simplified method assuming equal sharing remains the standard conservative design baseline for the PE Construction exam.

Worked Construction Load Calculation

Scenario: A multi-story commercial building is constructed using identical 8-inch thick concrete slabs (dead weight $D = 100\text{ psf}$). The construction live load during a pour is $L = 50\text{ psf}$. The weight of the shoring system is negligible. The construction sequence utilizes a $2S+1R$ system (two levels of shores and one level of reshores). Slabs 1, 2, and 3 have been poured, and the reshores under Slab 1 have been removed. Slabs 1, 2, and 3 are interconnected. We are now preparing to pour Slab 4.

Determine the maximum construction load carried by Slab 1 during the pouring of Slab 4.

Step 1: Determine the load state before the pour. Before Slab 4 is poured, Slabs 1, 2, and 3 support their own self-weights. Since they are identical and linked by active shores, they share the total weight of the system:

• Total weight in the system: $W_{total} = D_1 + D_2 + D_3 = 100 + 100 + 100 = 300 ext{ psf}$.
• Load on each slab: $P = \frac{300}{3} = 100 ext{ psf}$. So, Slabs 1, 2, and 3 are currently carrying exactly their own weight ($100 ext{ psf}$ each).

Step 2: Calculate the new load applied by Slab 4. The new load consists of the wet concrete of Slab 4 ($100 ext{ psf}$) and the construction live load ($50 ext{ psf}$):

• $W_{new} = 100 + 50 = 150 ext{ psf}$.

Step 3: Distribute the new load. This new load is distributed equally among the interconnected cured slabs. Note that Slab 4 is fresh (fluid) and does not contribute any bending stiffness to the system.

• Shared load per supporting slab: $P_{shared} = \frac{150 ext{ psf}}{3} = 50 ext{ psf}$.

Step 4: Calculate the total load on Slab 1.

• Total load: $P_{total} = P_{initial} + P_{shared} = 100\text{ psf} + 50\text{ psf} = 150\text{ psf}$.

Conclusion: Slab 1 must carry a total construction load of $150\text{ psf}$ (a load ratio of $1.5 D$). The construction engineer must verify that Slab 1 has cured sufficiently to safely withstand this load without cracking or excessive deflection.

Scheduling Safe Stripping and Stripping Strength

Stripping shores too early is a leading cause of construction collapses and serviceability failures (excessive long-term deflections). The decision to strip forms must be based on verified compressive strength, not calendar days.

• Vertical Elements: Wall and column forms can typically be stripped within 12 to 24 hours, as they carry minimal gravity loads.
• Horizontal Elements: Slabs and beams must typically achieve 70% to 85% of their 28-day design compressive strength ($f'_c$) before stripping.

Contractors verify in-place concrete strength using field-cured cylinders broken in a compression machine or non-destructive methods like concrete maturity meters (ASTM C1074), which correlate the slab's time-temperature history to strength development.