When I was asked to write an article about re-shoring, I had a flashback to my college days. As a freshman and sophomore, I took (and passed!) four quarters of calculus. The next series of math courses required a new book, Elementary Differential Equations and Boundary Value Problems. It was the most complicated textbook I ever used. It would take an hour to read one page and another week to understand it. There was nothing elementary about it.
Back to the present, I feel like I just passed four quarters of shoring and now I am looking for a book on re-shoring. I hope the title does not contain the word “elementary”.
The practice we call re-shoring was developed by builders trying to solve the fundamental problem with concrete; it is very heavy. An office floor is designed to safely support the weight of the floor (called dead load) and the weight of desks, file cabinets, and people (called live load). For concrete floors, the dead load is much greater than the live load. If we are depending on the live load capacity of a floor slab to support the weight of workers and fresh concrete from above, we will overload the slab. Even a fully cured slab may not be strong enough to safely support the workers and fresh concrete from above.
Shoring and re-shoring sound the same but they are performing different jobs. A quick definition will get us started;
SHORING—Shoring supports the forms, workers, & fresh concrete at the top level. The shore posts may be wood, aluminum, or steel. Shores distribute the loads from the form to the slab below which is the top surface of the re-shore system.
RE-SHORING—The re-shore system consists of three or more slabs separated by re-shore posts. The slabs and re-shore posts behave as an integrated structural system to support the load from the shore posts.
To further explain shoring and re-shoring, I will describe a step by step procedure to pour a floor in a multistory building. The building I will use as a model has one level of shores with two levels of re-shores.
All the slabs discussed below have the same stiffness and design strength. This procedure is used when construction is above the ground level (this will be discussed more in-depth later).
- Concrete is poured and cured in the slab form. The curing process may require several days depending on the concrete mix and the air temperature.
- After the slab concrete is strong enough, the slab forms and shores are removed. At this time, the slab is supporting its own weight and the weight of workers and materials on top of the new slab.
- After the shores are stripped, the load on the system of re-shores has been relieved. This allows the lower level of re-shores to be removed and carried up two floors where the shores were removed.