What is Shoring? What is Reshoring?
When I was asked to write an article about reshoring, 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 reshoring. I hope the title does not contain the word “elementary”.
The practice we call reshoring 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 reshoring sound the same but they are performing different jobs. See the quick definition on the left to help get us started.
To further explain shoring and reshoring, 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 reshores.
All the slabs discussed 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). We are examining a reshore system consisting of three slabs with reshore posts separating them. It is common to assume that the reshore posts do not deflect. When we apply a load (shores from above) to the system, the reshored slabs deflect equally. This tells us that the slabs share equal parts of the load. In this example, each slab will carry one third of the shored load. Usually, one third of the shored load is less than the live load capacity of the slab used in the reshore system.
In this procedure, we need to know the early strength of the uppermost concrete slab before we strip the form to prevent cracking or failure. We also need to know the strength of the slabs used in the reshore system before we place concrete in the forms above. These slabs will not have full design strength.
Estimating the strength and stiffness of slabs is an engineer’s job. This can be complicated. Concrete hardens over time and the process is dependent on ambient temperature. The building’s structural engineer designed the building frame with concrete that has fully hardened. The normal assumption is that the concrete will reach full strength in a month (28 day strength). However, concrete rapidly gains strength in the first week after the pour. The slab may have 70 to 75% of 28 day strength in seven days. After several days of curing, the slab may be strong enough to hold its dead load and some live load. The builder may then strip the slab form.
Monitoring concrete strength becomes critical to determine the speed and safety of construction. In the past we used field cured cylinders to estimate the strength of concrete. There were other methods employed but results were questionable. I am encouraged by the development of maturity methods. The maturity method involves placing a monitor directly into the slab as it is poured. The monitor records concrete temperature continuously. The data is compared with lab controls to estimate concrete strength. This method is becoming more popular with concrete professionals.
|Some of EFCO’s shoring and reshoring systems in use:|
What is Shoring? What is Reshoring?
Question & Answer
» Do builders always use two levels of reshores?
Speed of construction and climate will dictate the reshore requirements. In warm climates, two levels of reshoring may be adequate but the same building in a cooler climate may require three levels. If the pace of construction is increased, multiple levels of forms, shores, and reshores may be used. This is all about the strength of concrete.
» Do the slabs in a reshoring system actually carry the same load?
This depends on the stiffness of the reshore post and the concrete slab. There has been a lot of research on this question. I can offer simplified answers; is this assumption exactly correct? No. Approximately? Yes. Good enough for design? Most of the time, yes. This simplification has problems when you add more levels of shores and reshores. Wood reshore posts deflect more than steel posts and will shift more of the load to the top reshored slab.
» What is the problem with shoring at ground level?
Shoring and reshoring at the ground level is a special condition. The ground will not deflect. This means that the shored loads are taken through the reshores directly to the ground without using the live load capacity of the slabs. Bottom line, the loads on reshores to the ground will be higher than loads in reshores to flexible slabs. Note that mezzanines and mechanical floors may be designed with different strengths and stiffness that can cause reshoring problems similar to the ground floor conditions.
» Do reshore posts need to line up with shore posts?
If you want to start a lively discussion at an ACI convention, open with that question. Check with your engineers. With careful engineering, offset reshoring may be used.
» What happens to shores and reshores in a post tensioned slab?
This can be a problem in some structures, particularly post tensioned slabs with beams. As slab tendons are stressed, the weight of the slab may move from the slab shores to the beam shores, adding load to the beam shores and reshores below.
» What happens when we use a drop head shore post as a shore, then leave it in place to use as a reshore?
Depending on the drop head, shore posts hold the load of the slab while releasing the forms for early stripping. This forming sequence may be called pre-shoring or back-shoring. This is a problem because the procedure does not let the slab support itself. This will result in higher loads on posts and slabs This can be done, but the building’s structural engineer should work with the builder’s engineer to verify the strength of posts and slabs.
I will finish this article with two thoughts:
First, I want builders to employ the services of a forming and shoring specialist with experience in reshoring. There are many variables to consider in the design of shoring and reshoring systems. A qualified engineer is required.
Second, I recommend further reading on the subject. The document all builders of multistory structures need to study is ACI 347.2R-05, Guide for Shoring/Reshoring of Concrete Multistory Buildings. This is a product of the ACI 347 Formwork Committee. There are examples of design and good explanations to the questions I partially answered. And most important, the word “elementary” is not in the title.
> The author, Bob McCracken is a Licensed Professional Engineer and engineering graduate of Iowa State University, with over 30 years of form design experience at EFCO Corp.