# Concrete Structures

Kajewski, Stephen (2005) Multilevel Formwork Load Distribution with Posttensioned Slabs. Journal of Construction Engineering and Management 131(2):pp. 203-210

Copyright 2005 American Society of Civil Engineers

Accessed from: https://eprints.qut.edu.au/secure/00003700/01/CO-2003022775_Manuscript.pdf

Multi-Level Formwork Load Distribution with Post-Tensioned Slabs By Dr Stephen L Kajewski1

Abstract: Formwork and the associated shoring represent a significant proportion of the costs associated with the construction of multi-level concrete structures. To minimize these costs, a limited number of formwork and shoring sets are recycled up the structure as construction progresses, eliminating the need for a new set of formwork and shoring with each new slab. When a slab is post-tensioned using draped tendons, slab lift occurs as a portion of the slab self-weight is balanced. The formwork and shores supporting that slab are unloaded by an amount equivalent to the load balanced by the post-tensioning. This produces a load distribution through the structure that is inherently different from that of a conventionally reinforced slab. This paper presents two design methods suitable for modeling the multi-level formwork process for post-tensioned slabs: a modification to the simplified analysis method and a finite element model – both techniques will be of immediate use by industry practitioners and of interest to researchers examining the load distribution phenomenon. The paper also summarizes the findings of one of only a few research projects (Kajewski, 1997, 1998) in which actual shore loads were monitored during the construction of a multi-level post-tensioned building, which is used to validate the proposed design models.

1

A/Head, School of Construction Management & Property, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland, 4001, Australia. Phone: +61 7 3864 2676, Fax: +61 7 3864 1170, email: s.kajewski@qut.edu.au

CEDB Key Words: concrete construction, formwork, post-tensioned concrete, post-tensioning, structural design

INTRODUCTION

In multi-level building design, a fundamental principle of structural engineers designing post-tensioned slabs is that as the slabs are post-tensioned, a certain amount of slab lift occurs as a portion of the slab self weight is balanced. Extending this assumption, if the slab lifts, the shores supporting the slab must be unloaded by an amount equivalent to the slab load balanced by the post-tensioning. This unloading of the shores produces a situation entirely different from that of a conventionally reinforced slab system in which no load balancing occurs. The load distribution that occurs between interconnected post-tensioned slabs should therefore be significantly different from that of a conventionally reinforced slab system.

This phenomenon and the analysis thereof will be of particular interest to researchers and industry practitioners involved in the design of post-tensioned slabs and the specification of formwork cycle stripping times as the proposed analysis techniques potentially allow for significantly reduced cycle times, enhanced construction safety and a reduction in construction cost through the use of a lesser number of formwork and shoring sets.

POST-TENSIONING AND SLAB LIFT

When the shoring is stripped, all suspended concrete slabs will deflect elastically under the effect of self-weight and any applied loads. If the slab is conventionally reinforced, this deflection is irreversible. If a slab is posttensioned with tendons that are draped parabolically, some portion of the deflection is able to be reversed through a process commonly referred to as load-balancing.

When a slab is cast, it is fully supported by the formwork and shoring. The slab is unable to deflect resulting in a level slab that is not subjected to any bending stresses. If post-tensioning forces are adopted such that it produces stresses equal but...

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