1 © Copyright 2012 ADAPT Corporation all rights reserved

Effects of Post-Tensioning in Design of Columns and Walls of Multistory

Buildings

2012 PTI Convention, Nashville, TN

ADAPT Corporation | Redwood City, CA, USAADAPT Latin America | Miami, FL, USA

ADAPT International Pvt. Ltd. | Kolkata, IndiaADAPT Europe | Perugia, Italy

Dr. Florian Aalamiwww.adaptsoft.com

May 2012

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Agenda

• Motivation

• Redistribution of axial loads in columns and walls

• Differential column shortening

• Loss of PT force due to support restraints

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Motivation

To date, we have carried out the design of post-tensioned floor systems using 2D strip or finite element method analysis models that only model a substructure of the real structure (strip or single floor).

We cannot determine the effects of post-tensioning on the floor system, columns and walls as an integral structural building model because it is too time consuming and cost prohibitive.

ADAPT has developed new software that now makes it practical to model and analyze a fully integrated concrete building with post-tensioned floors and or beams.

The objective of this presentation is to begin quantifying the effects of post-tensioning on a complete building and identify areas where there is opportunity to improve our design process.

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Typical Single-Level Model for Floor

Using current finite element software, single-level slabs are modeled with columns and walls that are sitting on fixed rollers. This allows the slab to undergo in-plane elastic shortening but does not account for its interaction with the remainder of the building.

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Typical Single-Level Model for Floor

Deflection contour

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1.18kSafety design of columns and floor system should include secondary effects of post-tensioning (hyperstatic)

Diagram showing distribution of Hyperstatic axial force in columns calculated using single-level slab model.

Typical Single-Level Model for Floor

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Diagram showing distribution of Hyperstatic moment in design strip and columns for a single-level slab model.

Representative design moment:28.4 k/ft

Representative column moment:13.2 k/ft

Typical Single-Level Model for Floor

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Multi-Level Model

New design tools support the creation of fully integrated building models including detailed floor behavior and post-tensioning

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Distribution of Hyp Moment in Columns

21 k/ft 4.5

4.1

5.3 k/ftLevel 21

Level 1

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Distribution of Hyperstatic Axial Force

28k

1.3k

Level 21

Level 1

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Differential Support Shortening

As the deflection contours highlight, differential support shortening in tall concrete buildings can change the distribution of bending moments in slabs and may need to be accounted for in the design.

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Additional Moment from Diff Settlement

259 k/ft

235 k/ft

Level 21

Level 1

10% increase

Diagrams showing distribution of bending moment along design strips at different levels of a concrete building.

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Loss of PT Force Due to Restraints

Model of a typical multi-level frame

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Loss of PT Force by Level

Level Residual PT Initial PT Loss7 217 230 ‐6%6 217 230 ‐6%5 216 230 ‐6%4 215 230 ‐7%3 214 230 ‐7%2 211 230 ‐8%1 209 230 ‐9%

kips

This elevation of a building frame shows the residual post-tensioning force (red) at each level of a building. The model was creating using ADAPT-ABI software that models the time-dependent effects of concrete including creep and shrinkage as well as all support restraints.

Note that the loss reduces to 6% at level 5 and does not decrease any further at higher levels. It is highest at the base level.

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In-Plane Shortening at Each Level

Left Right in Difference0.34 0.036 0.38 0.0310.31 0.035 0.35 0.0390.36 0.024 0.38 0.0560.44 0.000 0.44 0.0700.51 0.000 0.51 0.0400.55 0.000 0.55 0.0480.60 ‐0.002 0.60 0.598

Lateral movement

This diagram shows the calculated long-term, in-plane movement due to shortening at each end of the slab. It illustrates that the difference in in-plane movement reduces as one moves up in the building.

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