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Concrete Slab Design

Requires: Design Level

Introduction

VisualAnalysis allows you to check flat reinforced concrete slabs (or walls in pure bending) according to ACI 318-11 requirements. There are no capabilities for shear walls and the software does not check footings or foundations (no punching shear checks).

Concrete Wall/Slab Capabilities

The Slab design uses 3-4 node plate/shell finite elements. These elements are grouped together in logical groups called a Design Mesh. A typical rectangular tank for example with open top might have five such meshes, one for each wall and one for the base slab.  The Slab design is also complatible with auto-meshed areas.  A Design Mesh may consist of a single auto-meshed area.  Manual plates and auto-meshed areas are not allowed to be grouped together in the same Design Mesh. Design meshes are created automatically if the Project Settings "auto-group" option is enabled. You may also manually construct design meshes for manual plate meshes.

Concrete Slab Checks

The Wall/Slab module will design wall and slab-type structures based on ACI 318 concrete code strength provisions. Strength checks made include shear and flexure.

Local Coordinate System

The Wall/Slab module attaches a local coordinate system to all panels designed. An individual slab has associated with it a local-x direction and local-y direction for rebar orientation, and a top and bottom face (used to reference the different mats of reinforcement). The local coordinate system has x and y in the plane of the mesh, with the +z formed normal to the plane according to the right-hand-rule. You can display the local x-y directions in the Design View window.

For design meshes associated with a meshed Area object, the local coordinate system will line up with the "Span Direction" specified for the area. For a mesh of manually created plate elements, the local-x and local-y directions are set by the software based on the local x-axis of the "first" plate in the mesh, which you have no control over, so it is useful to align ALL your plate elements the same way.

Steel Reinforcement

The slab, wall, or panel as it is referred to here will consist of a single region for rebar layout.  If you need more regions than this, you will need to manually create your mesh in different groups for design. Rebar is oriented with the local coordinate system for the Design Mesh. Rebar is specified as a bar size and spacing for each bar layer in each direction. The software simply checks the rebar layout and details that you specify, the "default" or starting rebar configuration is just the smallest bar at some fairly large spacing.

When two layers of steel are needed, four bar groups exist in the region:

  1. top layer local-x bars
  2. top layer local-y bars
  3. bottom layer local-x bars
  4. bottom layer local-y bars.

 When a single (centered in thickness) layer of steel is used, only two bar groups exist in each detail region; local-y and local-x bars.  The local x-bars are assumed to be the top layer.

Concrete Wall/Slab Limitations

This section summarizes the assumptions made in the concrete wall/slab design software module. It also includes several discussions that are very critical to the proper use of the design module. You are encouraged to read the attached sections before using the software.

Flat Slabs Only

There are no provisions for waffle slabs, ribbed slabs, or column capitals. Only flat walls or slabs are checked.

No In-Plane Forces

We check pure-bending only. No shear wall or bearing wall design. The Wall/Slab module does not look at in-plane forces in a slab. If walls have axial loads these are not considered in the design. Shear walls are not supported by this module. The software will check for 'significant' in-plane forces and report the problem with a warning.

Critical Section for Moment and Shear

The Wall/Slab module does not consider critical section for edge moment at the face of the support or shear at 'd' from a support point. The software currently does not try to detect or allow you to specify specific support locations.

Mesh Shape and Orientation

The Wall/Slab module has been written to design arbitrarily shaped mesh of connected elements, which all must lie in the same plane. Curved walls (even when approximated with flat plates), such as circular tank walls, cannot be designed.

Shear Strength

This program does not consider shear steel; therefore, all shear strength is attributable to the concrete shear strength. Normal weight concrete is assumed. The software does not consider punching shear near columns in the mesh.

Concrete Wall/Slab Parameters

The following parameters are used to control the unity checks made for the wall/slab design mesh:

Wall/Slab

Name: A name for you to use

Specification: ACI version

High Seismic: ACI seismic provisions are used.

ACI Wall Minimum: Minimum wall steel is checked. (ACI 14.3)

Concrete f'c: Set by plate or area in the Model

Details

Thickness: thickness of wall or slab to check, if you change this you can 'synchronize' to update the model, which will cause analysis results to be thrown out.

Bar details: Rebar you specify for checks.

Reinforcement

Steel Fy: Yield stress for rebar

Bar Layers: Two or one-center layer

Outside Layer: Specify either x- or y-direction bars

Top/Bottom Cover: Clear distance to outer bars from face.

 

There is no way to control the wall-width settings; we now "smartly" check shear (at the centroid of elements not at the perimeter nodes.) This allows for arbitrary geometry slabs, prior versions of VA were only accurate for rectangular-shaped slabs.

If you wish to, you could manually remove plate elements around the "perimeter" of mesh from the design-checks to prevent any local/artificial shear-spikes from interfering with good checks. This way the system is less of a "black box" and you can take-charge as necessary. To do this you would:

  1. Turn off the "Auto-Group" feature in Project Settings Settings
  2. Turn off the "Toggle Groups" feature in the Filter tab for the Design View
  3. Select some of the "perimeter" plates in a mesh and remove them from the design group.

The information in the parameters is accessible to you through the Modify tab in Project Manager. The parameter information you enter applies to the entire design mesh. You may select multiple design meshes and edit many of the parameters at one time. After making a change all of the unity checks will be recalculated unless there are no analysis results, or you have "Suppressed Unity Checks" [Advanced level only].

The "Design Group" feature that was available in VA 8.0 and prior is now gone because it is was longer necessary after the simplifications in VA 9.0, see below for details...

Wall/Slab Design "Procedure"

The "Design" of your slab consists of two things: the Slab Thickness that you set in the design view and the Rebar Layout you choose. When the checks pass you have a successful design! The only thing that "synchronizes" back to the model is the slab thickness, so if you change that you must synchronize the change and re-analyze to get updated forces/stresses in the slab.

Concrete Wall/Slab Reports

To understand the unity-check values displayed in the Design View, you can double-click on a Design Mesh in the Design View to generate a report. The report includes (optionally) the input parameters, the design details (rebar) and the code checks performed showing demand, capacity, intermediate values and unity check results. A code or specification reference is provided for each check. If there are errors or warnings this information will appear near the top of the report.

Concrete Wall/Slab Preferences

You will find some default values for concrete design in the preference settings under Edit | Preferences, Design, Concrete. These can streamline your setup of concrete design meshes.