Hollowcores can be defined in two ways, as dxf/dwg-import (recommended) or added via section measurements.

PRE-Stress supports both .dwg and .dxf for import of hollwocore sections. Once the import file has been chosen PRE-Stress scans the content and presents what it finds, here you have to choose the actual object to import by browsing the presented content.

The chosen section needs to be given a name and PRE-Stress tries to figure out a proper scale. Review the 'Section parameters' to make sure the scale is correct, then press Import.

For DWG/DXF import of hollowcores, make sure the drawing contains at least one block or layer with a closed polyline or a circle. An element may not contain holes which in turn contains holes. An element may not contain overlapping holes. Dwg import supports up to AutoCAD 2018.

The alternative to dwg/dxf import is adding the Section measures (see picture) which then are used to create a closed polygon which is used in calculations. The polygon is used to calculate section properties as A, I_y, I_z, b_w, arrange reinforcement and show as graphics. The measures B and H may also be used at calculation.

The measurements of the polygon are as follows: the top width b must be less than or equal to the bottom width B. For flanges, the values b_fb, h_fb1, h_fb2, b_ft, h_ft1 and h_ft2 may optionally be set to zero.

Positions of hollowcores are defined by number of hollowcores n, spacing s and top distance c. The hollowcores are positioned symmetrically. The shape of a hollowcore is defined by its height h, widths b_b and b_t, arc radii r_b and r_t, arc origins z_rb and z_rt and number of polygon sides at the arcs n_b and n_t. 

Widths b_b and b_t may be set equal or unequal in order to get either vertical or inclined hollowcore sides.

Arc radii r_b and r_t may be set larger than, equal to or smaller than respective width/2. When a radius is larger than a width/2 a segment of a circle is obtained. When a radius is equal to a width/2 a semicircle is obtained. When a radius is smaller than a width/2 and positive two quarter circles are obtained which are positioned leftmost and rightmost at the width with a horizontal connection side. When the radius is negative two quarter circles are obtained too as before, but in this case the absolute value of the radius may be larger than width/2. 

The origins of the arcs determine the position of the arcs vertically. If the distance z_t is larger than c + r_t it is replaced by z_rt = c + r_t. If the distance z_rt is smaller than c + r_t a segment of the arc corresponding to c + r_t - z_rt will be removed, e.g. the arc will not be tangent to the topside of the hollowcore. Bottom is treated correspondingly if z_rb is larger or smaller than H - c - h. 

The arcs are approximated by equilateral polygons with n_b and n_t sides where the polygons have the same area as the corresponding segment of the circle. In case any part of the polygon is extended outside of the height extension of hollowcore these parts are removed and replaced by a horizontal connection side. Selection of a large number of sides n_b and n_t will give graphical nice sections but increased calculation time.

This, example one, is represented by the above image. Here, the upper part of the hollowcore is represented by a semicircle replaced by a polygon with 8 sides. As the top corner of the polygon lies above the given distance c to the hollowcore from top side, the polygon is cut and a new smaller horizontal side inserted. 

The lower part of the hollowcore is represented by a segment of a circle as 2*r_b > b_b and replaced by a polygon with 8 sides which has been cut because it is extended outside of the bottom of the hollowcore.

The vertical line below the hollowcore is the bottom side connection to the hollowcore.

In example two, the upper part of hollowcore is represented by two quarter circles (2*r_t < b_t) replaced by two polygons with 4 + 4 = 8 sides + a connection side. As the top corner of the polygons lie above the given distance c to the hollowcore from top side, the polygons are cut and a new slightly longer horizontal side is obtained. 

The lower part of the hollowcore is represented by a two quarter circles (r_b < 0) and replaced by two polygons with 4 + 4 = 8 sides + a connection side and the polygons have been cut because they are extended outside of the bottom of the hollowcore. A horizontal connection side has been inserted. 

In example three, the upper part of hollowcore is represented by two quarter circles (2*r_t < b_t) replaced by two polygons with 4 + 4 = 8 sides + a connection side. As the top corner of the polygons lie above the given distance c to the hollowcore from top side, the polygons are cut and a new slightly longer horizontal side is obtained.

The lower part of the hollowcore is represented by a two quarter circles (r_b < 0) and replaced by two polygons with 4 + 4 = 8 sides + a connection side and the polygons have been cut because they are extended outside of the bottom of the hollowcore. A horizontal connection side has been inserted.

In example four, the upper part of hollowcore is represented by a semicircle replaced by a polygon with 6 sides. As the top corner of the polygon lies above the given distance c to the hollowcore from top side, the polygon is cut and a new smaller horizontal side inserted.

The lower part of the hollowcore is likewise represented by a semicircle replaced by a polygon with 6 sides which has been cut. Note that an increase of number of sides in the polygons to 12 would give a nicer graphical picture but a slower calculation while hardly improving the calculation result. See Calculation Performace for a discussion on how to improve calculation performance.

By clicking the help button (symbolized by a double-arrow and question mark), the section image can be toggled to display a representation of what the various measurements of the hollowcore mean. Clicking it again will bring the section image back. The actual set values are retained throughout, allowing the user to consult the schematics on the fly while making adjustments.

Once the values of the HD element have been properly configured, finish the configuration by clicking either Change or Add to store the data.