Modeling Precast Structures

StatiCAD offers modeling, analysis and connection design tools for precast reinforced concrete industrial buildings: nonprismatic (variable-section) members and parametric precast sections, assignment of pinned connections, roof diaphragm equivalent braces (TBDY 2018 Annex 8B), façade panel and crane masses, overhead travelling crane loads, different R and D for the upper and lower parts (TBDY 4.3.6), TBDY Chapter 8 connection design, façade panel seismic load, foundation sliding and reinforcement design of polygonal and nonprismatic beams.

These features were developed and compared on the example building in the Turkish Precast Association’s book “Analysis and Design of an Example Precast Reinforced Concrete Industrial Building According to TBDY-2018” (40×75 m, two bays, with a mezzanine floor, with a 200 kN overhead travelling crane); see Precast Structures Verification Studies. For a step-by-step solution of the book’s building with screenshots, see Worked Example: Precast Industrial Building.

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Which editions? The precast tools (the Precast Reinforced Concrete building type, the precast commands in the Macros menu, the Precast section templates, the connection design window, the column-based relative drift option and the connection page in the report) are available in the StatiCAD Ultimate edition. The precast data of a project prepared in Ultimate is preserved in the file; when the project is opened in another edition, the analysis runs with the same data. The nonprismatic (Nonprismatic) section definition is available in all editions.

1. General workflow

1. In General Project Settings, select Precast Reinforced Concrete as the building type (it uses the same solution type as the Reinforced Concrete Building and additionally turns on column-based storey drift for the semi-rigid diaphragm) or the Reinforced Concrete Building; enter the material and the seismic parameters. For buildings with a mezzanine floor, enter the coefficients of the upper part in the R and D boxes.
2. Define the sections: rectangular for columns; a nonprismatic section or the precast templates for sloped roof beams (Sections 2 and 3).
3. Draw the columns and beams. Select the beams that are connected with pins, such as roof beams, purlins and gutter beams, and run the Macros → Make ends of selected beams pinned (M2+M3) command (Section 4).
4. Decide on the roof diaphragm: rigid diaphragm (default) or Annex 8B equivalent braces (Section 5).
5. Enter the façade panel and crane bridge masses as additional seismic mass and the overhead travelling crane loads with the crane window; for buildings whose upper and lower parts have different structural systems, set the different R–D values (see Overhead Crane Loads, Additional Seismic Masses and Different R–D).
6. Check the analysis options and run the analysis. The reinforcement of polygonal and nonprismatic reinforced concrete beams is calculated automatically during the analysis.
7. Design the connections in the Macros → Precast connection design (TBDY Chapter 8)... window and add them to the report (see Precast Connection Design).

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All precast commands are at the end of the Macros menu in the main menu. The brace, crane and connection windows are non-modal: you can keep selecting members in the model while the window is open.

2. Parametric precast section templates

The following templates are available in the template type list of the Frame Member Section Properties window. When a template is selected, the Section Parameters table shows the dimensions (cm) and their descriptions; the preview is drawn instantly with the dimension labels.

Template	Use
Precast I	Precast I or bulb tee beam; flange widths, heights, chamfer and web thicknesses
Precast T	Floor beam with a single top flange and chamfer (the precast part of a composite beam topped with cast-in-place concrete)
Precast U	U-section precast beam or gutter beam
Precast Box, Box/Tube	Box sections: outer dimensions, wall and slab thicknesses
Trapezoidal	Trapezoidal section: top and bottom width, height

The section is stored in the program as a polygon section; area, moment of inertia and centroid are calculated automatically from the polygon. Since the parameters are stored as well, the section opens in parametric mode when reopened. The reinforcement layout can be given in the polygon editor, or, if left empty, it is generated automatically in the beam design. When exporting to SAP2000, the templates are sent with SAP2000’s own precast section definitions.

3. Nonprismatic (variable-section) beams

Members whose height varies along their length, such as a roof truss beam, are modeled as a single member; the stiffness is calculated according to the actual variation of the section. The definition is made in the Nonprismatic Section Definition window, which opens when the template type Nonprismatic is selected in the Frame Member Section Properties window:

Column	Description
Start Section / End Section	Section at the start and at the end of the segment (from previously defined prismatic sections; e.g. a 40×26 rectangle at the end and a 40×140 rectangle at mid-span, or two different Precast I sections)
Length / Length Type	Absolute: segment length in cm. Variable: proportional weight; after the Absolute segments are subtracted from the member length, the remaining length is distributed to the Variable segments in this proportion.
EI33 / EI22 variation	Variation of the flexural stiffness: Linear (EI linear), Parabolic (√EI linear; for an I section where the flange dominates and the height varies), Cubic (∛EI linear; for a rectangle with constant width and linearly varying height)

A double-pitched (gable) roof beam is defined with two segments: end section → middle section → end section. The area, torsional constant and shear areas vary linearly within a segment. It is enough to select the section name as the beam section; the analysis, the SAP2000 export (with all segments) and the reinforcement design use the nonprismatic section automatically.

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How is it calculated? The stiffness matrix of the member is built by flexibility integration of the section properties that vary along the length (including shear deformation); for a constant section this matrix equals that of a prismatic member to numerical precision. The fixed-end forces of uniformly distributed loads are also calculated according to the nonprismatic section; the thick end receives the larger fixed-end moment. Mass and self-weight are distributed along the length with the average area (the total volume is correct).

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The nonprismatic section is taken into account only in beams; assigning it to a column is not supported.

4. Pinned connections

In precast buildings, roof beams, purlins and gutter beams are usually connected to the columns with pinned hinges. To assign them in a single step, select the relevant beams (on all floors) and run the Macros → Make ends of selected beams pinned (M2+M3) command. The command releases the two bending moments (M2, M3) at both ends of each selected beam, sets the hinge spring coefficients to zero, makes the hinge active in all load cases and finally reports how many beams were processed. For a single end, the hinge can also be entered from the hinge table in the beam’s properties window.

Even when the beam has an axis offset (the beam sits below the joint line), the hinge is applied on the joint side; the pinned beam behaves like a true simple support. The composite axial force produced together by the rigid diaphragm and the axis offset is filtered out of the beam’s axial load evaluation.

5. Roof diaphragm: rigid diaphragm or Annex 8B equivalent braces

The default assumption is a rigid diaphragm for every floor (including the roof) (General Project Settings → Member Forces → Fully rigid diaphragm). In roofs with purlins and bracing bays where thousands of joints are connected to the diaphragm, the rigid diaphragm is solved with an exact master–slave transformation; no user setting is needed.

For roofs covered with trapezoidal sheeting or sandwich panels, TBDY Annex 8B allows the shear stiffness of the roof plane to be represented by equivalent brace pairs pinned at both ends. For this, use the Macros → Precast roof diaphragm equivalent braces (TBDY Annex 8B) window:

Input	Description	Default
Self-drilling screw diameter D (mm)	Diameter of the screw that fastens the sheet to the purlin	4
Top sheet thickness t1 (mm)	Sheet thickness for trapezoidal sheeting; top sheet for a sandwich panel	0.5
Bottom sheet t2 (mm)	Bottom sheet for a sandwich panel; 0 for trapezoidal sheeting	0.4
Screws per unit length n (pcs/m)	Number of screws per metre along the purlin	5.5
Sheet yield strength fy (MPa)		255
Safety factor γ		0.67
Number of divisions	Number of cells into which the space between two beams is divided along the beam length	4
Brace material E (MPa)	Modulus of elasticity of the equivalent member	33,227

1. Calculate: The equivalent axial stiffness (EA)e, the maximum force a brace can carry, the TBDY Eq. 8B.1 comparison, the equivalent member section and the Annex 8B applicability conditions (purlin spacing ≤ 2.0 m, brace angle ≈ 45°) are displayed.
2. In the model, select the two parallel beams between which the braces will be created (e.g. two adjacent roof beams) and press the Generate braces between the 2 selected beams button. The space between the two beams is divided into the selected number of cells and an X pair is placed in each cell. The braces are virtual members that are pinned at both ends, massless and unloaded; they do not enter the reinforced concrete design or the quantity takeoff.
3. The generated braces and the source beams remain selected. Run the Macros → Split selected beams that intersect on all floors command so that the brace ends join the purlins and beams at common joints.

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When working with a roof that has Annex 8B braces, General Project Settings → Member Forces → Semi-rigid diaphragm must be selected, and the column-based option on the same tab must be turned on for storey drift (Section 6). It is recommended to run the rigid diaphragm solution as well as the design reference.

6. Analysis options

Option	Location	Description
Fully rigid / Semi-rigid diaphragm	General Project Settings → Member Forces	Semi-rigid is selected for a roof with Annex 8B braces; otherwise the rigid diaphragm is recommended.
Semi-rigid diaphragm: column-based storey drift (top–bottom joint)	General Project Settings → Member Forces (bottom of the tab)	In the semi-rigid diaphragm, the relative storey drift is calculated from the difference between the top and bottom joints of the same column instead of the difference between the largest and smallest joint displacements of the storey; the purlin and brace joints in the flexible zone of the roof and the gable columns do not distort the result. It is checked automatically when the Precast Reinforced Concrete building type is selected; it is saved with the project.
Rigid diaphragm assignment in modal analysis	General Project Settings → Seismic Forces	With the “Yes” option, a reduced model with three degrees of freedom per floor (UX, UY, RZ); joints at the floor elevation (±2 cm) are connected to the floor.
Consider slabs in rigid-diaphragm modal analysis	General Project Settings → Seismic Forces	The contribution of slab shells to stiffness exists in the static solution but by default does not exist in rigid-diaphragm modal analysis; therefore modal periods come out slightly longer and static displacements slightly smaller (2–5%). It can be turned on in small models to equalize the two solutions; it is not recommended for large roofs with purlins and bracing bays because of the memory requirement.
Do not connect beam intermediate joints to the rigid diaphragm	General Project Settings → Seismic Forces	When beam axes are modeled below the joint line, the beams become composite-stiffened because the beam division joints are connected to the rigid diaphragm. When the option is turned on, only the division joints are left out of the diaphragm; column, shear wall and beam intersection joints remain connected. It is off by default and is saved with the project.

7. Scope limits

• The nonprismatic section is taken into account only in beams; prestressed section calculation is out of scope.
• Short corbel (haunch) and crane corbel checks, socket (pocket) foundation checks and erection-stage checks are not available in this edition.
• The shear stiffness of a semi-rigid roof with Annex 8B braces must be evaluated together with the rigid diaphragm solution.
• In the flange–wall junction detail of the Precast U template there is a small area difference compared with SAP2000 (−4% in the example section).

8. Frequently encountered situations

Symptom	Solution
The Macros menu has no precast commands; the Precast templates do not appear in the section list.	The precast tools are enabled in the Ultimate edition.
The generated braces do not join the purlins.	Run the Macros → Split selected beams that intersect on all floors command while the braces and beams are selected.
The relative storey drift is very large in a semi-rigid roof.	Flexible roof joints and gable columns inflate the value in the “largest − smallest” method. Turn on the column-based drift option in General Project Settings → Member Forces (Section 6); also run the rigid diaphragm solution as the design reference.
The dominant period is shorter than expected.	This is a result of the rigid roof diaphragm assumption; the explanation is in the verification article.