Patsy Moore's Communities Activities

ANALYSIS OF STEEL STRUCTURES IN STAAD.Pro

rozarker — Wed, 22 Jun 2016 17:32:24 GMT


	Applies To

	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	ANALYSIS OF STEEL STRUCTURES IN STAAD.Pro
	Subarea:	N/A
	Original Author:	Ravi Ozarker, P.Eng. Bentley Technical Support Group

Ravi Ozarker, P.Eng., Applications Engineer, Bentley Systems Inc.

Thanks to Ray Cutis, Senior Advisory Software Developer for all the help that he has offered me to write this article. Thanks to Dr. Bulent Alemdar, FEA Specialist and Santanu Das, VP, Integrated Engineering Group for reviewing this article in detail and providing their valuable feedback.

1.0 INTRODUCTION:

Features such as non-linear analysis of a structure seems to be a grey area to me sometimes. There are terms that will be thrown out at me such as; second order analysis, plastic analysis etc. I have found that the meaning of the word "non-linear" can vary depending upon what the engineer may want to do.

This blog is for engineers who are trying to explore structural analysis methods implemented in structural analysis software such as STAAD.Pro V8i. The goal of this article is to offer a basic overview of features such as Linear static, P-Delta, small P-Delta, stress stiffening, and geometric non-linearity and how they are implemented in STAAD.Pro.

Note that the non-linear and pushover analysis features in STAAD.Pro V8i are a part of the "Advanced Analysis License" which is an add-on.

2.0 LINEAR STATIC AND P-DELTA (P-Δ) ANALYSIS:

The following figure illustrates a steel frame with some gravity and lateral loading. This frame could be a new or existing structure. There are several analysis options available to the engineer to analyze this frame depending on what is the final goal. For example, if a new frame is to be designed to come up with the member sizes, engineers may use p-delta (P-Δ) analysis with effects of small p-delta (p-δ) included etc.

Figure 1: Frame subjected to lateral and gravity loads

Figure 2 shows a plot of levels of structural analysis and behavior they produce (i.e. applied load (H) vs. displacement (Delta) graph). This graph illustrates the response of the structure depending on what structural analysis method is used. Most engineers are used to the first-order (linear) elastic analysis method. This type of analysis used to be good enough to come up with the force distribution in a particular structure. Once the force distribution is obtained, engineers can obtain stresses in the members and compare them with the allowable stress which used to be 36 ksi. This is all good if you were using the AISC-ASD codes.

American Institute of Steel Construction (AISC) 13^th Edition 2005 Code introduced many new concepts to analyze steel structures. The code specifically addresses how to consider nonlinear effects (P-Delta) in analysis, and provide several guidelines for this purpose. In the past, engineers were more concerned with the stresses not exceeding a particular code defined value, displacements not exceeding a particular code defined value and same applied to slenderness, torsion etc. Today, stability and performance of a structure have become equally important.

Most civil engineering structures behave in a linear fashion under service loads. Exceptions are slender structures such as arches and tall buildings, and structures subject to early localized yielding or cracking.

Consider the structure shown in Figure 1. Note that this structure is subjected to lateral and vertical forces. The point loads shown in this example may be dead load applied to the structure. The lateral loads may be wind loads applied to the structure. If the lateral loads are applied while the dead loads are acting, the structure would displace laterally. The lateral displacement and the vertical forces would exert an extra moment on the columns which is not taken into account in the linear static analysis. The analysis that would take this moment into account is known as the P-Delta (P-Δ) analysis or the Big P-Delta analysis. This analysis is performed by first applying the loads laterally to create the displaced shape of the structure and then applying the vertical loads. Once the vertical loads have been applied, the additional moment is converted to lateral forces and is added to the existing lateral forces to obtain a set of updated lateral forces. The updated lateral forces are applied to the structure to obtain an updated displacement. The moment generated by the difference of the previous displacement and the updated displacement and the vertical loads is used to calculate a new moment which is again added to the lateral loads.

Engineers are aware of this concept but part of the reason why this is only being discussed recently in the codes is because of the easy availability of computational power and structural analysis software packages like STAAD.Pro. Availability of computational power and structural analysis products does not mean that engineers do not have to worry about the analysis part anymore. I would say that the engineers have to have a thorough understanding of the analysis procedures and how they are implemented in their structural analysis product of choice. It is critical to understand what the results mean in the analysis product; i.e. has the structure fallen down?, Is there global buckling? Is the structure unstable? etc. After all these analysis related issues have been sorted out, the engineer could then concentrate on the design issues.

Figure 2: Load vs. displacement graphs depending on analysis method being used

Figure 2.1 shows the P-Delta (P-Δ) Analysis graph after 35 iterations. Note that the displacement is still about 53 in. From this graph, it is clear that the structure is expected to be stable if it is subjected to lateral loads because the displacement value almost remains unchanged as the number of P-Delta (P-Δ) iterations is increased. In other words, the frame is reached to an equilibrium state so that further iterations are not needed.

Figure 2.1: P-Delta (P-Δ) analysis in STAAD.Pro

3.0 SMALL P-DELTA (p-δ) ANALYSIS:

When a column member is subjected to compressive loads, it can have localized deflections throughout it length. The local deflection of the column (known as small delta (p-δ)) and the gravity load together can result to an additional moment which must be taken into account to perform the P-Delta (P-Δ) analysis. The displaced shape of a structure is illustrated in Figure 3 based on which analysis procedure is used (i.e. P-Δ OR P-Δ with effects of p-δ included). Note that the red lines show local deflections of the column members if the P-Delta analysis takes the effects of small P-Delta into account (i.e P-Δ with effects of p-δ included).

Figure 3: Difference between a "regular P-Delta analysis (P-Δ)" and a "P-Delta analysis that takes effects of small P-Delta into account (P-Δ with effects of p-δ included)"

Note that in Figure 3, the left hand side columns have less localized deflections than the columns at the right. The lateral load applied to the building will induce a tensile loading in the columns at the left and try to straighten the columns. This effect is known as the stress stiffening effect. The lateral load applied to the building will induce additional compressive loads in the columns at the right and try to bend the columns more. This effect is also known as the stress stiffening effect but in this case the columns at the right have reduced stiffness.

The P-Delta (P-Δ) analysis capability in STAAD.Pro V8i has been enhanced with the option of including the above-mentioned stress stiffening effect of the Kg matrix into the member/plate stiffness. This implementation will also report any global bucking in the structure.

The AISC 360-05 Appendix 7 describes a method of analysis, called Direct Analysis, which accounts for the second-order effects resulting from deformation in the structure due to applied loading, imperfections and reduced bending stiffness of members due to the presence of axial load.

In STAAD.Pro, this feature is implemented as a non-linear iterative analysis as the stiffness of the members is dependent upon the forces generated by the load. The analysis will iterate, in each step changing the member characteristics until the maximum change in any Tau-b is less than the tau_tolerance. Note that the member stiffness will be changed depending on the load applied.

3.0 NON-LINEAR ANALYSIS:

P-Delta (P-Δ) analysis discussed above is a type of non-linear analysis but this section talks about what a true non-linear analysis is all about.

In linear elastic analysis, the material is assumed to be unyielding and its properties invariable, and the equations of equilibrium are formulated on the geometry of the unloaded structure. We assume that the subsequent deflections will be small and will have insignificant effect on the stability and mode of response of the structure.

Nonlinear analysis offers several options for addressing problems resulting from the above assumptions. There are two basic sources of nonlinearity:

1. Geometric Non-Linearity: Similar to the P-Delta (P-Δ) analysis feature discussed above but could apply to a structure with any geometry. P-Delta (P-Δ) analysis is not a pure non-linear analysis.

2. Material Non-linearity: Similar to the direct analysis feature in which the member properties are modified based on the load they experience. The direct analysis is not a true non-linear analysis. A true non-linear analysis would consider plastic deformation and inelastic interaction of axial forces, bending shear, and torsion.

Let us consider the following example to explain the implementation of "geometric non-linear analysis" feature in STAAD.Pro:

Figure 4: Three-hinged arch

The system shown above in Figure 4 is a shallow arch consisting of two axial force members. A linear static analysis on the structure using the applied loads will lead the engineer to believe that the structure is stable with the applied loading. A method that only considers material non-linearity may also lead to the same conclusion. The geometric non-linear analysis will illustrate if the shallow arch will snap-through to become a suspension system. Figure 5 shows a load vs. displacement at center node graph that was developed for this STAAD.Pro model. As the load approaches 16 kips, the shallow arch becomes a suspension system. The following video shows the same phenomenon.

Figure 5: Load vs. Displacement at center node

This example in literature is known as "Williams' Toggle" example, and it is one of the very fundamental example found almost in most structural analysis book as shown in Figure 5.1. It is worthwhile to mention that typical Newton Raphson algorithm fails to trace all post-buckling behavior, so Arclength method (or minimum residual method) is required to capture it. Figure 5 shows that the displacements jumps from one to another while snapping through.

Figure 5.1: Williams's Toggle example

Video 1: Non-Linear Analysis of a shallow arch system

A shallow roof system is illustrated in Figure 7. This structure is designed as per the AISC-2005 13th edition code and the loadings from IBC 2006/ASCE 7-05 code. We know that code based loadings are minimum requirements and it is upto the engineer to ensure that the stability and performance criteria are satisfied based on the importance of the structure and customer's needs. Normally, Civil Engineering structures similar to the one shown in Figure 7 will not experience large deformations and when service loads are applied, these structures will behave in a linear fashion. In some instances, engineers are required to study the performance of the structure in the event of an unusually high loading and predict the governing failure mode. This will require the engineer to perform a geometric non-linear or a critical load analysis on the structure. In this case, the model is expected to yield the "true" behavior of a structure closely.

Figure 7: Hanger Structure

In this structure, suppose the shallow arch system has to be analyzed for snap through for extremely high loading the engineer could easily utilize the geometric non-linear capability in STAAD.Pro to see if that failure mode is even possible. Figure 8 shows the displacement diagram of arch snap through and the failing members red. It is clear from this analysis that the columns and roof members have to yield before such a failiure mode could occur. Figure 9 shows a similar structure but in this case, the bottom chord members expereinced excessive local deflections during an event of excessive roof loading.

Figure 8: Displacement Diagram of arch snap through. Failing members shown in red.

Figure 9: Displacement Diagram. Bottom cord members deform first due to excessive roof loading.

4.0 ADDITIONAL COMMENTS:

Let us re-visit Figure 2. Figure 2 shows two yellow curves that represent inelastic analysis of structures. The pushover analysis feature in STAAD.Pro would produce the same curve as the first-order inelastic analysis. The second-order inelastic analysis part is covered in STAAD.Pro without the effects of plastic hinge formation taken into account.

In the first-order inelastic analysis option, equations of equilibrium are written in terms of the geometry of the undeformed shape. Inelastic regions can develop gradually. Development of inelastic regions or "plastic hinge" development is not handled by STAAD.Pro V8i's new "Geometric Non-Linear Analysis" feature. The pushover analysis is the only feature that currently handles plastic hinge formation in STAAD.Pro.

In the second-order inelastic analysis option, equations of equilibrium are written in terms of the geometry of the deformed shape. It has the potential for accommodating all of the geometric, elastic, and material factors that influence the response of a structure. Again, only the geometric non-linearity is implemented in STAAD.Pro.

The equations of equilibrium in STAAD.Pro V8i's new "Geometric Non-Linear Analysis" feature are written in terms of the deformed shape.

References:

(1) Matrix Structural Analysis, Second edition, William McGuire

(2) Structural Plasticity, CIV E 705 Course Notes, Dr. Don E. Grierson, University of Waterloo

Comunidad de Usuarios de Bentley en Español

Eduardo Cortes — Mon, 03 Apr 2017 06:31:12 GMT

Hola,

Para todo lo referente a productos de Bentley en Castellano existe una Comunidad de Usuarios específica. Esta Comunidad se llama "Usuarios de Bentley en Español (Spanish Users)" y está accesible en este enlace:

communities.bentley.com/.../

En ella publico toda la información referente a los productos de Bentley que utilizo, tanto de Plataforma (MicroStation, ProjectWise, Navigator, Generative Components) como de Building (Bentley Architecture, Structural, Mechanical, Electrical, etc.)

Dicha comunidad también dispone de un foro que cumple la función de los antiguos foros en Español, existentes hasta que se migraron a las nuevas BE Communities,

Un saludo,

Acceptable Environment Attribute Format String Options

Jo West — Tue, 31 Mar 2009 17:46:00 GMT

One of the most common questions I get when covering the topic of environment attributes in a ProjectWise Administrator class is "is there a list of items that work for the Format String option?" The typical answer is "You can use C formats to format the value of the attribute". Unfortunately, most of the users in the classes I have been in are not "C" programmers. They just want a simple list they can work with.

The following is from The Model Server Teammate Environment Editor, circa 1999. It is simple documentation that is still valid today.

Formatting fields

When you specify the settings for an Edit field, you can apply a format mask which ensures that the field displays the data in a particular format. Acceptable formats are:

Format              Effect
Empty String    String passed as entered.
%[Flags]           Type Restricts data to the set type and applies justification (See below)
date                   Only accepts a date in international date format (yyyy-mm-dd) eg 1999-12-01 for 1 Jan 1999
date,day            Only accepts a date in international date format (yyyy-mm-dd) eg 1999-12-01 for 1 Jan 1999
date,sec            Only accepts a date and time in international date format (yyyy-mm-dd hh:mm:ss) eg 1999-12-01
                            12:00:00 for noon on 1 Jan 1999
UpperCase      Converts entered text to upper case.
LowerCase      Converts entered text to lower case.

Flags

Flags can be used to define the justification of the field. Strings are always left justified. Numbers are right justified by default, but their justification can be amended by an optional flag. Acceptable entries are:

Flag      Effect
None    Right Justification.
-             Left Justification.
0            Fills empty leading spaces with zeros. (only with right justification)

Width sets the number of characters which can be entered or displayed.

Type

Type defines the data type which can be entered in the field. Acceptable entries are:

Type     Data type
s            String (any alpha-numeric character)
d            Decimal number. (Whole signed numbers eg -10)
u            Unsigned decimal number. (Whole unsigned numbers eg 10)
f              Floating point number. (eg 10.3)

For example, a field with the format string ‘%0u' accepts only unsigned decimal numbers and fills the leading empty spaces with zeros. If the user enters 1234 in a field whose length is set to 10, the number 0000001234 displays in the field.

A field with the format string ‘%-d' allows the user to enter a signed number and applies left justification to the field.

A field with the format string ‘%-s' allows the user to enter any character and applies left justification to the field.

Release Announcement: RM Bridge CE V11 Update 10 and RM Bridge CE V11 Update 8 Maintenance Release 2

Afshin Hatami — Wed, 02 Dec 2020 03:58:00 GMT

RM Bridge CONNECT Edition V11 Update 10 (V11.10.00.XXX) contains the following enhancements and error corrections:

BIM Workflow: Improve Model Transfer from OpenBridge Modeler (OBM)
New Wizard for Rail Structure Interaction
Enhancements in GUI:
- Display Modeler Axis Orientation
- New option for Ascending/descending Tendons in Modeler
- New Options for Substructure, Cable-Stayed, and Rail-Structure Wizards
- Updated Display Load Options in The Main Window
Error Corrections

Note: All enhancements are available in OpenBridge Designer (OBD) CE V10.09.00.10, and error corrections are available in RM Bridge standalone download (V11.08.02. 04).

How to place and update fields in MicroStation?

Mark Penn — Mon, 13 Jan 2014 13:06:35 GMT

Fields are basically self-updating text. A field points to a property of an object and display its value. When the value changes, the text update and show the new value.

How to place a field:

You can place fields using Place Text tool. Select Insert Field icon in Word Processor and select the desired property.

Several types of fields can be placed in MicroStation:

Element Property Fields - Display properties of selected element.
Model Property Fields - Display properties of active model.
File Property Fields - Display properties of active file.
Place Holder Cell Property Fields - Display properties of the parent cell that contains the field.
Place Holder Link Property Fields - Display properties of an object that the field points to with the help of a design link.
Place Holder Digital Signature Cell Property Fields - Display properties of a digital signature cell.

Let's see these types in detail.

Element Property Fields: These are properties of an identified element in the active model or its reference. These fields update to reflect changes whenever a change to the element causes the property to change. You can place different properties like Angle, Length, Area, Perimeter, Color, Line Style etc.

Here is video for placing a Length field of the Line.

Note 1: Double-click to view Full Screen

Note 2: To run this video in Fire fox Click Here to download plug in.

Model Property Fields: These are properties of active model. These fields update automatically depending on the "Update Fields Automatically" check box in Model Properties dialog.

File Property Fields: These are properties of active file. All these properties appear in "File - Properties" dialog. When a file property is changed, you have to run key-in "FIELD UPDATE ALL" to update the fields.

Place Holder Cell Property Fields: These fields are placed in cell libraries. When you place those cells, the fields evaluate themselves based on the placed cell's properties. These fields update if you make any change the corresponding property of the placed cell.

Procedure for placing Place Holder Cell Properties field:

From Task - Drawing Menu, select Place Text. This will open word processor dialog
Select "Insert Field" from menu items or right click in word processor and select "Insert Field".
From "Field Type" drop down, select "Place Holder Cell Properties" and click OK. Field editor dialog shows properties available for placing as fields.
Select property you want to place such as Origin, Cell Name etc. and click OK.
Place the field in view window.
Suppose you want to place this field in another model in active file, open that model. With CTRL key pressed, drag the model in which you placed the field to the active model. Place the Model as cell in active model. OR Open Cell Library dialog, select File - Attach file, navigate to the file in which you placed the field. Double click on the model and place the model as cell in view window.
Run keyin "FIELD UPDATE ALL" to update the field.

Digital Signature Place Holder Cell Property Field: This is similar to place holder cell properties, except they show properties of a digital signature cell. These fields display signing date or other information about digital signature. You can create these fields in digital signature cell libraries.

Place Holder Link Property field: These fields make use of design links to find their target object. You can create these fields in cell libraries. When you place that cell in your master model and add appropriate link to the field, then it will evaluate itself and display value of the property. You have run keyin "FIELD UPDATE ALL" after adding link the field.

Procedure for placing Place Holder Link Property field:

Here is video for placing a place holder link property field pointing to a model property, adding a model link and then updating the field.

In the video,

Place Holder Link Property field for a model property is placed in a cell library.
This cell library is placed in as a cell in a design model.
A link pointing to a design model is added to the cell.
"FIELD UPDATE ALL" key-in is executed to update the field.

Note 1: Double-click to view Full Screen

Note 2: To run this video in Fire fox Click Here to download plug in.

This procedure shown in video can be used for placing all types of Link Properties fields. You can place fields for saved views, references, drawing titles, PDF files, MS Office files, etc.

For adding model as cell, you can drag and drop models, with CTRL key pressed, from Project Explorer dialog, if the models in which field is placed, is in different file.

Another method to place cell is to attach the file in Cell Library dialog. Then double click on the model you want to place as cell. This will start the Place Cell tool and you can place the field as cell.

Configuration Variables to update fields:

MS_AUTO_UPDATE_FIELD - This configuration variable controls rules for updating the fields.
In this blog, we have seen different types of field we can place, how to update them.

More blogs are coming up so be there.