design parameters in staad

CIV

IL D

EP

AR

TM

EN

T

CIV

IL D

EP

AR

TM

EN

T

Dated:- 25-OCT-2007

INTRODUCTION

SOME BASIC DEFINITIONS

SIGNIFICANCE OF DESIGN PARAMETERS

STEEL DESIGN PARAMETERS

CONCRETE DESIGN PARAMETERS

CONCLUSION

CONTENTS

Keeping in mind the competition in the market that JGC-DESCON is facing with the globally recognized organizations. It is important to provide the efficient services to our clients.

Efficiency of our work associated with the correct and precise use of designing tools. We should be familiar with the latest modifications or updates made in these tools.

Some useful modifications are also made in STAAD, which are helpful for the users to use the software more effectively. I would like to discuss some of these modifications made in DESIGN PARAMETERS.

The preparation of this presentation is based on the study made of the followings,

AISC- ASD Steel Construction ManualSteel Structures by Dr. Zahid Ahmad SiddiqiDiscussions with STAAD Technical Support Group

INTRODUCTION


PARAMETERS

STRUCTURE (ELEMENTS & ORIENTATION)

DESIGN RULES FOR BEAM

DESIGN RULES FOR COLUMN

STAAD CONCRETE DESIGN

STAAD STEEL DESIGN

PARAMETERS

STAAD contains a large number of parameters which are needed to perform designing and code checking. These parameters communicate design decisions from Engineer to the program.

The default parameter values have been selected such that they are frequently used numbers for conventional design.

Depending on the particular design requirements of an analysis, some or all of these parameter values may have to be changed to exactly model the physical structure. For example, by default the Kz (k value in local z-axis) value of a member is set to 1.0, while in real structure it may be 1.5



STRUCTURE

STRUCTURAL ELEMENTS

- LINE ELEMENTS ~ FRAME STR (Beam, Columns etc.)

- SHELL ELEMENTS ~ PLATE (Slabs, FDN, Walls etc.)

- BRICK ELEMENTS ~ SOLIDS, ASOLIDS (Dam, Thick FDN etc.)

ORIENTATION OF AXES

- Global axis (X, Y, Z)

- Local axis (x, y, z OR 1, 2, 3)

- All the forces are referred as “ALONG THE AXIS.” (Global or local)

- All the moments are referred as “ABOUT THE AXIS.” (Global or local)


ORIENTATION OF LOCAL AXIS FOR PLATE & LINE ELEMENT


DESIGN RULES FOR BEAM

The member is designed using the rules of BENDING (about local z) + SHEAR (along local y) + TORSION (about local x), when designed as a beam (MZ, FY and MX).

DESIGN RULES FOR COLUMN

The member is designed using the rules of AXIAL FORCE (along local x) + BIAXIAL BENDING (about local y & z), when designed as a column (FX, MY and MZ).


STAAD CONCRETE DESIGN

In staad, concrete members are designed using the commands,

- Design Beam (Beam design rules are used.)

- Bending (about local z-axis)

- Shear (along local y-axis)

- Torsion (about local x-axis)

- Design Column (Column design rules are used.)

- Axial (along local x-axis)

- Biaxial Bending (about local y and z-axis)


STAAD STEEL DESIGN

Steel members are designed considering the following points,

Steel structure members are not categorized into beams or columns as done in case of concrete design.

Steel members are designed as Beam-Column, Considering both, design rules used for Beams and Columns.

A steel member is designed for FX, FY, FZ, MZ, MY and MZ.

We have to assign all the associated parameters to all steel members without considering that the member is behaving as beam or column.

Design parameters plays an important role in structural analysis & design. For all structural elements, appropriate parameter values are essential to be assigned.

STAAD results would not represent the original structural behavior, if parameter values are missed or wrongly assigned.

Significance of Design Parameters in STAAD can easily be understood considering a simple example.


EXAMPLE:-

Assume a simply supported beam,

Length of beam = 10m

Profile = W8X15 (American)

Loading (UDL) = 1 KN/m

Material = Steel

Let’s design this beam in STAAD using AISC-ASD codes. We will compare the STAAD Results with and without assigning the parameter values.



UNT 10 assigned to this beam


AISC DESIGN PARAMETERS

SOME IMPORTANT AISC PARAMETERS

UNL, UNT, UNB, LY, LZ, KY, KZ, DFF, DJ1, DJ2

AISC Parameters


SOME IMPORTANT AISC PARAMETERS:-

(UNL, UNT, UNB, LY, LZ, KY, KZ, DFF, DJ1, DJ2)

UNL, UNT, UNB are associated with BENDING.

LY, LZ, KY, KZ are associated with BUCKLING.

DFF, DJ1, DJ2 are associated with DEFLECTION.


UNL, UNT, UNB (FLEXURAL BENDING):-


UNL = Unsupported length for calculating allowable bending stress.

UNT = Unsupported length of the top flange for calculating allowable bending compressive stress. Will be used only if flexural compression is on the top flange.

UNB = Unsupported length of the bottom flange for calculating allowable bending compressive stress. Will be used only if flexural compression is on the bottom flange.

UNL represents the laterally unsupported of the compression flange. It is defined in Chapter F, page 5-47 of the specifications in the AISC 1989 ASD manual as the distance between cross sections braced against twist or lateral displacement of the compression flange. UNL is used to calculate the allowable compressive stress (FCZ and FCY) for behavior as beam.


In versions of STAAD prior to STAAD/Pro 2000, the mechanism for specifying unsupported length of the compression flange was through the means of the UNL parameter. However, the drawback of this command is that if the value for the top flange is different from that of the bottom flange, there wasn’t any means to communicate that information to STAAD.

Consequently, 2 new commands were introduced, UNT and UNB for top and bottom flange simultaneously.

To avoid the confusion that may arise from having 3 separate parameters to specify 2 items of input, we no longer mention the UNL parameter. However, to enable the current versions of STAAD to analyze input files created using the older versions of STAAD, the UNL parameter continues to work the way it did.



A member is designed for a total of 13 points along its length Start, End, and 11 intermediate sections. At each of those 13 points, Staad examines Mz to determine whether the moment produced compression on the top flange or bottom flange.

If the compression due to Mz is on the top flange, UNT is used in design. If the compression due to Mz is on the bottom flange, UNB is used.

How to assign UNB & UNT in staad


LY, LZ, KY, KZ (BUCKLING):-


LY = Length to calculate slenderness ratio for buckling about local y-axis.

LZ = Length to calculate slenderness ratio for buckling about local z-axis.

KY = K (Effective Length factor) value in local y-axis.

KZ = K (Effective Length factor) value in local z-axis.


Effective Length (Ly, Lz)

Effective length factors (Ly, Lz) are used to calculate the slenderness ratios, which are associated with Buckling phenomenon. All the steel structural members are required to satisfy the allowable slenderness limits as discussed below,

According to AISC,

For Tension Members, the slenderness ratio Kl/r ≤ 300

For Compression Members, the slenderness ratio Kl/r ≤ 200

If this check results in failure, the member is declared as Failed, and design for that member is immediately terminated. The code does not offer any guidelines on what must be the minimum magnitude of axial force for the member to become a candidate for this check.

How to assign Ly & Lz in Staad?



Effective Length Factors (Ky, Kz)

This factor gives the ratio of length of half sine wave of deflected shape after buckling to full-unsupported length of column. In other words, it is the ratio of effective length to the unsupported length. This depends upon the end conditions of the column and the fact that whether side sway is permitted or not. Greater the K-value, greater is the effective length and slenderness ratio and hence smaller is the buckling load. K-value in case of no side sway is between 0.5 and 1.0, whereas, in case of appreciable side sway, it is always greater than or equal to 1.0

1. K-Factor For Columns Having Well Defined End Conditions

2. K-Factor For Frame Or Partially Restrained Columns

3. K-Factor For Truss Members


1. K-Factor For Columns Having Well Defined End Conditions



2. K-Factor For Frame Or Partially Restrained Columns(ALIGNMENT CHART METHOD USING AISC-ASD)


2. K-Factor For Frame Or Partially Restrained Columns


2. K-Factor For Frame Or Partially Restrained Columns(ALIGNMENT CHART METHOD USING AISC-ASD)

3. K-Factor For Truss Members

The effective length factor, K, is considered equal to 1.0 for members of the truss subjected to static loads and equal to 0.85 if the moving loads are applied. Although welded and riveted connections provide some restraint at the ends and joint translation is prevented in the plane of the truss by other members and out of plane by the bracing, this restraint may not be considered fully effective as various members have a tendency to buckle at the same load. However, the possibility of all members buckling together is less in case of moving loads due to varying magnitudes of forces in such cases. Reduced K-value may thus be considered for truss members subjected to moving loads.


DFF, DJ1, DJ2 (DEFLECTION)

DFF = “Deflection Length”/Max. allowable local deflection

Example:- Allowable local Deflection = L / 360, L/1500, L/800

DJ1 = Joint No. denoting starting point for calculation of “Deflection Length”.

DJ2 = Joint No. denoting end point for calculation of “Deflection Length”.


CONCRETE DESIGN PARAMETERS

ACI-318 Parameters

CONCLUSION

CONCLUSION / SUMMARY

STAAD parameters/parameter values may vary depending upon the design codes and software version used, so the subsequent changes must be investigated in the start of the project.

Parameter UNL is no more available in STAAD steel design, it is replaced with UNT and UNB.

Parameters are of significant value regarding design, So all the parameters (Ky, Kz, Ly, Lz, UNB, UNT, DFF, DJ1, DJ2 etc.) must be calculated, if required (e.g. Ky, Kz etc.) and assign correctly.

In case of steel structure, we have to assign all the associated parameters to all the steel members without considering its beam or column behavior.

CIV

IL D

EP

AR

TM

EN

T

design parameters in staad

Documents

alignment

steel design

steel design

concrete design

partially

slenderness

effective

steel design