Steel Joist Design

Computers and Structures, Inc.Berkeley, California, USA

Version 8January 2002

ETABS®

Integrated Building Design Software

Steel Joist Design Manual

Copyright Computers and Structures, Inc., 1978-2002.The CSI Logo is a trademark of Computers and Structures, Inc.

ETABS is a trademark of Computers and Structures, Inc.Windows is a registered trademark of Microsoft Corporation.

Adobe and Acrobat are registered trademarks of Adobe Systems Incorporated

Copyright

The computer program ETABS and all associated documentation are proprietary andcopyrighted products. Worldwide rights of ownership rest with Computers andStructures, Inc. Unlicensed use of the program or reproduction of the documentation inany form, without prior written authorization from Computers and Structures, Inc., isexplicitly prohibited.

Further information and copies of this documentation may be obtained from:

Computers and Structures, Inc.1995 University Avenue

Berkeley, California 94704 USA

Phone: (510) 845-2177FAX: (510) 845-4096

e-mail: [email protected] (for general questions)e-mail: [email protected] (for technical support questions)

web: www.csiberkeley.com

DISCLAIMER

CONSIDERABLE TIME, EFFORT AND EXPENSE HAVE GONE INTO THEDEVELOPMENT AND DOCUMENTATION OF ETABS. THE PROGRAM HASBEEN THOROUGHLY TESTED AND USED. IN USING THE PROGRAM,HOWEVER, THE USER ACCEPTS AND UNDERSTANDS THAT NO WARRANTYIS EXPRESSED OR IMPLIED BY THE DEVELOPERS OR THE DISTRIBUTORSON THE ACCURACY OR THE RELIABILITY OF THE PROGRAM.

THIS PROGRAM IS A VERY PRACTICAL TOOL FOR THE DESIGN/CHECK OFSTEEL STRUCTURES. HOWEVER, THE USER MUST THOROUGHLY READ THEMANUAL AND CLEARLY RECOGNIZE THE ASPECTS OF STEEL JOIST DESIGNTHAT THE PROGRAM ALGORITHMS DO NOT ADDRESS.

THE USER MUST EXPLICITLY UNDERSTAND THE ASSUMPTIONS OF THEPROGRAM AND MUST INDEPENDENTLY VERIFY THE RESULTS.

i

©COMPUTERS AND STRUCTURES, INC., BERKELEY, CALIFORNIA JANUARY 2002

STEEL JOIST DESIGN

Contents

Steel Joist Design

1 General Design InformationUnits 1-1Beams Designed as Steel Beams 1-1

Section Requirements for Steel Joists 1-1Material Property Requirements for

Steel Joists 1-2Other Requirements for Steel Joists 1-2Frame Elements Designed by Default

as Steel Joists 1-2Overwriting the Frame Design

Procedure for a Steel Joist 1-3How the Program Optimizes Design Groups 1-3Analysis Sections and Design Sections 1-4Output Stations 1-6

2 Steel Joist Design ProcessDesign Process 2-1

3 Interactive Steel Joist DesignMember Identification 3-1Section Information 3-2Acceptable Sections List 3-3ReDefine 3-4Temporary 3-4Show Details 3-5

4 Steel Joist PropertiesGeneral 4-1Joist Properties for Standard Design 4-1

Steel Joist Design Manual

ii

Joist Properties for Envelope Design 4-3

5 OverwritesGeneral 5-1Using the Steel Joist Overwrites Form 5-1Overwrites 5-2

6 Design Load CombinationsGeneral 6-1

7 Design Checks for Standard DesignJoist Span 7-1Total Load Check 7-1Live Load Check 7-1Design of Joists When the Load is Not

Uniform 7-2

8 Design Checks for Envelope DesignMoment Capacity 8-1Shear Capacity 8-1Live Load Deflection Check 8-1Maximum Equivalent Uniform Load Check 8-1

9 Data Plotted Directly on the ModelOverview 9-1Labels Displayed on the Model 9-2Design Data 9-2Design Ratios 9-2

10 Output DetailsSummary of Steel Joist Output 10-1Steel Joist Detailed Output 10-2Steel Joist Design Check Messages 10-5

Units Technical Note 1 - 1


STEEL JOIST DESIGN

Technical Note 1General Design Information

This Technical Note presents some basic information and concepts that areuseful when performing steel joist design using this program.

UnitsFor steel joist design in this program, any set of consistent units can be usedfor input. Typically, design codes are based on one specific set of units. Thedocumentation in the Steel Joist Design series of Technical Notes is pre-sented in kip-inch-seconds units unless otherwise noted.

You can change the system of units at any time using the pull-down menu onthe Status Bar or pull-down menu on individual forms where available.

Note:

You can use any set of units in steel joist design and change the units "on the fly."

Beams Designed as Steel JoistsSection Requirements for Steel JoistsOnly beams that are assigned joist sections can be designed as steel joists.The joist sections can be selected from the built-in program joist section da-tabase, or they can be user defined. The user-defined sections can be speci-fied using the Define menu > Frame Sections command and clicking theAdd Steel Joist option.

Note that beam sections that are defined in Section Designer are alwaystreated as general sections. Thus, if you define a joist-type section in SectionDesigner, the program will consider it as a general section, not a joist section,and will not allow it to be designed as a steel joist.

General Design Information Steel Joist Design

Technical Note 1 - 2 Beams Designed as Steel Joists

Note:

Beam sections defined in the section designer utility cannot be designed as steel joists.

Material Property Requirement for Steel JoistsIf a beam is to be designed as a steel joist, the Type of Design associatedwith the Material Property Data assigned to the beam must be Steel. Use theDefine menu > Material Properties > Modify/Show Materials commandto check your beams.

Other Requirements for Steel JoistsThe line type associated with the line object that represents a steel joist mustbe "Beam." In other words, the beam element must lie in a horizontal plane.Right click on a line object to bring up the Line Information form to check theLine Type.

For steel joists, the beam local 2-axis must be vertical. The Local axis 2 Angleis displayed on the Assignments tab of the Line Information form.

The line object representing a steel joist should span from support to support.Steel joists should not be modeled using multiple, adjacent line objects be-tween supports for a single joist.

Frame Elements Designed by Default as Steel JoistsThe program will design certain frame elements using the design proceduresdocumented in these Technical Notes by default. Those elements must meetthe following criteria:

The beam must meet the section requirements described in the subsectionentitled "Section Requirements for Steel Joists" in this Technical Note.

The beam must meet the material property requirement described in thesubsection entitled "Material Property Requirement for Steel Joists" in thisTechnical Note.

The beam must meet the other requirements described in the subsectionentitled "Other Requirements for Steel Joists" in this Technical Note.

The beam must be assigned a steel joist section.

Steel Joist Design General Design Information

How the Program Optimizes Design Groups Technical Note 1 - 3

The beam must not frame continuously into a column or a brace. Bothends of the beam must be pinned for major axis bending (bending aboutthe local 3-axis).

Overwriting the Frame Design Procedure for a Steel JoistThe design procedures possible for a beam that is assigned a steel joist sec-tion design are:

Steel joist design

No design

All beam elements that meet the requirements described in the previous sub-section entitled "Frame Elements Designed by Default as Steel Joists" are bydefault designed using the steel joist design procedures.

Change the default design procedure used for a beam(s) by selecting thebeam(s) and clicking Design menu > Overwrite Frame Design Proce-dure. This change is only successful if the design procedure assigned to anelement is valid for that element. For example, if you select two steel beams,one an I-section and the other a joist section, and attempt to change the de-sign procedure to Steel Joist Design, the change will be executed for the joistsection, but not for the I-section because it is not a valid section for the steeljoist design procedure.

How the Program Optimizes Design GroupsThis section describes the process the program uses to select the optimumsection for a design group. In this description, note the distinction betweenthe term section, which refers to a joist section in an auto select section list,and the term beam, which refers to a specific beam element in the designgroup.

When considering design groups, the program first discards any beam in thedesign group that is not assigned an auto select section list.

Next, the program looks at the auto select section list assigned to each beamin the design group and creates a new list that contains the sections that arecommon to all of the auto select section lists in the design group. The pro-


Technical Note 1 - 4 Analysis Sections and Design Sections

gram sorts this new common section list in ascending order, from smallestsection to largest section based on section weight.

Note:

When designing with design groups, the program attempts to quickly eliminate inade-quate beams.

The program checks all beams in the design group for the first section (small-est by weight) in the section list. If the section is adequate for all beams inthe design group, the optimum section has been identified. If the section isnot adequate for a beam, the next higher section in the section list is trieduntil a section is found that is adequate for all beams in the design group.

If all sections in the section list are tried and none of them are adequate forall of the beams in the design group, the program proceeds to design eachbeam in the design group individually based on its own auto section list andignores the rest of the design group.

Analysis Sections and Design SectionsAnalysis sections are those section properties used to analyze the modelwhen you click the Analyze menu > Run Analysis command. The designsection is whatever section has most currently been designed and thus desig-nated the current design section.

Tip:

It is important to understand the difference between analysis sections and design sec-tions.

It is possible for the last used analysis section and the current design sectionto be different. For example, you may have run your analysis using a 16K5joist and then found in the design that a 16K3 joist worked. In that case, thelast used analysis section is the 16K5 and the current design section is the16K3. Before you complete the design process, verify that the last usedanalysis section and the current design section are the same. The Designmenu > Steel Joist Design > Verify Analysis vs Design Section com-mand is useful for this task.

The program keeps track of the analysis section and the design sectionseparately. Note the following about analysis and design sections:

Steel Joist Design General Design Information

Analysis Sections and Design Sections Technical Note 1 - 5

Assigning a beam a frame section property using the Assign menu >Frame/Line > Frame Section command assigns the section as both theanalysis section and the design section.

Running an analysis using the Analyze menu > Run Analysis command(or its associated toolbar button) always sets the analysis section to bethe same as the current design section.

Assigning an auto select list to a frame section using the Assign menu >Frame/Line > Frame Section command initially sets the design sectionto be the beam with the median weight in the auto select list.

Unlocking a model deletes the design results, but it does not delete orchange the design section.

Using the Design menu > Steel Joist Design > Select Design Combocommand to change a design load combination deletes the design results,but it does not delete or change the design section.

Using the Define menu > Load Combinations command to change adesign load combination deletes the design results, but it does not deleteor change the design section.

Deleting the static nonlinear analysis results also deletes the design re-sults for any load combination that includes static nonlinear forces. Typi-cally, static nonlinear analysis and design results are deleted when one ofthe following actions is taken:

Use the Define menu > Frame Nonlinear Hinge Properties com-mand to redefine existing or define new hinges.

Use the Define menu > Static Nonlinear/Pushover Cases com-mand to redefine existing or define new static nonlinear load cases.

Use the Assign menu > Frame/Line > Frame Nonlinear Hingescommand to add or delete hinges.

Again, note that these actions delete only results for load combinations thatinclude static nonlinear forces.


Technical Note 1 - 6 Output Stations

Output StationsFrame output stations are designated locations along a frame element. Theyare used as locations to report output forces and to perform design, and asplotting points used for graphic display of force diagrams. When force dia-grams are plotted, exact forces are plotted at each output station and thenthose points are connected by straight lines. Output stations occur at user-specified locations and at point load locations along a beam. Designate theoutput stations for a frame element using the Assign menu.

Design Process Technical Note 2 - 1


STEEL JOIST DESIGN

Technical Note 2Steel Joist Design Process

This Technical Notes describes a basic steel joist design process using thisprogram. Although the exact steps you follow may vary, the basic design pro-cess should be similar to that described herein. Other Technical Notes in theSteel Joist Design series provide additional information.

Design ProcessThe following sequence describes a typical steel joist design process. Notethat although the sequence of steps you follow may vary, the basic processprobably will be essentially the same.

1. Create the building model.

2. Run the building analysis using the Analyze menu > Run Analysiscommand.

3. Assign steel joist overwrites, if needed, using the Design menu > SteelJoist Design > View/Revise Overwrites command. Note that youmust select beams before using this command. Also note that default val-ues are provided for all steel joist design overwrites so it is unnecessary todefine overwrites unless you want to change some of the default values.Note that the overwrites can be assigned before or after the analysis isrun. See Steel Joist Design Technical Note 5 Overwrites.

4. Designate design groups, if desired, using the Design menu > SteelJoist Design > Select Design Group command. Note that you musthave already created some groups by selecting objects and clicking theAssign menu > Group Names command.

5. To use design load combinations other than the defaults created by theprogram for steel joist design, click the Design menu > Steel Joist De-sign > Select Design Combo command. Note that you must have al-

Steel Joist Design Process Steel Joist Design

Technical Note 2 - 2 Design Process

ready created your own design combos by clicking the Define menu >Load Combinations command.

See Steel Joist Design Technical Note 6 Design Load Combinations formore information.

6. Click the Design menu > Steel Joist Design > Start Design WithoutSimilarity command to run the steel joist design.

7. Review the steel joist design results by doing one of the following:

a. Click the Design menu > Steel Joist Design > Display DesignInfo command to display design input and output information on themodel. See Steel Joist Design Technical Note 9 Data Plotted Directly onthe Model for further discussion.

b. Right click on a beam while the design results are displayed on it toenter the interactive design mode and interactively design the beam.Note that while you are in this mode, you can also view diagrams(load, moment, shear and deflection) and view design details on thescreen. See Steel Joist Design Technical Note 3 Interactive Steel JoistDesign for more information.

If design results are not currently displayed (and the design has beenrun), click the Design menu > Steel Joist Design > InteractiveSteel Joist Design command and then right click a beam to enter theinteractive design mode for that beam.

c. Use the File menu > Print Tables > Steel Joist Design commandto print steel joist design data. If you select beams before using thiscommand, data is printed only for the selected beams. See Steel JoistTechnical Note 10 Output Details for more information.

d. Use the Design menu > Steel Joist Design > Verify all MembersPassed command to verify that no members are overstressed or oth-erwise unacceptable.

8. Use the Design menu > Steel Joist Design > Change Design Sectioncommand to change the steel joist design section properties for the se-lected joists.

Steel Joist Design Steel Joist Design Process

Design Process Technical Note 2 - 3

9. Click the Design menu > Steel Joist Design > Start Design/Check ofStructure command to rerun the steel joist design with the new sectionproperties. Review the results using the procedures described in Step 8.

10. Rerun the building analysis using the Analyze menu > Run Analysiscommand. Note that the section properties used for the analysis are thelast specified design section properties.

11. Click the Design menu > Steel Joist Design > Start Design/Check ofStructure command to rerun the composite beam design with the newanalysis results and new section properties. Review the results using theprocedures described in Step 8.

12. Again use the Design menu > Steel Joist Design > Change DesignSection command to change the steel joist design section properties forselected joists, if necessary.

13. Repeat Steps 11, 12 and 13 as many times as necessary.

Note:

Steel joist design in the program is an iterative process. Typically, the analysis and de-sign will be rerun multiple times to complete a design.

14. Select all beams and click the Design menu > Steel Joist Design >Make Auto Select Section Null command. This removes any auto selectsection list assignments from the selected beams.

15. Rerun the building analysis using the Analyze menu > Run Analysiscommand. Note that the beam section properties used for the analysis arethe last specified design section properties.

16. Click the Design menu > Steel Joist Design > Start Design/Check ofStructure command to rerun the steel joist design with the new sectionproperties. Review the results using the procedures described above.

17. Click the Design menu > Composite Beam Design > Verify Analysisvs Design Section command to verify that all of the final design sectionsare the same as the last used analysis sections.

Steel Joist Design Process Steel Joist Design

Technical Note 2 - 4 Design Process

18. Use the File menu > Print Tables > Steel Joist Design command toprint selected steel design results if desired. See Steel Joist Technical Note10 Output Details for more information.

It is important to note that design is an iterative process. The sections used inthe original analysis are not typically the same as those obtained at the endof the design process. Always run the building analysis using the final joistsection sizes and then run a design check using the forces obtained from thatanalysis. Use the Design menu > Steel Joist Design > Verify Analysis vsDesign Section command to verify that the design sections are the same asthe analysis sections.

Member Identification Technical Note 3 - 1


STEEL JOIST DESIGN

Technical Note 3Interactive Steel Joist Design

Interactive steel joist design is a powerful feature that allows the user to re-view the design results for any steel joist and interactively revise the designassumptions and immediately review the revised results.

Note that a design must have been run for the interactive design mode to beavailable. To run a design, click the Design menu > Steel Joist Design >Start Design/Check of Structure command.

To enter the interactive design mode and interactively design the joist, rightclick on a joist while the design results are displayed in the active window. Ifdesign results are not displayed (and the design has been run), click the De-sign menu > Steel Joist Design > Interactive Steel Joist Design com-mand and then right click a joist.

The following sections describe the features that are included in the Interac-tive Steel Joist Design and Review form.

Member IdentificationStory IDThis is the story level ID associated with the steel joist.

Beam LabelThis is the label of the line object to which the steel joist section is assigned.

Design GroupThis is the name of the design group that the line object is assigned to if thatdesign group was considered in the design of the steel joist. If the line objectis part of a design group but the design group was not considered in the de-sign, N/A is displayed. If the line object is not assigned to any design group,"NONE" is displayed.

If a joist is redesigned as a result of a change made in the Interactive SteelJoist Design and Review form, the design group is ignored and only the single

Interactive Steel Joist Design Steel Joist Design

Technical Note 3 - 2 Section Information

beam is considered. Thus, as soon as you design a beam in the InteractiveSteel Joist Design and Review form, the Design Group box either displays N/Aor None.

Section InformationAuto Select ListThis drop-down box displays the name of the auto select section list assignedto the beam. If no auto select list has been assigned to the beam, NONE isdisplayed. You can change this item to another auto select list or to NONEwhile in the form and the design results will be updated immediately. If youchange this item to NONE, the design is performed for the Current De-sign/Next Analysis section property.

OptimalIf an auto select section list is assigned to the beam, this box displays the op-timal section as determined by joist weight. If no auto select list is assignedto the beam, N/A is displayed for this item.

Last AnalysisThis box displays the name of the section that was used for this beam in thelast analysis. Thus, the beam forces are based on a beam of this sectionproperty. For the final design iteration, the Current Design/Next Analysis sec-tion property and the Last Analysis section property should be the same.

Current Design/Next AnalysisThis box displays the name of the current design section property. If thebeam is assigned an auto select list, the section displayed in this form initiallydefaults to the optimal section.

Tip:

The section property displayed for the Current Design/Next Analysis item is used by theprogram as the section property for the next analysis run.

If no auto select list has been assigned to the beam, the beam design is per-formed for the section property specified in this box.

It is important to note that subsequent analyses use the section propertyspecified in this box for the next analysis section for the beam. Thus, theforces and moments obtained in the next analysis are based on this joist size.

Steel Joist Design Interactive Steel Joist Design

Acceptable Sections List Technical Note 3 - 3

The Current Design/Next Analysis section property can be changed by clickingthe Sections button that is described later in this Technical Note.

Important note: Changes made to the Current Design/Next Analysis sectionproperty are permanently saved (until you revise them again) if you click theOK button to exit the Interactive Composite Beam Design and Review form. Ifyou exit the form by clicking the Cancel button, those changes are consid-ered temporary and are not permanently saved.

Acceptable Sections ListThe Acceptable Sections List includes the following information for each joistsection that is acceptable for all considered design load combinations.

Joist section name

Controlling design load combination

Ratio type (flexural, shear or live load deflection)

Controlling ratio

Joist unit weight

Tip:

A single beam displayed in a red font in the Acceptable Sections List means that none ofthe sections considered were acceptable.

Typically, the ratio displayed is the largest ratio obtained considering flexure,shear and live load deflection.

If the beam is assigned an auto select list, many joist sections may be listedin the Acceptable Sections List. If necessary, use the scroll bar to scrollthrough the acceptable sections. The optimal section is initially highlighted inthe list.

If the joist is not assigned an auto select list, only one joist section will belisted in the Acceptable Sections List. It is the same section as specified in theCurrent Design/Next Analysis edit box.

At least one joist will always be shown in the Acceptable Sections List, even ifnone of the joists considered are acceptable. When no joists are acceptable,

Interactive Steel Joist Design Steel Joist Design

Technical Note 3 - 4 ReDefine

the program displays the section with the smallest controlling ratio in a redfont. Thus, a single beam displayed in a red font in the Acceptable SectionsList means that none of the sections considered were acceptable.

ReDefineSections ButtonUse the Sections button to change the Current Design/Next Analysis sectionproperty. This button can designate a new section property whether the sec-tion property is or is not displayed in the Acceptable Sections List.

When you click on the Sections button, the Select Sections form appears.Assign any joist section property to the beam by clicking on the desired prop-erty and clicking OK. Note that if an auto select list is assigned to the beam,using the Sections button sets the auto select list assignment to NONE.

Overwrites ButtonClick the overwrites button to access and make revisions to the steel joistoverwrites and then immediately see the new design results. Modifying someoverwrites in this mode and exiting both the Steel Joist Design Overwritesform and the Interactive Steel Joist Design and Review form by clicking theirrespective OK buttons permanently saves changes made to the overwrites.

Exiting the Steel Joist Design Overwrites form by clicking the OK button tem-porarily saves changes. Subsequently exiting the Interactive Steel Joist De-sign and Review form by clicking the Cancel button cancels the changesmade. Permanent saving of the overwrites does not occur until the OK but-tons in both the Steel Joist Design Overwrites form and the Interactive SteelJoist Design and Review form have been clicked.

TemporaryCombos ButtonClick this button to access and make temporary revisions to the design loadcombinations considered for the joist. This is useful for reviewing the resultsfor one particular load combination, for example. You can temporarily changethe considered design load combinations to be just the one you are interestedin and review the results.

Steel Joist Design Interactive Steel Joist Design

Show Details Technical Note 3 - 5

The changes made to the considered design load combinations using thecombos button are temporary. They are not saved when you exit the Interac-tive Steel Joist Design and Review form, whether you click OK or Cancel toexit it.

Show DetailsDiagrams ButtonClicking the Diagrams button displays a form with the following four types ofdiagrams for the beam.

Applied loads

Shear

Moment

Deflection

The diagrams are plotted for specific design load combinations specified in theform by the user.

Details ButtonClicking the Details button displays design details for the joist. The informa-tion displayed is similar to the detailed output that can be printed using theFile menu > Print Tables > Steel Joist Design command. See Steel JoistDesign Technical Note 10 Output Details for more information.

General Technical Note 4 - 1


STEEL JOIST DESIGN

Technical Note 4Steel Joist Properties

This Technical Note provides an overview of steel joist properties, includingtheir design capacities.

GeneralSteel joist properties are defined using the Define menu > Frame Sectionscommand. Use this command to import joist properties from the joist sectiondatabase file named joists.pro that is provided with the program, or to defineyour own joist sections. The joist sections available in the database includethe K-Series, KCS-Series, LH-Series and DLH-Series as defined by the SteelJoist Institute (SJI).

Two types of joist design capacities are considered by the program: Standardand Envelope design. Standard design is based on specified total uniform loadcapacities and live uniform load capacities for a range of spans. Envelope de-sign is based on specified moment and shear capacities and a specified mo-ment of inertia that is used to calculate deflection.

The K-Series, LH-Series and DLH-Series joists use the Standard design ca-pacities. The KCS-Series joists use the Envelope design capacities. Userspecified joists can use either Standard design or Envelope design. Each ofthe design types is described in more detail later in this technical note.

Joist Properties for Standard DesignFigure 1 shows the Joist Section Properties form for a joist using Standarddesign. For Standard design, a series of joist spans is specified along with atotal load capacity for each span and a live load capacity that produces anL/360 deflection for each span.

In addition, a moment of inertia is provided for each span. This moment ofinertia is only used when reporting the joist deflection in Interactive Steel

Steel Joist Properties Steel Joist Design

Technical Note 4 - 2 Joist Properties for Standard Design

Joist Design and Review (see Steel Joist Design Technical Note 3 InteractiveSteel Joist Design); it is not actually used in the design of the joist.

The joist depth item is specified but is not currently used in the design.

The joist unit weight is used both to calculate an area for the section that is inthe analysis, and to optimize the design when auto select section lists areused. The optimum joist is the lightest joist that passes all of the designchecks.

The Analysis I33 item is used as the moment of inertia for the joist in theanalysis.

Figure 1: Joist Properties - Standard Design

Steel Joist Design Steel Joist Properties

Joist Properties for Envelope Design Technical Note 4 - 3

Joist Properties for Envelope DesignFigure 2 shows a the Joist Section Properties form for a joist using Envelopedesign. For Envelope design a moment and shear capacity are specified alongwith a minimum and maximum span for which the joist will be designed.

The joist depth item is specified but is not currently used in the design.

The joist unit weight is used both to calculate an area for the section that is inthe analysis, and to optimize the lightest joist that passes all of the designchecks.

The Analysis I33 item is used both as the moment of inertia for the joist inthe analysis and also to calculate the joist deflection, both for design checksand when the joist deflection is reported in Interactive Steel Joist Design andReview (see Steel Joist Design Technical Note 3 Interactive Steel Joist De-sign).

Figure 2: Joist Properties - Envelope Design



STEEL JOIST DESIGN

Technical Note 5Overwrites

This Technical Note provides instructions on how to use the Steel Joist DesignOverwrites form and describes each overwrite.

GeneralThe steel joist design overwrites are basic assignments that apply only tothose steel joists to which they are assigned. After selecting one or more steeljoists, use the Design menu > Steel Joist Design > View/Revise Over-writes command to access the Steel Joist Design Overwrites form where youcan view and revise the steel joist design overwrites.

Default values are provided for all steel joist overwrite items. Thus, it is notrequired that you specify or change any of the overwrites. However, at leastreview the default values for the overwrite items to make sure they are ac-ceptable. When changes are made to overwrite items, the program appliesthe changes only to the elements to which they are specifically assigned; thatis, to the elements that are selected when the overwrites are changed.

Using the Steel Joist Overwrites FormAfter selecting one or more steel joist, use the Design menu > Steel JoistDesign > View/Revise Overwrites command to access the Steel Joist De-sign Overwrites form.

The steel joist overwrites are displayed with a column of check boxes and atwo-column spreadsheet. The left column in the spreadsheet contains thename of the overwrite item. The right column in the spreadsheet contains theoverwrite value.

Initially, the check boxes are all unchecked and all of the cells in the spread-sheet have a gray background to indicate they are inactive and that the itemsin the cells currently cannot be changed. The names of the overwrite items inthe first column of the spreadsheet are visible. The values of the overwrite

Overwrites Steel Joist Design

Technical Note 5 - 2 Overwrites

items in the second column of the spreadsheet are visible if only one joist wasselected before the Steel Joist design Overwrites form was accessed. If multi-ple joists were selected, no values show for the overwrite items in the secondcolumn of the spreadsheet.

After selecting one or multiple joists, check the box to the left of an overwriteitem to change it. Then left click in either column of the spread sheet to acti-vate a drop-down box or to highlight the contents of the cell in the right col-umn of the spreadsheet. If the drop-down box appears, select a value fromthe box. If the cell contents becomes highlighted, type in the desired value.The overwrite will reflect the change. You cannot change the values in thedrop-down boxes.

When you check a check box or left click in one of the columns in the spread-sheet, a short description of the item in that row displays in the large text boxjust below the list of items. This description helps you recall the purpose ofthe overwrite item without referring to the manual.

When changes to the steel joist overwrites have been made, click the OKbutton to close the form. The program then changes all of the overwrite itemswhose associated check boxes are checked for the selected joist(s). You mustclick the OK button for the changes to be accepted by the program. If youclick the Cancel button to exit the form, any changes made to the overwriteswill be ignored and the form will be closed.

OverwritesFor purposes of explanation in this Technical Note, the overwrite items arepresented in tables. The column headings in these tables are described asfollows.

Item: The name of the overwrite item as it appears in the cells at the leftside of the Steel Joist Design Overwrites form.

Possible Values: The possible values for the associated overwrite item.

Default Value: The built-in default value that the program assumes forthe associated overwrite item.

Description: A description of the associated overwrite item.

Steel Joist Design Overwrites

Overwrites Technical Note 5 - 3

Table 1: Steel Joist Overwrites

ItemPossibleValues

DefaultValue Description

Current designsection

Any definedjoist section

The currentjoist design

section

The current design section for the joist.

LL deflectionlimit (L/)

>0 360 Deflection limitation for live load. Input-ting 360 means that the limit is L/360where L is the length of the joist.

Reduced liveload factor

Programcalculated oruser-defined

Programcalculated

Reducible live load is multiplied by thisfactor to obtain the reduced live load. Ifzero is selected, the program calcu-lated valued is used.

Ignoresimilarity

Yes, No No This item is Yes if story level similarity(to a master story level) is to be ignoredwhen designing the joist.



STEEL JOIST DESIGN

Technical Note 6Design Load Combinations

This Technical Note described the default design load combinations for steeljoist design in the program.

GeneralTwo default design load combinations created by the program for steel joistdesign are:

Σ(DL) + Σ(SDL) Eqn. 1

Σ(DL) + Σ(SDL) + Σ(LL) + Σ(RLL) Eqn. 2

where

Σ(DL) = Sum of all dead load applied to the joist.

Σ(SDL) = Sum of all superimposed dead load applied to the joist.

Σ(LL) = Sum of all live load applied to the joist.

Σ(RLL) = Sum of all reducible live load applied to the joist.

You can replace or supplement these default design load combinations withyour own design load combinations, if desired.

Joist Span Technical Note 7 - 1


STEEL JOIST DESIGN

Technical Note 7Design Checks for Standard Design

This Technical Note describes the design checks performed by the programfor Standard joist design.

Joist SpanThe joist span used to determine the capacity of K-Series and User joists isthe full center-of-support to center-of-support span.

For LH-Series and DLH-Series joists, the joist span is taken as the full center-of-support to center-of-support span minus 8 inches.

Total Load CheckFor the total load check, the program calculates the ratio of the maximumtotal uniform load acting on the joist divided by the total uniform load capac-ity of the joist. This ratio is reported in the output.

Live Load CheckFor the live load check, the program first calculates the uniform live load ca-pacity using Equation 1.

LL Capacity = L/360 Capacity *(360 / LL Defl Limit) Eqn. 1

where,

LL Capacity = Maximum uniform live load capacity

L/360 Capacity = Uniform live load capacity specified for the joist spanin the joist section properties

LL Defl Limit = Live load deflection limit specified in the overwrites

Design Checks for Standard Design Steel Joist Design

Technical Note 7 - 2 Design of Joists When the Load is Not Uniform

The program then calculates the ratio of the maximum uniform live load act-ing on the joist divided by the uniform live load capacity of the joist. This ratiois reported in the output.

Design of Joists When the Load is Not UniformFor Standard design, the program checks if the applied load is uniform. If theload is not uniform, the design section is displayed with SP appended to itsname.

The program uses the following method to determine if the load applied in aparticular design load combination is uniform.

1. Find the maximum moment in the joist.

2. Calculate the uniform load that produces this maximum moment.

3. Calculate the moment cause by the uniform load calculated in step 2 ateach design output station along the joist.

4. Compare the actual moments obtained from the analysis at each designoutput station with the moments calculated in step 3. If any of the mo-ments differ by more than 5%, the load is reported as not uniform.

When the load is not uniform, the program uses an equivalent uniform load todetermine the design ratios. The equivalent uniform load is the one calculatedin step 2 above, and is equal to 8 * Mmax / Span2.

Moment Capacity Technical Note 8 - 1


STEEL JOIST DESIGN

Technical Note 8Design Checks for Envelope Design

This Technical Note describes the design checks performed by the programfor Envelope joist design.

Moment CapacityFor the moment capacity check, the program calculates the ratio of themaximum moment anywhere in the joist divided by the moment capacity ofthe joist. This ratio is reported in the output.

Shear CapacityFor the shear capacity check, the program calculates the ratio of the maxi-mum shear in the joist divided by the shear capacity of the joist. This ratio isreported in the output.

The program also checks that the largest point load anywhere on the joistdoes not exceed the shear capacity of the joist.

Live Load Deflection CheckFor the live load deflection check, the program calculates the ratio of the ac-tual maximum live load deflection, calculated using the Analysis I33, dividedby the live load deflection limit specified in the steel joist overwrites.

Maximum Equivalent Uniform Load CheckFor Envelope design, the program checks that the maximum equivalent uni-form load on the joist does not exceed 550 plf. The maximum equivalentuniform load is equal to 8 * Mmax / Span2, where Mmax is the maximummoment and Span is the joist span.

Overview Technical Note 9 - 1


STEEL JOIST DESIGN

Technical Note 9Data Plotted Directly on the Model

This Technical Note describes the steel joist design input and output data thatcan be plotted directly on the model.

OverviewUse the Design menu > Steel Joist Design > Display Design Info com-mand to display on-screen output plotted directly on the model. If desired,the screen graphics can then be printed using the File menu > PrintGraphics command.

The on-screen display data is organized into three data groups, as follows.

Labels

Design Data

Design Ratios

Each of those data groups is described in more detail later in this TechnicalNote. It is important to note that items from different data groups cannot bedisplayed simultaneously.

Tip:

The colors related to the joist ratios can be modified by clicking the Options menu >Colors > Output command.

When design information is displayed directly on the model, the frame ele-ments are displayed in a color that indicates the value of their controlling ra-tio. (Note that this controlling ratio may be a flexural, shear or live load de-flection ratio.) The colors associated with various ranges of ratios are speci-fied in the Steel Ratios area of the Assign Output Colors form, which is ac-cessed using the Options menu > Colors > Output command.

Data Plotted Directly on the Model Steel Joist Design

Technical Note 9 - 2 Labels Displayed on the Model

Labels Displayed on the ModelBeam labels and associated beam design group labels can be displayed on themodel. A beam label is the label that is assigned to the line object that repre-sents the steel joist.

Tip:

Long labels may not display or print properly (fully).

If a beam has been assigned to a group that has been designated as a steeljoist design group, the group name for the beam will be displayed when re-quested. If a beam is not part of a steel joist design group, no group namewill be displayed for that beam. Note that you can assign steel joist designgroups by clicking the Design menu > Steel Joist Design > Select DesignGroup command.

Tip:

The design data and ratios output that is plotted directly on the model is also available intext form in the printed output, which is described in Technical Note Output Details.

Design DataThe following design data can be displayed on the model:

Joist section (e.g., 16K3)

Joist end reactions

One or both of these items can be displayed at the same time.

Design RatiosThe following design data can be displayed on the model:

Controlling ratio

Controlling ratio type

The controlling ratio is the maximum design ratio obtained for the joist. Thecontrolling ratio type is either flexure, shear or LL Deflection, indicating thetype of design check that yielded the maximum design ratio.

Summary of Steel Joist Output Technical Note 10 - 1


STEEL JOIST DESIGN

Technical Note 10Output Details

This Technical Note describes the steel joist design output that can be printedto a printer or to a text file. It also documents the design check messages, forexample, CHK#2, that indicate the program is unable to design a particularjoist. All the output data documented in this Technical Note, except for thesummary output data, also appears when the Details button in the ShowDetails area of the Interactive Steel Joist Design and Review form is clicked.See Steel Joist Design Technical Note 3 Interactive Steel Joist Design formore information.

The program provides the output data in a series of tables. The heading ortitle for each piece of output data and a description of the data are providedin the tables in this Technical Note.

Summary of Steel Joist OutputThis output is printed when you click the File menu > Print Tables > SteelJoist Design command and check the Output Summary checkbox on the re-sulting form. Similar output also appears on screen if you click the Detailsbutton in the Show Details area of the Interactive Composite Beam Designand Review form. See Steel Joist Design Technical Note 3 Interactive SteelJoist Design for more details on the interactive design.

Table 1 Summary of Steel Joist OutputOUTPUT ITEM DESCRIPTION

Story Level Story level associated with the joist.

Beam Label Label associated with the line object that represents the joist. Atypical label beam would appear as "B23." Do not confuse thiswith the Design Section Label, which might be identified as"16K3."

Output Details Steel Joist Design

Technical Note 10 - 2 Steel Joist Detailed Output

Table 1 Summary of Steel Joist OutputOUTPUT ITEM DESCRIPTION

Design Section The joist section selected as a result of the design process, forexample, 16K3.

Maximum Ratio The maximum design ratio for the joist. See Steel Joist DesignTechnical Note 7 Design Checks for Standard Design and SteelJoist Design Technical Note 8 Design Checks for Envelope De-sign for a description of how the ratio is calculated.

Ratio Type This is either flexure, shear or LL Deflection, indicating the de-sign check that produced the Maximum Ratio.

Steel Joist Detailed OutputThis output is printed when you click the File menu > Print Tables > SteelJoist Design command and check the Detailed Output checkbox on the re-sulting form. Similar output also appears on screen if you click the Detailsbutton in the Show Details area of the Interactive Steel Joist Design and Re-view form. See Steel Joist Design Technical Note 3 Interactive Steel Joist De-sign for more details on the interactive design.

Table 2 Steel Joist Detailed OutputOUTPUT ITEM DESCRIPTION

Basic Beam Information

Joist The joist section selected as a result of the design process, forexample, 16K3.

Beam Label Label associated with the line object that represents the joist. Atypical label beam would appear as "B23." Do not confuse thiswith the Design Section Label, which might be identified as"16K3."

Story Story level associated with the joist.

Length Length of the joist.

Steel Joist Design Output Details

Table 2 Steel Joist Detailed Output Technical Note 10 - 3


Loc X Global X coordinate of the center of the joist.

Loc Y Global Y coordinate of the center of the joist.

Group Name of the design group (if any) to which the beam has beenassigned.

Joist Overwrite Data

LL Deflection Limit The live load deflection limit specified for the joist.

Ignore Similarity This item is Yes if story level similarity (to a master story level)is to be ignored when designing the joist.

RLLF A reducible live load is multiplied by this factor to obtain the re-duced live load.

Joist Design Results - Standard Design

Analysis section The joist section used in the last analysis of the structure.

Design section The joist section selected as a result of the design process.

Span used to selectcapacities

The joist span that the program used to select the joist UniformTL Capacity and Uniform LL Capacity.

Load type This is either Uniform or Not Uniform, indicating the type of loadassumed by the program. See Steel Joist Design TechnicalNote 7 Design Checks for Standard Design for a description ofhow the program determines if a load is uniform.

Controlling Ratio The controlling design ratio for the joist. It is the larger of theUniform TL Ratio and the Uniform LL Ratio.

Uniform TL Combo The design load combination that produces the largest UniformTL (total load) Ratio.

Equivalent Uniform TL The equivalent uniform total load acting on the joist. See SteelJoist Design Technical Note 7 Design Checks for Standard De-sign for a description of equivalent uniform total load.

Uniform TL Capacity The specified uniform total load capacity of the joist.


Technical Note 10 - 4 Table 2 Steel Joist Detailed Output


Uniform TL Ratio The ratio obtained by dividing the Equivalent Uniform TL by theUniform TL Capacity.

Uniform LL Combo The design load combination that produces the largest UniformLL (live load) Ratio.

Equivalent Uniform LL The equivalent uniform live load acting on the joist.

Uniform LL Capacity The specified uniform live load capacity for an L/360 deflectionof the joist.

Uniform LL Ratio The ratio obtained by dividing the Equivalent Uniform LL by theUniform LL Capacity and then multiplying by a factor to accountfor the possibility that the specified live load deflection limit maybe different from L/360. See Steel Joist Design Technical Note7 Design Checks for Standard Design for a description of howthis ratio is calculated.

Joist Design Results - Envelope Design

Analysis section The joist section used in the last analysis of the structure.

Design section The joist section selected as a result of the design process.

Load type This is either Uniform or Not Uniform indicating the type of loadassumed by the program. See Steel Joist Design TechnicalNote 7 Design Checks for Standard Design for a description ofhow the program determines if a load is uniform.

Equivalent uniformtotal load

The equivalent uniform total load acting on the joist. See SteelJoist Design Technical Note 8 Design Checks for Envelope De-sign for a description of how this is calculated.

Minimum joist length The minimum span for which the joist design section will bedesigned by the program.

Maximum joist length The maximum span for which the joist design section will bedesigned by the program.

Controlling Ratio The controlling design ratio for the joist. It is the larger of theMoment Ratio, Shear Ratio and the LL Deflection Ratio.

Steel Joist Design Output Details

Steel Joist Design Check Messages Technical Note 10 - 5


Maximum MomentCombo

The design load combination that produces the Maximum Mo-ment.

Maximum Moment The maximum moment acting on the joist.

Moment Capacity The specified moment capacity of the joist.

Moment Ratio The ratio obtained by dividing the Maximum Moment by theMoment Capacity.

Maximum ShearCombo

The design load combination that produces the MaximumShear.

Maximum Shear The maximum shear acting on the joist.

Shear Capacity The specified shear capacity of the joist.

Shear Ratio The ratio obtained by dividing the Maximum Shear by the ShearCapacity.

LL Deflection Combo The design load combination that produces the Maximum LLDeflection.

Maximum LL Deflec-tion

The maximum live load deflection for the joist.

LL Deflection Limit The specified live load deflection limit for the joist in Lengthunits.

LL Deflection Ratio The ratio obtained by dividing the Maximum LL Deflection bythe LL Deflection Limit.

Steel Joist Design Check MessagesThis output is displayed on the model and in the printed output when the pro-gram is unable to design a particular joist. The check messages, such asCHK#2, are a "shorthand" way of indicating the design problem.


Technical Note 10 - 6 Table 3 Steel Joist Design Check Messages

In the printed Steel Joist Detailed Output and the output obtained from theInteractive Steel Joist Design and Review form, descriptive messages are pro-vided along with the shorthand messages. In the printed Summary of SteelJoist Output and for display on the model, only the shorthand messages areprovided.

Table 3 shows that descriptive messages that correspond to the shorthandcheck messages.

Table 3 Steel Joist Design Check MessagesOUTPUT ITEM DESCRIPTION

CHK# 1 Error: No section in the auto select list is adequate for this joist.

CHK# 2 Error: The element is not designed because it is not a joist sec-tion.

CHK# 3 Error: This joist is not designed because there is negative mo-ment.

CHK# 4 Error: This joist is not designed because the equivalent uniformload exceeds 550 plf.

CHK# 5 Error: This joist is not designed because a single concentratedload exceeds the shear capacity.

CHK# 6 Error: This joist is not designed because no load data exists forspans this short.

CHK# 7 Error: This joist is not designed because no load data exists forspans this long.

Documents

Steel Joist Design