Joist Models in Standard Structural Software

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Joist Models in Standard Structural

Software

September 24, 2015

Dave Samuelson, P.E. – Nucor New Product & Market Development Joe Pote, P.E. – New Millennium Building Systems

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Learning Objectives

•  What are the advantages and disadvantages to using classic tools to select conventional standard joists and Joist Girder designations?

•  What are a Virtual Joist and Virtual Joist Girder and how are they used? •  In what situation might it be advantageous for a Structural Engineer to

use the SJI Virtual Joist Table or Virtual Joist Girder Table?

•  When a Virtual Joist or Virtual Joist Girder is used in the building design model, how should it be specified on the contract structural drawings?

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Joist Models in Standard Structural Software

Joists and Joist Girders in a RAM-Steel Rendering

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Learning Objectives

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Learning Objectives

•  What are a Virtual Joist and Virtual Joist Girder and how are they used?

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Learning Objectives

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Learning Objectives

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Advantages and Disadvantages to using Classic Tools

to select Standard Joists and Joist Girders

•  Classic Roof and Floor Framing Optimizers such as RAM-Steel or RISA-Floor

•  General Structural Analysis Software such as STAAD, RISA or STRUDL

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Classic Roof and Floor Framing Optimizers such as

RAM-Steel or RISA-Floor

•  Excellent job of selecting optimum “Standard” joists (Pro)

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Standard Joists Selected based on Span and Loading

RAM-Steel Partial Framing Plan

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RAM-Steel or Risa-Floor

•  Cannot actually ‘select’ Joist Girders, as there are no ‘standard’ Joist Girders to select from. Joist Girder Designation is specified based on loading and user-provided depth. (Con)

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Joist Girder Designations Assigned based on Loading

and User-Provided Depth

RAM-Steel Partial Framing Plan without User-Provided Depth

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RAM-Steel

•  Joists and Joist Girders automatically appear on the Structural Drawings with correct designation needed by the Joist Manufacturer for quoting, designing and manufacturing. (Pro)

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Joist and Girder Designations displayed in

Conventional Format

RAM-Steel Partial Framing Plan

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Joist and Girder Designations transferred to CAD in

Conventional Format

AutoCAD Partial Framing Plan

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RAM-Steel

•  Joists and Joist Girders automatically appear on the Structural Drawings with correct designation needed by the Joist Manufacturer for quoting, designing and manufacturing. (Pro)

•  Joists can only be managed as simple-span infill joists supporting vertical gravity loads. ‘Standard’ joist designations and definition do not address negative moments, end moments, or axial loads. (Con)

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Advantages and Disadvantages to using Classic Tools

to select Standard Joists and Joist Girders

•  Classic Roof and Floor Framing Optimizers such as RAM-Steel

•  General Structural Analysis Software such as STAAD, RISA or STRUDL

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General Structural Analysis Software such as STAAD,

RISA, or STRUDL

•  No Section Properties available for standard joists and girders. Therefore, the joists and girders cannot be included in the stiffness matrix model. (Con)

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Steel Joist and Joist Girder Stiffness are not included

in the Structural Model

Quote from STAAD-Pro Technical Reference Manual

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General Structural Analysis Software such as STAAD,

RISA, or STRUDL

•  No Section Properties available for standard joists and girders. Therefore, the joists and girders cannot be included in the stiffness matrix model. (Con)

•  Some general analysis software does include tools for selecting joists and treats the joist as a load distribution link…an infinitely rigid simply supported infill component between supporting members. (Pro)

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Joists Treated as Links with Simple Selection Tools

Screen Shot of STAAD-Pro Joist Database Table Tool

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Learning Objectives

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What are a Virtual Joist and Virtual Joist Girder?

•  Virtual Joists and Virtual Joist Girders are a set of pseudo section

properties based on commonly available joist chord pairs.

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What are a Virtual Joist and Virtual Joist Girder?

Excerpt from Virtual Joist Girder Table

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•  The major axis moment of inertia (I_Z) is based directly on the pair of double angle joist chords and pre-adjusted for approximate deflection contribution from web strain. The Area is likewise based directly on the pair of double angle joist chords.

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What are a Virtual Joist and Virtual Joist Girder?

A

_x

: Total Area of Top and Bottom Chords – Sum of top and

bottom chord areas

D: Total VJG Depth

I

_z

:

VJG Strong-Axis Effective Moment of Inertia – Classically

calculated Moment of Inertia of the Chords divided by

1.15 to account for deflection effects from the joist web

members.

Virtual Joist Girder Section Properties:

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Virtual Joist Section Properties, I

_z From SJI 43rd_{Edition Joist Catalog, Page 14:}

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Virtual Joist Section Properties, Izz

From SJI Technical Digest 11, “Design of Lateral Load Resisting Frames using Steel Joists and Joist Girders”:

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•  All other section properties are equivalent wide-flange beam properties manipulated to encourage selection of the appropriate Virtual Joist and/ or Virtual Joist Girder when checked against a Wide-Flange Beam

selection routine.

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What are a Virtual Joist and Virtual Joist Girder?

TD: Web Thickness – Total Depth/30; Ensures that the sec7on is

treated as "compact" when considering web shear.

B: Flange Width – 2*Chord Angle Leg + 1" Chord Gap

TB: Flange Thickness – (Chord angle thickness)/(chord angle

leg)

* B/2; This value results in the correct width/thickness ra7o when

STAAD checks (B/2)/TB.

Sample VJ & VJG Section Properties:

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•  All other section properties are equivalent wide-flange beam properties manipulated to encourage selection of the appropriate Virtual Joist and/ or Virtual Joist Girder when checked against a Wide-Flange Beam

selection routine.

•  The Virtual Joist and Virtual Joist Girder tables provide approximate

section properties suitable for use in a stiffness based structural analysis program. Testing at South Dakota School of Mine and Technology has verified that Virtual Joists and Virtual Joist Girders selected via STAAD and/or RAM using a broad range of loading scenarios result in section properties reasonably close to those in the final joist and Joist Girder designs.

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SDSM&T Comparison Study of VJ and VJG

K-Series Span/Load Test Matrix

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SDSM&T Comparison Study of VJ and VJG

Moment of Inertia Variance –vs- Depth for Simply Supported K-Series Tests

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LH-Series Span/Load Test Matrix

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Moment of Inertia Variance –vs- Depth for Simply Supported LH-Series Tests

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Learning Objectives

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When might it be advantageous to use the SJI Virtual

Joist Table or Virtual Joist Girder Table?

•  When sizing joists for special loads (anything other than full-length uniform design loads)

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Not all Design Loads are Uniformly Distributed

Sample Special Load Diagram from SJI COSP

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When might it be advantageous to use the SJI Virtual

Joist Table or Virtual Joist Girder Table?

–  A more accurate estimate of appropriate joist and/or Joist Girder depth and weight can be found using the Virtual Joist and Virtual Joist Girder tables than by using equivalent uniform loads and standard joist load tables.

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•  When sizing joists for conditions with negative moments, such as

cantilevered joists and/or joists with intermediate supports (multi-span conditions).

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Not all Joists are Simply Supported

Cantilevered Joist Diagram from SJI

43rd_{ed Catalog}

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•  Anytime it is desirable to include the joists and/or Joist Girders directly in the structural model with appropriate approximate section properties.

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When might it be advantageous to use the SJI Virtual

•  Anytime it is desirable to include the joists and/or Joist Girders directly in the structural model with appropriate approximate section properties.

–  Particularly when joists and/or Joist Girders are used as part of a lateral load resisting frame. The VJ and VJG tables allow easy

selection of appropriate depths with appropriate approximate section properties for use in the structural model.

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Not all Joists are Simply Supported

Frame Model with Joist Girders

24 @ 5'-0" = 120'-0"

23'-6"

FRAME MODEL @ GRID LINE B & E

P P P WIN D = 1000 plf 1 2 3 4 5 6 7 WIN D = 1000 plf G3 G4 G3 P : DL = 2.4 kips LL = 4.0 kips TL = 6.4 kips WL = 7.0 kips 44

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Not all Joists are Simply Supported

Joist Girder End Moment Schedule

Mark

Target Ix [in4_]

Fixed-End Moments (Pos. is Counter-Clockwise) Couples between Top & Bottom Chord

Dead [ft-k] Live [ft-k] Wind [ft-k]

Acting to Right Wind [ft-k] Acting To Left Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. G1 931 G2 931 24.5 -24.5 38.2 -38.2 -204.9 -71.2 71.2 204.9 G3 1347 11.2 -51.5 17.8 -81.8 -55.5 -106.6 -5.8 396.0 G4 1521 G5 1521 G6 1236 44.0 -44.0 70.2 -70.2 -398.9 -153.3 153.3 398.9 45

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Learning Objectives

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How should a Virtual Joist or Virtual Joist Girder be

Specified on the Structural Drawings?

•  It is important to specify all joists and Joist Girders using conventional SJI designation nomenclature.

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50’ Span 28” _{Joist Depth}

ASD 28K7 Joist

185/100 Uniform Load (PLF)

Standard Joist Designation

ASD Allowable Strength Design 28 Joist Depth In Inches K Joist Series 7 Chord Size 48

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50’ Span 28” _{Joist Depth}

ASD 28K Load/Load Joist

185/100 Uniform Load (PLF)

Load/Load Joist Designation

ASD Allowable Strength Design 28 Joist Depth In Inches K Joist Series 185 Total Load Capacity PLF 100 Live Load Capacity PLF ASD 28K 185/100

185 = Total safe uniformly distributed load carrying capacity in PLF of ASD K-Series Steel Joists 100 = Nominal LIVE loads per lineal foot of joist.

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Joist Girder Span (C.L. of Column to C.L. of Column)

44” Depth

Standard Joist Girder

Standard Joist Girder Designation

ASD Allowable Strength Design 44G Depth of Girder in Inches 8N Number of Joist Spaces 11.9K Load in Kips at Each Panel Point Joist Space 11.9K 11.9K 11.9K 11.9K 11.9K 11.9K 11.9K 50

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How should a Virtual Joist or Virtual Joist Girder be

•  The VJ and/or VJG selections cannot be used as a substitute for the joist and Joist Girder designations. It may, at times, be helpful to include

these as additional information. However, they do not provide the

information needed by the joist manufacturer to design and manufacture the joists and joist girders required for the project.

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Virtual Joist Girder Designation

An Example from a set of Structural Drawings

Standard Designation Above VJG Designation Below

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How should a Virtual Joist or Virtual Joist Girder be

•  For Joist Girders, specify the designation based on depth, number of joist spaces, and joist reaction. If the loading is unequal, unequally spaced, or includes special loads, use a load diagram and/or load schedule.

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Virtual Joist Girder

Mark Designation

Panel Point Loads

Axial (Pos. causes Tens.) Split between Top & Bottom

Chord Dead [kips] Live [kips] Total (D+L) [kips] Wind [kips] Dead [kips] Live [kips] Wind [kips] G1 36G8N 1.2 2.0 3.2 ±3.5 ±5.88 G2 36G8N 1.2 2.0 3.2 ±3.5 1.04 1.63 ±2.84 G3 36G8N 2.4 4.0 6.4 ±7.0 0.48 0.76 ±9.39 G4 36G8N 2.4 4.0 6.4 ±7.0 1.71 2.72 ±4.85 G5 36G8N 2.4 4.0 6.4 ±7.0 ±11.75 G6 36G8N 2.4 4.0 6.4 ±7.0 1.87 2.99 ±5.23 Reporting Loads 54

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Virtual Joist Girder

Mark

Target Ix [in4_]

Fixed-End Moments (Pos. is Counter-Clockwise) couples Between Top & Bottom Chord

Acting to Right Wind [ft-k] Acting To Left Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. G1 931 G2 931 24.5 -24.5 38.2 -38.2 -204.9 -71.2 71.2 204.9 G3 1347 11.2 -51.5 17.8 -81.8 -55.5 -106.6 -5.8 396.0 G4 1521 G5 1521 G6 1236 44.0 -44.0 70.2 -70.2 -398.9 -153.3 153.3 398.9 Reporting Loads 55

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How should a Virtual Joist or Virtual Joist Girder be

•  For Joist Girders, specify the designation based on depth, number of joist spaces, and joist reaction. If the loading is unequal, unequally spaced, or includes special loads, use a load diagram and/or load schedule.

•  For joists, specify the designation using either a standard joist designation or a load/load joist designation. For special loading

conditions, show all loads on the framing plan, in load diagrams and/or a load schedule.

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How should a Virtual Joist or Virtual Joist Girder be

•  It is important to specify all joists and Joist Girders using conventional SJI

designation nomenclature.

•  The VJ and/or VJG selections cannot be used as a substitute for the joist and Joist

Girder designations. It may, at times, be helpful to include these as additional information. However, they do not provide the information needed by the joist

manufacturer to design and manufacture the joists and joist girders required for the project.

•  For Joist Girders, specify the designation based on depth, number of joist spaces,

and joist reaction. If the loading is unequal, unequally spaced, or includes special loads, use a load diagram and/or load schedule.

•  For joists, specify the designation using either a standard joist designation or a load/

load joist designation. For special loading conditions, show all loads on the framing plan, in load diagrams and/or a load schedule.

•  For joists and/or Joist Girders used as part of a lateral load resisting frame, provide

the Joist Moment of Inertia used in the structural model (from the VJ and/or VJG tables) as a target value for the Joist Manufacturer. It is recommended to specify that the Joist Manufacturer is to contact the Structural Engineer of Record regarding moments of inertia that deviate from the target values by more than 10%.

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Virtual Joist Girder

Reporting Loads Mark Target Effective Ix [in4_]

Fixed-End Moments (Pos. is Counter-Clockwise) couples Between Top & Bottom Chord

Acting to Right Wind [ft-k] Acting To Left Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. Left Ext’r Col. Right Int’r Col. G1 931 G2 931 24.5 -24.5 38.2 -38.2 -204.9 -71.2 71.2 204.9 G3 1347 11.2 -51.5 17.8 -81.8 -55.5 -106.6 -5.8 396.0 G4 1521 G5 1521 G6 1236 44.0 -44.0 70.2 -70.2 -398.9 -153.3 153.3 398.9

Note: Please contact the Project Structural Engineer of Record if the final effective Joist Girder Moment of Inertia (Ix) varies from the tabulated target value by more than 10%

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Current Access to VJ and VJG in Structural Software

•  STAAD / STAAD-Pro

–  VJG Table available as a built-in

–  VJ Table available as a User Defined Table which can be downloaded from the SJI website

•  RAM / RAM-Structural

–  VJG Table available as a built-in •  RISA and RISA-Floor

•  Scia Engineer (scheduled for October release) –  VJG Table available as built-in

•  Uses VJG Depth for Conventional Girder Designation –  VJ Table available as built in

•  Uses VJ Depth for Load/Load Joist Designation

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Current Access to VJ and VJG in Structural Software

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Virtual Joist Girder

RAM-Steel Screen Shot with VJG Designations

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Current Access to VJ and VJG in Structural Software

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Virtual Joist Girder

RISA-Floor Screen Shot with VJG Designations

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Virtual Joist Girder

RISA-3D Screen Shot with VJG Designations

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Current Access to VJ and VJG in Structural Software

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Virtual Joist and Virtual Joist Girder

Scia uses VJ & VJG Designations for Stiffness Modeling

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Virtual Joist and Virtual Joist Girder

Scia overlays the VJ/VJG Designations with Conventional Designations

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Virtual Joist and Virtual Joist Girder

Scia uses the VJ/VJG Depths in Defining Special Joist and Joist Girder Designations

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Learning Objectives

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Conclusions

•  The classic tools, though limited, provide a very effective means of selecting standard joists with appropriate designations.

•  Popular structural design packages are beginning to incorporate Virtual Joists and Virtual Joist Girders, thereby allowing more modeling

flexibility.

•  For simple-span uniformly loaded conditions, the classic tools are simple, accurate and easy to use.

•  For complex loading and/or complex support conditions, the VJ and VJG tables provide a good tool for incorporation in the structural model.

•  Current use of VJ and VJG requires the user to manually follow up with their own standard format designation for use on Structural Framing Plan.

•  Some structural software packages are now working toward incorporating VJ and VJG in such a way as to automate the accompanying standard format designation.

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Conclusions

flexibility.

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Conclusions

flexibility.

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Conclusions

flexibility.

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Conclusions

flexibility.

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Conclusions

flexibility.

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