Virtual Joist Girder PP 091912

VIRTUAL JOIST VIRTUAL JOIST

GIRDERSUsing Virtual Joist Girders for Building

Modeling and Design

Presenters: David Samuelson, PE, Nucor R & D

Walter Worthley, PE, Valley Joist

0

Virtual Joist Girders

Introduction & Purpose

User Table Information

General Information General Information

Implementation in STAAD

Implementation in RISA

Example


IntroductionIntroduction


Joist Girders (JG) are custom designed trusses

designed and manufactured for use in specific

locations on specific construction projects.

The Project Structural Engineer of Record (SER) The Project Structural Engineer of Record (SER)

provides the JG designation as well as any special

loads or other design criteria.

The Joist Manufacturer designs and manufactures

the JG to meet the specified project requirements.

Introduction and Purpose

There are many instances where the SER has a

need to incorporate the JG into the structural

model of the overall building design.

For example:For example:

In designing a lateral load resisting frame using JGs,

the SER must include the JG in the frame model, and

must provide member end-moments and member

end-forces to the Joist Manufacturer for

incorporation in the final JG design.

4


The process requires good communication

between the SER and the Joist Manufacturer to

ensure compatibility between the JG design and

the overall frame design model.

Historically, the coordination process has been

hampered because the SER did not know, at the

time of modeling the overall building structure,

the design properties for the JG.

5


The Steel Joist Institute (SJI) has developed a

table that provides the approximate section

properties for Virtual Joist Girders for use by

the SER in preparing the building structural

models.models.

6


Virtual Joist Girders (VJG) are equivalent-beam section properties based on commonly available top and bottom chord angle sizes.

The tabulated VJGs do not represent the final JG design and cannot be used to specify the JG JG design and cannot be used to specify the JG design requirements.

They do yield reasonably close approximations of the final JG chord area, effective moment of inertia, and weight, for use in structural models.

7


The intent is for the VJG tables to be used in the design models through use of the design programs Member Selection and Code Check functions.

Using these functions, the VJGs are treated as custom wide-flange beams, the appropriate members can be selected, section properties displayed, and approximate weights calculated.weights calculated.

The VJG sections in the tables are not stock sections for any given Joist Manufacturer. They are a guide for use in the design of structural systems using JGs.

The sections have demonstrated to yield results reasonably close to the final JG designs.

8

Introduction & Purpose - Tables

After the appropriate VJG selections are made

using the tables, the SER must specify the JG

design requirements using conventional JG

nomenclature as directed by SJI Standard nomenclature as directed by SJI Standard

Specifications and Code of Standard Practice.

Special loading requirements, i.e. Axial Loads,

Fixed-End Moments, must also be specified.

9

Introduction & Purpose - Tables

If the JG stiffness (effective Moment of Inertia) is

significant to the overall building structural model

(such as in a lateral load resisting frame) then;

a) the SER must specify the VJG moment of a) the SER must specify the VJG moment of

inertia as the target JG effective moment of

inertia, and

b) Directions to notify the SER if the final JG

design moment of inertia varies by more than 10%

from the target value should also be specified.

10


USER TABLE INFORMATIONUSER TABLE INFORMATION

User Table Information- Excerpt

12

User Table Information(Format By Line)

First Line Property units

Second Line Member classification

Third Line VJG designation, Example: VJG20-12 (20 inch deep VJG with chord combination #12).(20 inch deep VJG with chord combination #12).

Fourth Line Designated VJG Properties as shown in the next slides.

Repeat third and fourth line for each available VJG.

13

Virtual Joist GirderSection Properties

The section properties for a each VJG as they read from left

to right are:

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

bottom chord areas

D: Total VJG Depth

TD: Web Thickness Total Depth/30; Ensures that the section is TD: Web Thickness Total Depth/30; Ensures that the section 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 ratio when STAAD checks (B/2)/TB.

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Virtual Joist GirderSection Properties

Iz: 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.

Iy: VJG Weak-Axis Moment of Inertia 2*Top Chord Moment Iy: VJG Weak-Axis Moment of Inertia 2*Top Chord Moment

of Inertia; Based on flange (chord) that would typically be in

compression.

Ix: VJGTorsion Constantapproximate based on chord

elements

15

Virtual Joist Girder Section Properties

Sz: Section Modulus About Strong-Axis Minimum Chord

Area*VJG Effective Depth; Reduces the over-estimation of

chord (flange) stresses. The method substitutes an "effective

section modulus" based on a stress distribution used in classic

truss theory of uniform stress distribution across the cross truss theory of uniform stress distribution across the cross

section of the member.

Sy: Section Modulus About Weak-Axis Section modulus of top

chord; a reasonable conservative value used when a JG is used

in out-of-plane bending. Note that it is not recommended to

apply out-of-plane loading to joists or JGs, and if this is done it

must be carefully coordinated with the JG Manufacturer.

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Ay: Shear Area in Y Direction Ax*.25; Based on an

approximation of the shear area used in SJI spec's for chord

shear checks.

Az: Shear Area in Z Direction Ax*.25; Based on an

approximation of the shear area used in SJI spec's for chord approximation of the shear area used in SJI spec's for chord

shear checks.

Pz: Plastic Modulus About Strong-Axis Equals Sz; Stress distribution is always uniform across the chord in classic truss analysis, whether in a plastic or elastic state.

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Py: Plastic Modulus About Weak-Axis Unity; is not a significant factor in current analysis.

HSS: Warping Constant approximate based on chord elements

DEE: Depth of Web Equals Top Chord Angle Leg Length

User Table Information- Excerpt

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Virtual Joist Girder

General Information General Information

Virtual Joist Girder Material Properties

The sections defined by the VJG table use

the same material properties as structural

steel, with the exception of density, which steel, with the exception of density, which

is 15% higher, to account for approximate

weight of web members and

miscellaneous materials.

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Material Properties

The explicit material properties are:

Youngs Modulus (E) = 2.9e+007 (lb/in2)

Poissons Ratio (nu) = 0.3 Poissons Ratio (nu) = 0.3

Density = 0.333 (lb/in3)

Thermal Coeff (alpha) = 6e-006

Critical Damping = 0.03

Shear Modulus (G) = 1.1154e+007 (lb/in2)

22

Virtual Joist Girder Bracing Condition To achieve the best performance for the VJG approximation,

typical VJG members should be modeled with an Unbraced

Length of zero.

Research1 indicated that setting the unbraced length to zero

yielded selection of VJGs with properties closest to the final

JG design properties, for conditions with normal Joist JG design properties, for conditions with normal Joist

Girder loading and bracing.

This recommendation does not apply (and could be

unconservative) for very unusual conditions, such as a JG

being used in a lateral load resisting frame with no joists

attached to provide intermediate lateral support.1. Development of an Improved Design Method for Joist Girder Selection in a Generalized Structural Design Program, by Phillip A. Knodel, MSCE thesis,

South Dakota School of Mines and Technology, Rapid City, SD, 2011

23

Selecting an Appropriate

Span/Depth Ratio

In using the Virtual Joist Girder Tables, it is important to select only girder depths that are appropriate to the span.

The span of the girder should be no less than The span of the girder should be no less than 12 times its depth and no more than 24 times its depth.

Usually, the most economical selections have a span/depth ratio of between 12 and 18.

24

Managing Girder Self Weight

Standard SJI procedure is to include the approximate JG self weight

in the JG design loads provided to the joist manufacturer.

As an alternative, if the JG self weight is not included in the

specified design loads, then the JG design specifications must

include a note to the joist manufacturer stating that JG self weight

is not included in the specified design loads and must be added to is not included in the specified design loads and must be added to

the JG designs.

In order for the VJG selection properties to match the final JG

Design properties as closely as possible, it is important to ensure

that self weight is included in the analysis and member selection.

In STAAD this can be done by using the Member Self Weight load

option, so that the member self weight is automatically updated

with each design iteration.

25

Joist Girder Stiffness Accuracy for Moment Frame Design and Analysis

Since the JG stiffness (effective Moment of Inertia) is significant to the overall building structural model the SER must specify the VJG moment of inertia as the target or effective Moment of Inertia.

Directions to notify the SER if the final JG design Moment of Directions to notify the SER if the final JG design Moment of inertia varies by more than 10% from the target value for model accuracy verification. (Note that this effective Moment of Inertia is the JG chord Moment of Inertia divided by 1.15 to account for the reduced shear rigidity of the open web system. To avoid potential confusion, it is very important to denote this value as the target/Effective JG Moment of Inertia.)

26

Joist Girder Designation

The JG Designation specified by the SER on the contract structural

drawings must follow SJI convention from the SJI Standard

Specifications for Joist Girders and Code of Standard Practice for

Steel Joists and Joist Girders, NOT the VJG designation from the VJG

Selection Tables used in the structural modeling.

For example, if a VJG22-20 is chosen for use in the structural model, For example, if a VJG22-20 is chosen for use in the structural model,

this is NOT an appropriate JG designation for the structural contract

drawings. Rather the JG designation on the contract structural

drawings must follow standard SJI convention, such as 22G5N10K.

Please reference the SJI Standard Specifications for Joist Girders and

SJI Code of Standard Practice for Steel Joists and Joist Girders, for

more information on appropriate JG designations.

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Chord Axial Loading and/or End

Moments from Wind or Seismic

For specification of JG design loading information that includes chord axial loads and/or end moments from wind or seismic, reference Chapter 6 of SJI loads and/or end moments from wind or seismic, reference Chapter 6 of SJI Technical Digest 11, Design of Lateral Load Resisting Frames Using Steel Joists and Joist Girders.

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IMPLEMENTATION IN STAAD

User Table Information

STAAD Implementation

The following instructions are applicable

for the use of the VJG user table in STAAD

Pro.Pro.

The information/section properties in the

User Table are equally applicable to other

analysis and design software based on

the softwares requirements.

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Installing User Table File

1. Create directory (folder) for STAAD.Pro design

files that will be using the VJG user table.

2. Place a copy of the user table file into the

same directory.same directory.

3. If design files are saved in a separate directory

from the user table file then the file path

must be changed to the user table file name

in order to access the user table data.

31

Installing User Table File

1. Create directory (folder) for STAAD.Pro design files

that will be using the VJG user table.

2. Place a copy of the user table file into the same

directory.

32

Activating User Table

(One method described. See STAAD documentation for other methods).

1. In Modeling mode select Tools.

2. Select Create User Table.

3. In the pop-up window select the New Table button.

4. Checkmark the External Table box and select the Browsebutton.

4. Checkmark the External Table box and select the Browsebutton.

5. Select the user table file and click Open.

6. In the Select Section Type drop down menu, choose General and press OK.

7. The user table should automatically be given a number. Press Close.

33

134

2


3

3. Select the New Table button.

35


4

4

4. Checkmark the External Table box and select

the Browse button.

36


5. Select the user table file and click Open.

37

5


6

6. In the Select Section Type drop down menu, choose General and press OK.

38

6


7

7. The user table should automatically be given

a number. Press Close.

39

7

Assigning User Table Data

(One method described. See STAAD documentation for other methods)

1. In Modeling mode select the General tab.2. In the Properties Whole Structure window,

select the User Table button.3. Choose the previously assigned user table

number.number.4. Select a section and assign the appropriate

material5. Select Add and close the window.6. Assign VJG sections from the Properties

Whole Structure window in the same manner as with ordinary member sections

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1. In Modeling mode select the General tab.

2. In the Properties Whole Structure window, select the User Table button.

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2


3

4

3. Choose the previously assigned user table number.

4. Select a section and assign the appropriate material

5. Select Add and close the window.

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5


6

6. Assign VJG sections from the Properties Whole Structure window in the same manner as with ordinary member sections.

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IMPLEMENTATION IN RISA IMPLEMENTATION IN RISA

RISA Virtual Joist Girder

Users GuideDesigning Virtual Joist Girders Using the RISA Building System

Required Files

Accessing the Database

Using Virtual Joist Girders in RISAFloor

Using Virtual Joist Girders in RISA-3D

45


Using RISA Design Software

Required FilesRequired Files

VJG_SJI.FILVJG_SJI.FIL

This is a binary database fileThis is a binary database file

Normally placed in the C:Normally placed in the C:\\RISARISA\\ directory with all the other directory with all the other

database filesdatabase filesdatabase filesdatabase files

VJG sections will be viewed as Wide Flange membersVJG sections will be viewed as Wide Flange members

US_VJGBeamLst.ascUS_VJGBeamLst.asc

This is a ReThis is a Re--Design list to promote optimization within the VJG Design list to promote optimization within the VJG

shapesshapes

Normally placed in the C:Normally placed in the C:\\RISARISA\\risa__ReDesign_Listsrisa__ReDesign_Lists\\ directorydirectory

46


Using Risa Design Software

Accessing the databaseAccessing the database

Click the Edit Shape Database iconClick the Edit Shape Database icon

Select Virtual Joist Girder as the Database / Manufacturer in the Select Virtual Joist Girder as the Database / Manufacturer in the

Shape Selection DialogShape Selection Dialog

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From the Draw or Modify Beam dialog select the Shape From the Draw or Modify Beam dialog select the Shape

Group / Design List as Virtual Joist GirderGroup / Design List as Virtual Joist Girder

This option is only available in version 6.0.2 (Beta) or This option is only available in version 6.0.2 (Beta) or

newernewer

The program will automatically optimize the member for The program will automatically optimize the member for


Using RISA Floor Design Software

The program will automatically optimize the member for The program will automatically optimize the member for

stress and deflection requirementsstress and deflection requirements

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Using RISAFloor

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Using RISA-3D

From the Draw or Modify Beam dialog select

the Virtual Joist Girder Database and select a

VJG member.

Select the Virtual Joist Girder Design list and Select the Virtual Joist Girder Design list and

the program will automatically suggest

alternate shapes for bending, axial and shear

code checks once the model has been solved.

50


Using RISA-3D

51


Example

Virtual Girder Example

This example is to show how to use the Virtual Girder Table in modeling building frames using STAAD software

The loads are ASD level and typical of what might occur in a building design but are not determined from ASCE /SEI 7-10 or any building code; however ASCE/SEI 7-10 load or any building code; however ASCE/SEI 7-10 load combinations will be used.

The girders from the example will be identified using the Virtual Girder designation and the designation that the Engineer needs to provide to the joist manufacturer.


A B C D E F

4

3 S

PA

CE

S @

40'-0"

= 1

20'-0"

5 @ 40'-0" = 200'-0"

G1

G1

G3

G3

G5

G5

DESIGN DATA: Roof Loads:

Dead = 12 psfLive = 20 psfWind = 35 psf

Walls: Wind = 25 psf

COLUMNS:Interior = W14X903

2

1

BUILDING STEEL FRAMING PLAN

3 S

PA

CE

S @

40'-0"

= 1

20'-0"

G1

G2

G2

G1

G3

G4

G4

G3

G5

G6

G6

G5

Interior = W14X90Exterior = W14X74

Girders = 36 deepJoists = 22 deep

Fixed End

Column Connections

TYP. TYP.

GIRDER CONNECTION DETAIL

@ PINNED ENDS

Column Connections

TYP. TIE PLATETIE PLATE TYP.

TYP.TYP.

GIRDER CONNECTION DETAIL

@ FIXED ENDS


Loads on Joists:

DL = 12 x 5 = 60 plf

LL = 20 x 5 = 100 plf

TL = DL + LL = 60 + 100 = 160 plf

WL = 35 x 5 = 175 plf

Steel Joist InstituteLecture 4 - 57

WL = 35 x 5 = 175 plf


Determine Girder Panel Point Loads

Interior Girders:

DL = .060 klf x 40 = 2.4 kips

LL = .100 klf x 40 = 4.0 kips

TL = DL + LL = 2.4 + 4.0 = 6.4 kips

WL = 0.175 klf x 40 = 7.0 kipsWL = 0.175 klf x 40 = 7.0 kips

Perimeter Girders:

DL = .060 klf x 20 = 1.2 kips

LL = .100 klf x 20 = 2.0 kips

TL = DL + LL = 1.2 + 2.0 = 3.2 kips

WL = 0.175 klf x 20 = 3.5 kips


Frame Models

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

23'-6"

P P P

WIN

D =

50

0 p

lf

1 2 3 4

5 6 7

WIN

D =

50

0 p

lf

G1 G2 G1P : DL = 1.2 kips

LL = 2.0 kips

TL = 3.2 kips

WL = 3.5 kips

23'-6"

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

FRAME MODEL @ GRID LINE C & D

FRAME MODEL @ GRID LINE A & F

P P P

1 2 3 4

5 6 7

WIN

D =

100

0 p

lf

WIN

D =

100

0 p

lfG5 G6 G5

P : DL = 2.4 kips

LL = 4.0 kips

TL = 6.4 kips

WL = 7.0 kips


Frame Models

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

P P P

WIN

D =

100

0 p

lf

5 6 7

WIN

D =

100

0 p

lf

G3 G3G4P : DL = 2.4 kips

LL = 4.0 kips

23'-6"

FRAME MODEL @ GRID LINE B & E

WIN

D =

100

0 p

lf

1 2 3 4

WIN

D =

100

0 p

lf

LL = 4.0 kips

TL = 6.4 kips

WL = 7.0 kips


Each Frame was modeled in STAAD.

For simplicity we have chosen the columns and used the

Virtual Joist Girder Tables to select the Virtual Joist Girders.

ASCE7-10 ASD Load Combinations were used for the design.

Each frame was then analyzed for each load type (DL, LL WL) Each frame was then analyzed for each load type (DL, LL WL)

to obtain the axial loads and fixed-end moments for that load

type.

All reported loads are UNFACTORED


Mark VJG

Selected

Approx.

Target/Effective Ix

[in4]

Approx.

Weight

[lbs]

The following table shows the Virtual Joist Girders (VJG)

selected by STAAD for the Analysis/Design of the frames

G1 VJG36-1 931 642

G2 VJG36-1 931 642

G3 VJG36-22 1347 949

G4 VJG36-28 1521 1061

G5 VJG36-28 1521 1061

G6 VJG36-17 1236 860


For comparison, the approximate Moment of Inertia, Ix, is determined from the equation given in the SJI COSP Section 2.2(b)

Ix = 0.027NPLd Ix = 0.027NPLd

where: N = no. of joist spaces = 8

P = Total Load (in kips)

L = Girder Length (in feet)

d = Girder depth (in inches)Note that this equation is for a simply supported Joist Girder under static loading and must be divided by 1.15 to obtain the effective Ix.


The approximate effective Moment of Inertia for the girders are:

IxG1 = IxG2 = 0.027(8)(3.2)(40)(36)/1.15 = 866 in4

IxG3 = IxG4 = IxG5 = IxG6= 0.027(8)(6.4)(40)(36)/1.15 = 1731 in4

Virtual Joist GirderReporting Loads

The following tables provide a format for reporting the

design loads for the Joist Girders.

The axial loads and fixed-end moments are from the The axial loads and fixed-end moments are from the

STAAD analysis results for the structure for each load

condition. Note that combining loads is not required.

Virtual Joist GirderReporting Loads

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 5.88G1 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

Virtual Girder ExampleReporting Loads

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

Extr Col.

Right

Intr Col.

Left

Extr Col.

Right

Intr Col.

Left

Extr Col.

Right

Intr Col.

Left

Extr Col.

Right

Intr 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

Virtual Joist Girder - Example Comparison of Results

Using the tabulated Joist Girder loads, the specified

Joist Girders were designed by a joist manufacturer to

check the final results.

The tables on the following slides show a comparison of The tables on the following slides show a comparison of

the Moments of Inertia and Weights for the Virtual Joist

Girders chosen by STAAD and the manufacturers final

designs.

Virtual Joist Girder - Example

Effective Moment of Inertia Comparison

Mark

Target/

Effective Ix

From

STAAD

Ix from

SJI-

CSOP

Equation

%

Diff

Design

Ix from

Manuf.

Effective

Ix from

Manuf.

%

Diff

G1 931 866 -7.0 1009 878 -5.7

G2 931 866 -7.0 1009 878 -5.7

G3 1347 1731 28.5 1621 1410 4.7

G4 1521 1731 13.8 1895 1648 8.3

G5 1521 1731 13.8 1895 1648 8.3

G6 1236 1731 40.0 1484 1290 4.4

Virtual Joist Girder - Example

Mark

VJG Weight

From STAAD

Final JG

Weight

%

Diff

G1 642 651 1.4

Weight Comparison

G1 642 651 1.4

G2 642 687 6.6

G3 949 1036 8.4

G4 1061 1135 6.5

G5 1061 1131 6.2

G6 860 985 12.7


SummarySummary


Summary

The VJG tables provide equivalent-beam section properties

for building models that can be used in structural analysis and

design software and will provide reasonably accurate results .

The VJG Member Classification cannot be used for The VJG Member Classification cannot be used for

specifying the final Joist Girder, JG, on the contract drawings.

The Joist Girder must be designated as described in the SJI

Code of Standard Practice Section 2.2(b).


Summary

When there are multiple or complex loading conditions that must be provided to the joist manufacturer i.e. Axial Loads, Fixed-End Moments, a load table (Refer to Slides 56 & 57) is the best method for supplying this information.method for supplying this information.

The Target or Effective Moment of Inertia should be specified when it will affect the accuracy of the structural analysis.


Downloads

The Virtual Joist Girder Table can be

downloaded from the SJI web site.

www.steeljoist.org

When you go to the website look across the When you go to the website look across the

options at the top of the web page. On the far

right is Virtual Joist Girders.


Further Developments

Development is underway for inserting the VJG

Tables within the next release of Computers

Structures Incorporated ETABS.

Working with Bentley for insertion within RAM Working with Bentley for insertion within RAM

Structural System.

SJI is currently working with the South Dakota School

of Mines and Technology to develop Virtual Joist

user tables for steel joists.

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Virtual Joist Girder PP 091912

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Transcript of Virtual Joist Girder PP 091912