Steel Girder with web holes procedure by OmGrbA

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									                     PROCESS FOR REVIEWING VIRTIS ANALYSIS
                  STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                  JUNE 9, 2007

PURPOSE
This document outlines a process that can be used to review the results of a Virtis rating for a
steel structure with a member that has holes in the web. It has been developed to address
concerns regarding the practicality of the web deterioration approach used in Virtis.


DESCRIPTION
Virtis allows deterioration of the flanges and web to be input using percentages. This is
acceptable for all conditions except when there is a complete loss of web section, hereafter
referred to as a hole, in the web. Using a percentage can not approximate this situation as a
portion of the web does not exist and thus the percentage reduction would theoretically be 100%.
Inputting such a value into Virtis results in a failed analysis because the web is, in effect, no
longer present.

PCBars, the rating analysis program used prior to implementation of Virtis, allowed the location
of the hole to be entered and then recalculated the section properties within the range of the hole
prior to analysis. This is regarded as a more realistic approach for holes.


APPLICATION
This process is intended to be used only with steel structures that have holes in the web, and
whose rating is controlled by flexure (flexural strength). However it can also be used for review
of any steel member section as it provides section property comparisions between the Virtis and
PCBars methods.


BACKGROUND
In order to evaluate the difference in deterioration approach, a computational spreadsheet was
developed. A brief description of the size combinations reviewed, the results, and the
conclusions drawn are shown below. Additional documentation is available upon request.

        Comparison of section properties were made for three depths of web (24”, 48” and 72”)
        Varying web thicknesses (1/16” increments) and flange plate thicknesses (1/8”
         increments) were used as noted in the table below

              Web                         Top Flange                        Bottom Flange
 Depth        t min      t max      Width   t min    t max            Width      t min    t max
  24”          3/8”        1”        12”     3/8”      2”              14”        3/8”      2”
  48”          3/8”        1”        16”     3/8”      2”              20”        3/8”      2”
  72”          3/8”        1”        20”     3/8”      2”              22”        3/8”      2”

        All properties considered the effect of a 4” high by 6” wide hole at the bottom or top of
         the web


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                      PROCESS FOR REVIEWING VIRTIS ANALYSIS
                 STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                           JUNE 9, 2007
       Virtis web thickness was reduced by the percentage of the hole height vs. the web depth
       Overall results in % difference for the comparisons, without removing unacceptable
        combinations, are noted below.

                                     Hole in Bottom of Web
              Moment of Inertia            Top Flange Modulus           Bottom Flange Modulus
Depth
            min    max       avg          min     max      avg           min     max     avg
  24”       0.0    16.6       3.8         0.0      8.7     2.6           1.6     23.6    8.4
  48”       0.0    10.0       2.7         0.0      4.3     1.4           2.0     14.9    6.3
  72”       0.0     7.6       2.6         0.0      3.0     0.9           2.0     11.3    5.3


                                       Hole in Top of Web
              Moment of Inertia           Top Flange Modulus            Bottom Flange Modulus
Depth
            min    max       avg         min       max    avg            min     max     avg
  24”       0.0    18.6       4.8         2.2      25.9   9.5            0.0      7.8    2.3
  48”       0.0    12.3       4.6         3.2      18.1   8.7            0.0      5.2    1.4
  72”       0.0     9.0       4.1         3.2      13.5   7.4            0.0      4.0    1.1

Conclusions:
   1. Difference in properties can be significant for certain girder combinations
   2. The flange near the hole will have the biggest difference in section modulus
   3. It is generally safe to assume that shear will not be an issue with holes as shear will travel
      around the deterioration, although holes at the member supports could result in a shear
      issue
   4. The section property differences are only an issue when flexural strength controls the
      rating




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                PROCESS FOR REVIEWING VIRTIS ANALYSIS
            STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                             JUNE 9, 2007
THIS OUTLINE IS PROVIDED ON SEPARATE SHEETS FOR EASY REFERENCE

OUTLINE OF STEPS
NOTE: This section assumes that the structure is a schedule based girder system. Steps involved
with a cross section based girder system are very similar. The steps are explained in more detail
in Appendix A.

INPUT
1. Input all steel structure data into Virtis if necessary, or open the existing file corresponding to
    the steel structure that requires analysis with deterioration
2. Find the member or members that have deterioration
3. COPY the existing member alternative by highlighting the alternative, right-clicking and
    selecting copy or using the top menu copy command or icon
4. Highlight the Member Alternative, right-click and choose Paste or use the top menu paste
    command or icon
5. Rename the new member alternative to distinguish it from all other member alternatives.
    Using deterioration as part of the new name is recommended
6. Double-click on the member designation (ex. G1)
7. Change the deteriorated member to the E and C alternative
8. Expand the deteriorated member alternative in the tree
9. Select and open (double-click) the deterioration entry under the deteriorated member
    alternative
10. Click on New to open a row for input
11. Compute the percentage (%) loss for the web by dividing the height of the hole by the depth
    of the web, and input it
12. Locate the deterioration by providing the starting point on the member and the length of the
    reduced section
13. Do the same for the width and thickness losses in the flanges if applicable
14. Select and open the point of interest entry under the deteriorated member alternative
15. Input the location or locations near or within the deteriorated area so that the moment will be
    computed and output
16. Save the structure and run Virtis

OUTPUT
1. The rating results for the structure member(s) will include the controlling limit states for both
   Inventory and Operating rating. Highlight the deteriorated member alternative and click on
   the View analysis report icon (resembles a spreadsheet)
2. If the limit state is something other than Flexural Strength, the remainder of this review
   process may be ignored
3. If the user wants to compare the results received from Virtis with what might have been the
   results from BARS, the spreadsheet Fundamental Section Properties.xls (FSP) should be
   used
4. The FSP spreadsheet requires the input of several girder dimensions in order to function. The
   dimensions can be found under the girder profile for the member being investigated



                                             Page 3 of 21
                    PROCESS FOR REVIEWING VIRTIS ANALYSIS
               STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                         JUNE 9, 2007
5. The Virtis web thickness can be determined by checking the deterioration section and
   modifying the original thickness by the percentage shown for the web
6. The Hole information should be available on the Inspection report
7. The UNFACTORED moment MMNT is shown on the LFD Analysis Output report. It is
    accessed by clicking on the icon with the sheet and gavel in the menu
8. When the dialog opens, the BWS Report will be the default. Click on the down arrow and
    select the LFD Analysis Output report
9. Choose the items you want to see in the report by checking the boxes by the various output
    items. The default is all of the items. You will only need the moments for this process
10. Click on the Generate button to generate the report
11. Under the moments provided in the results, find the point you wish to investigate. This will
    normally be a Point of Interest if the data is entered as noted above
12. Record the value provided under the Stage 1 loading. We are working with the member only
    to determine the variance between deterioration methods for the web
13. Input the moment shown in the MMNT blank of the input area
14. The FSP spreadsheet will then compute several items that can be verified either from the
    Virtis output or by hand

This concludes the steps necessary to make the comparison using the FSP spreadsheet.

Descriptions of the FSP output items are shown on one of the worksheets, as are the formulas
that were used for calculation. A brief description of the most important areas to review is
included on the next pages for convenience.




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




FSP SPREADSHEET ITEMS TO REVIEW
SECTION PROPERTIES
The moment of inertia and the section modulus for the top and bottom of the member are shown
to the right of the member drawing just above the highlighted box showing the stresses. Values
are shown for Virtis, based on the percent reduction web deterioration method, and BARs, based
on direct property calculation taking the deterioration due to holes into account.

The third column shown is the percentage difference between the two results. This percentage is
computed using the formula:

                                   (Virtis result – BARs result)
                                            BARs result

As a general rule of thumb, if the % DIFF is less than 5 %, the overall effect of the difference in
method may be regarded as negligible. In other words, the Virtis deterioration method closely
approximates the BARs method.

If the % DIFF is greater than 5 %, then further investigation may be required. Depending on the
frequency of high percentage differences, additional spreadsheets may be developed to assist in
the additional investigation. One step that can immediately be taken is to increase (or decrease)
the percentage of web thickness reduction shown in the spreadsheet. This will recalculate the
Virtis properties so that another comparison can be made. Once the % DIFF is within an
allowable tolerance, the Virtis analysis can be rerun using the modified web thinning.


STRESSES
The highlighted, boxed area provides the flange stresses that result from the Stage 1 moment
provided as part of the Virtis analysis. The moment used is the factored moment for stage 1,
which is dead load only. The factor used is 1.3.

Although a % DIFF calculation is not provided for the stresses, it is easy to recognize if there
are major differences. The same 5 % rule of thumb should be applied here.


SPECIFICATION CHECKS
The FSP spreadsheet includes three basic checks on the member.
1. A compactness check based on AASHTO formulas 10.48.1.1-a (formula modified to work
   using entire flange width, not ½) and formula 10.48.1.1-b.
2. A compression flange check based on AASHTO formula 10.34.2.1.3
3. A web thickness check based on AASHTO formula 10.34.2.1.1


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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007

All checks use the BARs web thickness as the Virtis web thickness is reduced for deterioration.


SUPPORTING CALCULATIONS
The values shown in the upper right are intermediate calculations that are used to generate the
section properties. They are available for the user if they desire to check the section property
calculation.




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                    PROCESS FOR REVIEWING VIRTIS ANALYSIS
                 STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                 JUNE 9, 2007

APPENDIX A – DESCRIPTION OF STEPS
NOTE: This section assumes that the structure is a schedule based girder system. Steps involved
with a cross section based girder system are very similar.


INPUT
1. Input all steel structure data into Virtis if necessary, or open the existing file corresponding to
   the steel structure that requires analysis with deterioration

2. Find the member or members that have deterioration (G1 will be used for this example)




3. COPY the existing member alternative by highlighting the alternative, right-clicking and
   selecting copy or using the top menu copy command or icon




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




4. Highlight the Member Alternative, right-click and choose Paste or use the top menu paste
   command or icon




5. Rename the new member alternative to distinguish it from all other member alternatives.
   Using deterioration as part of the new name is recommended




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




6. Double-click on the member designation (ex. G1)

7. Change the deteriorated member to the E and C alternative




8. Expand the deteriorated member alternative in the tree




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                    PROCESS FOR REVIEWING VIRTIS ANALYSIS
               STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                         JUNE 9, 2007
9. Select and open (double-click) the deterioration entry under the deteriorated member
   alternative




10. Click on New to open a row for input




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




11. Compute the percentage (%) loss for the web by dividing the height of the hole by the depth
    of the web, and input it




12. Locate the deterioration by providing the starting point on the member and the length of the
    reduced section (See above picture)

13. Do the same for the width and thickness losses in the flanges if applicable



                                           Page 11 of 21
                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007

14. Select and open the point of interest entry under the deteriorated member alternative




15. Input the location or locations near or within the deteriorated area so that the moment will be
    computed and output (See above picture)

16. Save the structure and run Virtis

OUTPUT
1. The rating results for the structure member(s) will include the controlling limit states for both
   Inventory and Operating rating. Highlight the deteriorated member alternative and click on
   the View analysis report icon (resembles a spreadsheet)




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                     PROCESS FOR REVIEWING VIRTIS ANALYSIS
                 STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                         JUNE 9, 2007
2. If the limit state is something other than Flexural Strength, the remainder of this review
   process may be ignored. This example was not controlled by flexural strength, however the
   process will be continued as an example
3. If the user wants to compare the results received from Virtis with what might have been the
   results from BARS, the spreadsheet Fundamental Section Properties.xls (FSP) should be
   used


                                         L:\Virtis Sect Prop
                                        Comp\Fundamental Section Properties.xls




4. The FSP spreadsheet requires the input of several girder dimensions in order to function. The
   dimensions can be found under the girder profile for the member being investigated




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




5. The Virtis web thickness can be determined by checking the deterioration section and
   modifying the original thickness by the percentage shown for the web




6. The Hole information should be available on the Inspection report

7. The UNFACTORED moment MMNT is shown on the LFD Analysis Output report. It is
   accessed by clicking on the icon with the sheet and gavel in the menu



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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                      JUNE 9, 2007
8. When the dialog opens, the BWS Report will be the default. Click on the down arrow and
   select the LFD Analysis Output report




9. Choose the items you want to see in the report by checking the boxes by the various output
   items. The default is all of the items. You will only need the moments for this process




10. Click on the Generate button to generate the report (See above picture)

11. Under the moments provided in the report, find the point you wish to investigate. This will
    normally be a Point of Interest if the data is entered as noted above


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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




12. Record the value provided under the Stage 1 loading. We are working with the member only
    to determine the variance between deterioration methods for the web

13. Input the moment shown in the MMNT blank of the input area

14. The FSP spreadsheet will then compute several items that can be verified either from the
    Virtis output or by hand

This concludes the steps necessary to make the comparison using the FSP spreadsheet.

Descriptions of the FSP output items are shown on one of the worksheets, as are the formulas
that were used for calculation.




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   PROCESS FOR REVIEWING VIRTIS ANALYSIS
STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                JUNE 9, 2007




                Page 17 of 21
                  PROCESS FOR REVIEWING VIRTIS ANALYSIS
               STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                               JUNE 9, 2007




VERIFICATION USING VIRTIS
1. Open the output from BRASS by clicking the eyeglass icon

2. Choose the LFD output file from the dialog that opens




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                  PROCESS FOR REVIEWING VIRTIS ANALYSIS
               STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                               JUNE 9, 2007




3. The output file will open in NotePad




4. Click on Edit in the top menu and select Find

5. When the Find dialog appears, type Section Moduli in the text box and click Find Next




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                    PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                        JUNE 9, 2007
6. The output file will move to what should be the initial point of interest designated in the
   member alternative




7. Scroll down to see the moments and stresses for the selected point




8. The value shown for M DL1 (Stage 1) corresponds to the factored moment in the example
   spreadsheet (92823.1 in-k/12 in/ft = 7735.26 kip-ft)




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                   PROCESS FOR REVIEWING VIRTIS ANALYSIS
                STEEL STRUCTURE MEMBERS WITH HOLES IN WEB
                                JUNE 9, 2007




9. The value shown for the Top Flange section modulus corresponds to the Virtis section
   modulus in the example spreadsheet

10. The value shown for the Bottom Flange section modulus corresponds to the Virtis section
    modulus in the example spreadsheet

11. The value shown for the Top Flange stress at Stage 1 corresponds to the stress in the example
    spreadsheet

12. The value shown for the Bottom Flange stress at Stage 1 corresponds to the stress in the
    example spreadsheet




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