1. General Input
Q: Do I need to input a decimal point all the time?
A: The current version should insert the decimal point wherever needed.
2. Geometry Input
Q: How is the point of contraflexure determined and what is it used for?
A: DASH uses DL+SDL to determine the contraflexural points and the dividing line between using composite and non-composite sections for the stiffness analysis as well as the section property calculation.
Q: Would the locations of composite and non-composite vary depending on the placement of live loads? Therefore, I could place a live load so that it causes positive bending in a region, but based on the set locations from the girder DL, it would use the steel girder only for section properties instead of the composite one.
A: You may get different answers from different code writers. My explanation, as well as some of my colleagues, is this:
- It is conservative to use non-composite section properties in the negative moment region. From the stiffness analysis point of view, the negative moment is less than assuming full composite in the negative moment region. (This option can be controlled by the program input, % of composite in the negative moment region). From the stress calculation point of view, depending on the shear studs in the negative moment region, the stress can be calculated by using steel section only or steel + rebar section.
- It is true that the moving live load will cause negative bending in the negative moment region so as to cause moving of the contraflexural points. If you want to put that scenario into consideration, you then have millions of possibilities. So, in consensus, the contraflexural points are usually fixed for the stiffness analysis. But for the stress calculation, there are two schools of thought:
- Still use fixed contraflexural points; beyond those, use non-composite section properties;
- Use the summation of moment to determine use of non-composite section properties or not; if the sum is negative, use non-composite section properties. Otherwise, use composite section properties. Compared to the first option, this one is more conservative.
3. Section Input
Q: Stated in the User’s Manual, “two utility programs are available to alter the contents of Steel Section Table (FILEW.BIN). Please contact your user support.” I would like to update the Steel Section Table. Do you know where I can obtain these utility programs?
Q: Further, I updated the SECTION.DAT file, but I am not sure how to create a new FILEW.BIN file. The readme file states “when the SECTION.DAT file editing has been completed, a new FILEW.BIN will be created by typing WRITEF.” Could you please provide more information about this step? Where do you type WRITEF?
A: It is an execution file. You can either go to the DOS environment and type “WRITEF” or use Windows Explorer to show all the files and click on “WRITEF.EXE.” Check the date of the FILEW.BIN created in the same folder, it should be today. The new FILEW.BIN should reside in the same folder where the application files located.
Note: Please override the section data file in the last few nominal depth sections because every section is mapped to one corresponding cost data for the design. If you override one of the sections, the corresponding cost data will not map to the right section. Please keep the original FILEW.BIN file and re-name it to the FILEW.BAK. After finishing your job, you can delete the new bin file and re-name the backup bin file back to the original file name so that every section has one correct corresponding cost data. If you forgot, the new bin file will stay there. The program will still work fine because you change only the last few nominal depth sections and nobody will use those sections.
4. Load Input/Output
Q: When using LRFD analysis, TABLE 0.0.6.1 AASHTO LIVE LOADING – LOAD TYPE (A) shows… HL-93 VEHICLE X FACTOR OF 0.00 . What is this X factor? I don’t remember seeing this before.
A: It’s a special request from one of the States which wants to manipulate HL-93 by a certain factor, say 1.1, which is not recommended by AASHTO. We also did not publicize it, but you noticed it.
Q: (LFD) I input HS loading and special vehicle at the same time for a LFD code check and the results look weird.
A: The program ignores the HS loading when you input the special vehicle (Load Type “C”) in LFD.
Q: How does the program analyze a pedestrian bridge? For a two-span pedestrian bridge, does loading (such as live load) at one span (one side of bridge) determine the maximum positive moment?
A: Input “HS-1” to turn off the vehicular loading and then input or let the program default to the sidewalk loading. See the description for sidewalk loading. Sidewalk loading is a live load applied to get the maximum effect like any vehicular loading.
Q: Same question about the sidewalk loading, isn’t it true that sidewall LL moments should result from the sidewalk LL input in XX k/ft. for both runs HS-1 and H-1?
A: For HS-1, the result is from HS-1 + sidewalk because HS loading can be factored, such as HS-25. For H-1, the result is from H-20 + sidewalk because H cannot be factored so there is no control of H-XX where XX is not 10, 15 or 20.
Q: When you have sidewalk loading, the program applies it directly to the beam that you are analyzing, regardless of whether it is an interior or exterior beam. The User’s Manual states that the program will apply the sidewalk loading directly to the exterior beam. Should the program leave off the sidewalk loading when the user specifies the beam as an interior beam, or should the user manually remove the sidewalk loading when analyzing an interior beam?
A: Based on users’ requests, the program has been modified to such a case:
- For interior beam, several users stated that when the sidewalk loading is applied to the exterior beam, the interior might be under uplifting force. So, the program now allows positive (downward) and negative (upward) loading input for the interior beam based on engineering judgment.
- For exterior beam, you may input 100. to indicate 100 percent of the AASHTO sidewalk loading where it varies by span length, or input loading (if <10) directly to override AASHTO sidewalk loading (but with constant magnitude regardless of the span length.) All should be positive to indicate downward loading.
So, to answer your question, yes, the sidewalk loading could be removed for interior beam if there is no effect based on engineering judgment. The effect of the sidewalk to the interior beam is based on location of the sidewalk.
5. Design Input
Q: How to input the data for a rolled beam design?
A: In Data Type 12042, you need to select either 1 (compact wide flange) or 2 (braced non-compact wide flange) in Section ID. Then, input a fixed nominal depth in Data Type 12052 under Const. Web Depth for a specific nominal depth you are interested in or input nominal depth range in Data Type 12062 in the fields of Maximum Web Depth and Minimum Web Depth if the rolled sections can be in same range.
Q: I am doing a design for WVDOH. They are requesting the use of Grade 70 steel in the negative moment region and Grade 50 steel in the positive moment region. Can Merlin Dash design a steel girder in this fashion or do I have to do a design in all Grade 70 and then in all Grade 50 and then combine the results? I would like to keep a constant web depth but the flanges can vary.
A: DASH can do the mixed design (different grades of steel in different regions)
but can’t do hybrid design (different grades of steel within one section, flange and web). Code check and rating are allowed for both (mixed and/or hybrid sections). However, I recommend you use the design section to go back and code check again.
6. Distribution Factor/Impact Factor Input/Output
Q: (LFD) Does the MERLIN-DASH program assume S/5.5 as per AASHTO 188.8.131.52? Does it assume a distribution of S/5.5 for the wheel at the support or does it assume the deck acts as a continuous beam for the end wheel? If it does, what is the logic used in the program?
A: S/5.5 is used for all moment and shear distribution factors, except for end shear. For end shear and reaction, the wheel on the top of the support is using simple beam action distribution but the rest is still S/5.5. Reactions have been verified and proved to be correct by us and many of our users. If you use the assumption stated above, you should get the same answer.
7. Analysis Output
Q: A simple analysis using AISC Continuous Beam tables for a 3-span bridge and redistributing 10% of the negative moment to the positive moment sections gives me approximately 625, 470, and 690 kip ft respectively for three points, the 4/10 point on span 1, over the pier (0/10) on span 2, and the 5/10 point on span 2. MERLIN DASH yielded approximately 640, 390, and 750 for the same points. The total moment agrees, but MERLIN DASH has redistributed the moment somewhat differently.
A: Due to different practices by different states, we gave an option for the structure analysis of a continuous beam in MERLIN-DASH. You may select “% of composite in the negative moment region” on screen marked “Structural Details”. If you give 100%, the program will use 100% of the composite section properties in the negative moment region when the stiffness is formed. In this case, more moment will be attracted to the negative moment region. I guarantee that you will get closer results. If left blank, as in your case here, non-composite section properties are used in the negative moment region and lower moments in this region are obtained, as you observed.
Q: Further, I entered 0% since the bridge is not composite in the negative moment areas. I expected this to lower the moment in the negative steel area, although I was somewhat surprised at the amount it was lowered. Was the moment also redistributed by the 10% rule? Also, I was surprised that most of the moment which ended up in the center span is somewhat counterintuitive. Any thoughts on this?
A: There generally are three rules for the negative moment region:
- “Moment distribution based on stiffness” rule: As I stated before, you want to use composite or non-composite section properties in the negative moment region while calculating the moment. Your SDL and LL moment envelopes will be “readjusted” between the two assumptions, as you observed.
- “Calculation of stress” rule: The moment calculation is based on Rule #1. Rule #2 will use whatever you got from Rule #1 and divide the moments by whatever section properties you specified by Rule #2. If you specify shear studs and rebars in this region, the stress calculation will use “steel + rebar” section properties. This will affect your stress calculation in this region.
- “10% redistribution” rule: This one says that if a compact section is used in the negative moment region, especially at the pier location, AASHTO allows you to “redistribute” 10% of the negative moment to the positive moment region. In other words, you may allow 10% over the capacity at the pier if the redistribution shows that your positive moment region capacities are enough after redistribution. For “redistribution,” please see MERLIN-DASH output Table 184.108.40.206. If non-compact at the pier, you may see no change between “before” and “after” moment envelopes.
These 3 rules are independent and shouldn’t be treated as the same.
Q: Questions concerning the LRFD reactions on Table 220.127.116.11 – Live load Reactions. How to separate truck and lane?
A: There is a hidden message here, the impact factor. You know the impact for truck is 1.33 and the impact for lane is none. The program gives you an “equivalent” impact factor, as well as LL and LL+I. Following is an example on how to separate truck and lane reactions.
- From Table 18.104.22.168, LL=368.51, (Equivalent) IF=1.237, LL+I=455.88
- Assuming truck reaction is X, we now have the equation: 1.33X + (368.51-X) = 455.88 (=368.51 x 1.237)
- X is solves as 264.77. So, LL for truck is 264.77 and LL for lane is 103.74.
- To back check LL+I for truck is 264.77 x 1.33 = 352.14 and LL+I for lane is still 103.74. So, the total LL+I is 455.88, matched with the table readout.
Q: MERLIN-DASH seems to have a new table now (Table 22.214.171.124C Camber Information at 30th Point) that it didn’t include before. The columns for Steel and Other Non-composite Dead Loads (slab + any other non-composite dead loads) and Composite Dead Loads seem to contain the correct values. But, I think the values for the Total Dead Load are incorrect. This column seems to be the sum of the Other Non-composite Dead Loads plus Composite Dead Loads, but does not include the beam Dead Load. Compare this table with 126.96.36.199A (Camber Information). Also, why is the program using 30th points?
A: From time to time we accommodate various States’ requests to implement certain features. The table is a request from NCDOT for any span length over 200′. In the future, if you have any request, you may send me a note and we may consider it in the future version.
8. Stress Output
Q: When I check the moment capacity in Table 188.8.131.52 it showed OK but the stress in Table 184.108.40.206 is over the yield stress. What is wrong with the program?
A: You may have a compact section with moment capacity Mp. In this case, the stress in Table 220.127.116.11 using linear relationship is invalid.
9. Code Check Output
Q: Table 18.104.22.168 under “strength category for lateral bracing,” #2 says “Braced non-compact section (so far), need to further check AASHTO LRFD compact requirement.” Is this a bad thing?
A: It is a misunderstanding. “So far” means that the first check for lateral bracing uses the “braced non-compact” criteria, which does not involve moments at bracing. The next 4 Tables 22.214.171.124A-D involve finding the moments at bracing and satisfying the compactness requirement. Please see the last two columns for lateral bracing compactness requirement. Then, all are summarized on Table 126.96.36.199. The program has done all the checking for you. So, you are supposed to know the status by reading Table 188.8.131.52.
Q: Table 184.108.40.206a under Status says it needs to be checked. What does this mean? And why does it also say “NG” under Table 220.127.116.11 – Code check?
A: When Table 18.104.22.168 was calculated and established, the process was to be stopped and engineer’s judgment made. However, when there is no interruption, Table 22.214.171.124a has to be made with internal judgment. Our judgment is to find the minimum spacing between (1) abutment and max positive moment, (2) max positive moment and point of contraflexure, and (3) the negative range. Based on the minimum spacing calculated from Table 126.96.36.199, we may get the total numbers within these three regions. It happened here that based on the fatigue criteria, the total number so shear connectors within region 1 is ok, region 2 is not and region 3 has no connector. So, you may do your own calculation on the side, based on your own spacing, to check the established N1 (or N2) based on ultimate strength criteria.
10. Design Output
Q: (LFD) MERLIN-DASH table 188.8.131.52 list Vr=119.8Kips and Q/I=0.01876 and Sr=1.5. Sr=Vr*Q/I = 119.8x.01876=2.25 Kips and the pitch = 3×6.01/2.25=8.01 In. instead of 12.05″. Can you explain to me if there is a mistake in the program in computing Sr and the spacing of the connectors? Keep in mind that shear range listed in Table 184.108.40.206 matches the shear range from Table 220.127.116.11A.
A: Shear connector design by fatigue criteria follows:
- For road type =2 or 3 only one case each has to be checked;
- For road type =1, according to AASHTO Table 10.3.2A, there are two cases to be checked for truck loading: 2 million cycles and over 2 million cycles.
- 2 million cycle case is for multi-lane loading case and over 2 million cycle case is for single-lane loading case. Their corresponding distribution factors are different (WSD/LFD: S/5.5 vs S/7) and their corresponding shear ranges are reduced shown in Sr, but not in Vr shown in Tables 18.104.22.168A and 22.214.171.124.
- Alpha values are also different for 2 million cycles (7,850) and over 2 million cycles (5,500). So, I recommend you to input diameter size only, not Zr, unless you want to overwrite the internal set values (7,850 and 5,500).
- If you plug both numbers in, you will find for fatigue criteria check of road type 1, the over 2 million cycle case always governs because
Pitch = Zr/Sr
Case 1: Pitch = (Vr * Q/I) (7.85 * d^2) = 7.85 (Vr * Q/I) (d^2)
Case 2: Pitch = (Vr * [(S/5.5)/(S/7)] (5.5 * d^2) = 7 (Vr * Q/I) (d^2)
So, you may see Case 2 (over 2 million cycles) gives smaller pitch (7 vs 7.85) and governs. Probably not too many people caught the small footnote shown on Table 10.3.2A (as well as the Alpha for over 2 million cycles shown in Art. 10.38.5.1.1). But this is the governing case. If you input connector diameter, instead of Zr, the program will calculate everything correctly.
Q: (LFD) Is the maximum allowable shear connector pitch (fatigue criteria) in Table 126.96.36.199 based on an HS25 truck if this is the design vehicle? Or does it revert to HS20 as indicated in Tables 188.8.131.52 and 184.108.40.206?
A: For shear connector fatigue design, we are conservatively using no reduction from HS25 and single lane loaded (over 2 million cycles) for road type 1 to get the pitch distance. HS20, instead of HS25, used in Tables 220.127.116.11 and 18.104.22.168 was discussed in AASHTO Specs.
Q: I am using MERLIN-DASH for Windows to design a 42″ plate girder for a simple span composite structure. I chose program flow 6 (Design + Recycle + Code check) for the design. I’m specifying 50ksi steel, but when I check the output, several tables are using 36ksi as the yield stress.
A: On the input screen group there are two places for the yield strength input (actually there are three for the new version). For the homogeneous/mixed steel code check and rating, use “Yield stress and lateral bracing data” screen in the “Detail” input screen group. For design, use “Material and fabrication cost” screen in the “Design” input screen group. (The third one is for the hybrid steel code check and rating. It should use “Definition of members” in the “Beam definition” input screen group to replace the 1st one.) All defaults are 36ksi steel.
So, I’m guessing that when you use the design option you didn’t specify the “design material” on the “Material and fabrication cost” screen. Please specify.
11. Rating Output
Q: (LFD) Does Merlin Dash calculate the HS 20 lane load when it is entered on the AASHTO live load input screen or just the truck load? If it only calculates the truck load, how is the lane load entered? I have an inconsistency between BRASS and Merlin Dash. They both agree nearly exactly for the truck load, but BRASS has lower numbers and they are attributed to the lane load. The summary for the Merlin Dash output says “Rating information for AASHTO Truck”. Where do I look for the lane load ratings?
A: Merlin-DASH uses both truck and lane loading in the evaluation. The title should be “Rating information for AASHTO Loading” instead. I checked your output and found that lane loading governs at the interior support for Rating based on Maximum Design Load (Table 22.214.171.124), but Rating based on Serviceability Strength (Table 126.96.36.199) at the same location governs. You may use the detailed output level to give you more information for comparison.
Q: Which table shows the constructibility and serviceability for live load?
A: For constructability: Tables 188.8.131.52 shows the capacity to the section under construction and Table 184.108.40.206A shows the web buckling and compression flange buckling checks while under construction.
For live load deflection rating: Table 220.127.116.11A shows two lines of output; the 1st is for “possible” 2-lane loading and the 2nd is for “possible” 3-lane loading. You may pick the one fit the actual case.
12. Staging Input/Output
Q: I tried to use the staging features but it doesn’t work.
A: In Data type 01032, you need to select either 7 (DL STAGE ANALYSIS) or 8 (DL STAGE + LL ANALYSIS). If you input other options, the slab load is per span.
13. Splice Input/Output
Q: (WSD) It appears as though the required number of bolts in the top and bottom flanges, as well as the web are inspected for slip-critical criteria only. That is, the allowable load on the bolt was inspected by AASHTO 10.32.3.2.1. Why are the bolts not inspected for the criteria under AASHTO T10.32.3B? Is it due to the fact that the A325 bolts are generally being used with a Class A contact surface, thus resulting in the slip resistance per bolt of 15 ksi which is less than the shear per bolt of 19 ksi outlined in AASHTO T10.32.3B?
A: Yes, this is the presumption when we programmed it.
Q: (WSD) In reviewing AASHTO Sections 10.18.1.3, 10.18.2.2 & 10.18.2.2.3, it appears as though for each element (flange or web), there are two criteria to inspect for: 1) for the inspection of the bolts for shear & the plates for bearing and 2) for the inspection of the slip force.
- With this in mind and referring to AASHTO 10.18.1.3, it states that “¡?For this case, the shear strength of the connections shall be checked for the maximum calculated splice plate force acting on a single shear plane¡?” Why a single shear plane? Isn’t the flange splices and web splice a double shear application requiring the inspection of both shear planes?
- With reference to AASHTO 10.18.2.2.3, last paragraph, it states “¡?As a minimum, high-strength bolted connections shall also be proportioned to prevent slip at a force¡?for the small section at the point of splice times the smaller value of the gross flange area on either side of the splice¡?” Is it AASHTO’s intent not to employ the effective area, Ae, found in AASHTO 10.18.2.2.4 in the computations involving slip critical criteria? Are we only to use the effective area, Ae, when inspecting for bolt shear and plate bearing?
I am reading the AASHTO 17th Edition, 2002. My reference to the sections may not be absolutely clear. Where I find the conflict between gross and net area references is within the last paragraph of Section 10.18.2.2.3, page 274. For the slip force criteria, AASHTO states “¡?As a minimum, high-strength bolted connections shall also be proportioned to prevent slip at a force equal to the maximum elastic flexural stress due to D + (L+I) at the mid-thickness of the flange under consideration for the smaller section at the point of splice times the smaller value of the gross flange area¡?” It seems as though AASHTO refers to net section for checking the shear on the bolt, but gross section for the slip resistance. Is this a misinterpretation, or is this AASHTO’s intent?
This same concept is later repeated in AASHTO 10.18.2.3.9 where it states, “¡?In addition, as a minimum, high-strength bolted connections for web splices shall be proportioned as eccentrically loaded connections to prevent slip¡?the horizontal force resultant shall be computed using the gross section of the member¡?” This is different from the one mentioned before.
- Since you already divided the flange design force to two parts. Each one is considered single shear.
I re-read the section and got the same conclusion as you got without reading your comments first. This probably doesn’t make too much sense with WSD. But when designed by LFD or LRFD, slip critical design due to D+(L+I) means using un-factored loading with gross section and shear/bearing design is using factored loading with net section.
- This sentence is just to give the guideline for the calculation of flexural moment and horizontal force resultant of the web and should be treated differently from previous article. AASHTO is indicating to use gross section beam properties in determining the moment and service load stresses, fcf & fncf, in the check of the slip resistance for the load combination of D+(L+I).
Q: (WSD) With respect to fillers, in AASHTO 10.18.1.2, first paragraph, it states that filler plates over ?ù inch need not be extended, provided the fasteners are reduced by a factor R. It later goes on in the last paragraph and states that for slip-critical connections shall not be adjusted for the effect of the fillers. Is it AASHTO’s intention to treat fillers differently depending on if you are inspecting shear in the bolt or plate bearing versus the slip-resistance criteria?
A: Yes, R is only for shear and bearing and not for slip resistance. I would have to concur at this point in the interpretation of AASHTO. Which means for a Class A, slip critical connection with A325 bolts, a designer should not reduce the slip resistance of the bolt from 15ksi. This means that the shear on the bolt would have to be inspected to ensure that R*19ksi doesn’t fall below 15ksi, or it will control over the slip critical criteria.
Q: Did you have different installation packages for different computer operating systems?
A: For the single-user version, there are two installation versions, one for NT/98 and one for the others. For the network user version, there are one concurrent user and three concurrent users by using different HARDLOCK keys
Q: (Network Version) When I open the software, and it is finished loading, I click on “Input” and I get the following error message: Run-time error “372”: Failed to load control “SSTab” from TABCTL32.OCX. Your version of TABCTL32.OCX may be outdated. Make sure you are using the version of the control that was provided with your application.
A: It is the OCX (path) registration problem for your PC. It can be easily fixed by following the steps listed below:
- Connect to the File Server from end user’s workstation. (If there is a privilege problem, please log in as the System Administrator.)
- Go to the server folder where the MERLIN-DASH program was installed.
- Click on the “RegTab.exe” to install the TABCTL32.OCX. Message “Registration of OCX attempted” will show up.