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surface modelling problems

March 13, 2014, 4:40 am
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Hi,

please see attached model where I am trying to creat a polygonal surface element with a polygonal meshing.

i have following queries:

1. once i have created the surface and then meshed it then staad divides it into various plates. now in order to assign properties can i use plate thickness?

2. when to use surface thickness option? i.e. if staad is going to divide the meshed surface into plates then what's the point of having availability of surface thickness option?!!

3. once created the surface I tried to model a random polygonal opening into surface but struggling to do that. please advise. do i need to create another surface within polygonal profile for hole and then define a second surface as hole?!!

4. the stress types on surface -are they only membrane or plate ie. including across thickness bending (as staad divides surface into plates anyways)!

thanks.

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RE: RAM Connection

March 13, 2014, 9:49 am
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The Single Plate or Shear Tab connection is still only possible in Ram Connection with bolts on the beam side. I checked the pre-release version 9.0 for this in the beam to column configuration and as part of the Column-Beam-Brace type and using the US codes.

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RE: RAM Concept Load History Deflections

March 13, 2014, 10:50 am
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When the Load history deflection predicts significantly larger deflection than a standard combinations times the creep factor, it's usually a matter of cracking. I use the Service Design - ECR plots to gauge the amount of predicted cracking, and in your model at many of the supports, the cracking results in approximately double curvature, over 50% cracked.

The other thing to watch out for are sections with no equilibrium. In your Load History Calc log there are dozens of these warnings. Here is the first:

Calculating Instantaneous Cross Section Load History Curvatures for Load Step Maximum Short Term Load Iteration 3 Cross section 437 has no equilibrium at (-251,9.199) (-257.6,9.199). Setting element stiffness to be very small.

So with all these fully cracked sections it's no surprise that the final load history results are so large. To improve the behavior, you might have to add some more user reinforcement, assuming there are no problems with the loads, load history steps or strip layout. As a test I increase the #5 @ 12 bottom bars to #6 at 12 and the max long term deflection dropped slightly to 3.0".

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RE: STAAD Pro units

March 13, 2014, 10:54 am
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Thank you but, when I was using the previous version of  STAAD Pro, this wasn't a problem. That's why i called it bug. because when i updated my STAAD Pro to the latest version this problem showed up.

hopefully someone from bently tecnical support will look at this problem.

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RE: RAM Concept Load History Deflections

March 13, 2014, 1:57 pm
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Taking your suggestions from above, we tried to remove those two issues.

If we set our slab thickness to 18" in that same model, and rerun, we no longer get ECR spikes at the columns and equilibrium errors are reduced to just a couple that are not located near the maximum deflection point.

With those two errors/issues worked out, we were left with the following deflection values:

ACI 9.5 Method: 0.4153"

Load History Final Instantaneous: 1.17"

Any other thoughts on what might be causing the large discrepancy? 

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RAM Concept Load History Deflections

March 13, 2014, 9:06 am
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We are currently working on a RC garage model (11” slab thickness, max span ≈34’).  We are noticing that our load history deflections are much larger than we are expecting.

When we run load history deflections we get a Final Instantaneous Deflection of 3.257” in our worst span.

We created a load combination based off of ACI 318-9.5.2.5 to compare this to.  The load combination we created essential takes sustained loads (DL + 10%LL + 50%ParkingLL), multiplies them by the long-term multiplier from ACI, and then adds in Service loads (DL+LL).  Using 2.0 as a conservative value for the long-term multiplier, our resulting load combination is as follows:

Using this formulation, we get a maximum deflection of 1.044" from the 'Max Deflection' plot under the load combination.

We are aware of some of the reasons stated in Section 65 of the Concept Manual as to why some deflection results are lower than those calculated in Load History deflections, but we are having a difficult time making sense of the difference being 3x what ACI predicts.

Are there any other reasons that could explain the large discrepancy?

I have attached the model in question for reference.

Thanks

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RE: RAM Concept Load History Deflections

March 13, 2014, 3:06 pm
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Your Creep factor is 3.35, but your Dead load factor is only 3, reducing the creep factor to 3 would align better with your custom combo.

It's also informative to look at the second step "Sustained Load" vs. the "Final Instantaneous Load" step since your custom combo only represents half the Parking load sustained.

When I even the playing field in those ways, the load history sustained result is 1.0", still significantly larger that your combo.

We can deduce that the effects of shrinkage are significant. As a test I reduced the shrinkage restraint percentage from 10 to 1% and then the sustained deflections are dramatically reduced to 0.36". For more on the use of shrinkage restraint in load history calculations, check out this article:

communities.bentley.com/.../6191.aspx

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RE: Cross Section Trimming in 2-Way PT Slabs with Drop Panels and Drop Caps

March 13, 2014, 4:10 pm
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It's a bit of work, but if you segment your design strips into 3 segments, then you can use different trimming options at the ends. For example, if there is a segment from the outer panel edge to the column, and that segment uses "None" trimming + inter cross section slope limits = 0.00, then you'll get what you are after.

Using Max Shear Core trimming could also work, but only if the area of the panel section is greater than the slab rectangle.

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Cross Section Trimming in 2-Way PT Slabs with Drop Panels and Drop Caps

March 13, 2014, 1:43 pm
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I have an 11" PT slab with a 9'x9'-14.5" thick drop panel and 4'x4'-20" thick drop cap at the columns.  I am having trouble getting the proper cross section trimming at locations close to the columns where the drop caps exist.  The cross section trimming for each of the design strips is set to "T or L".  This works fine at the drop panels since there is only one bottom elevation to find.  However, when at the drop caps there are two elevations and Concept is choosing to use the drop cap depth and width for the shear core and for the section used in flexural analysis.  Is there any way to force Concept to trim this section so that it is using the drop panel width and thickness and disregard the drop cap section for shear core and flexural analysis purposes?  I don't want to use the full drop cap section for flexural design and any other cross section trimming options (max rectangle, slab rectangle, etc.) don't help either.

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RAM Structural - Toggle composite warnings

March 13, 2014, 5:03 pm
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Is there a way to shut off composite warnings? 

Flange width inadequate, Not enough studs for 25%, Cannot fit enough studs for 25%

I'm glad it lists these but they aren't on the same level as moment capacity or deflection warnings.  It's hard to go through beams with warnings when the vast majority are simply telling you the beam will not be composite. 

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RE: Fabricated beam using plate command

March 14, 2014, 4:23 am
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Hi,

Plate element should me be modeled using some rules. A plate element’s aspect ratio should be as near to 1:1 as possible.

Aspect ratio is defined as the ratio of the length of the shortest side to the longest side of the element.  Aspect ratios in excess of 4:1 should be avoided.  In other words, a triangular (3-noded) plate element should be shaped as near to an equilateral triangle as possible, and a quadrilateral (4-noded) element should be shaped as near to a square as possible.

You may consider breaking the plates into smaller pieces. 

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Fabricated beam using plate command

March 14, 2014, 3:23 am
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In staad pro,  Fabricated " i " Section to be Modeled by Plates. I getting this Warning message.    *WARNING* PLATE NO.     5(JOINTS      1 -      2 -      7 -      8)
          IS BADLY SHAPED, WARPED, NOT CONVEX, OR NOT NUMBERED COUNTER-CLOCKWISE.

How to solve the warning message. Give the suggestion & procedure to model a fabricated "i " section using plates.( section depth 750, width 200, flange & web thickness 10mm)

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RE: Fabricated beam using plate command

March 14, 2014, 4:40 am
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RE: surface modelling problems

March 14, 2014, 4:57 am
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xyz111,

You may want to have a look at the information at the following posts

communities.bentley.com/.../171554.aspx

communities.bentley.com/.../194290.aspx

communities.bentley.com/.../204643.aspx

communities.bentley.com/.../202064.aspx

a) The underlying constituents of surfaces are just 3 and 4-noded plate elements, the same ones that you can create manually, or using the mesh generation facilities.

b) The only benefit of using surfaces is that you deal with a physical object and leave the conversion of that object to plate elements to the program. But, unless you are careful in ensuring that the underlying elements get properly connected to the beams and columns of the structure, the results can go wrong. Hence, our suggestion is to model the wall or slab using a mesh of plate elements instead.

c) It is essential to use surfaces only if you intend to use the program's shear wall design capability. If that is not your intention, better to stick to plate elements.

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RE: Tower Model_

March 14, 2014, 5:01 am
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The supports I model are just tentative -they might not be correct!! Possibly using FIXED BUT support option!! The boom is pivoted at locations where I have shown the supports. Similar to crane boom fixed against translational movements but pivoted so could have luffing motions.

Assume I am analysing a crane boom fro various vertical rotation positions in a single analysis then how can I achieve this?!!

Thanks.

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RE: Tower Model_

March 14, 2014, 5:07 am
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To the best of my knowledge, in such case you need to model the crane boom separately at different vertical angles.

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RE: Tower Model_

March 14, 2014, 5:16 am
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So different analyses!

Is it not possible to use CHANGE command here somehow?!...

thanks.

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RE: surface modelling problems

March 14, 2014, 5:17 am
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RE: RAM Structural - Toggle composite warnings

March 14, 2014, 5:18 am
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+1

One of the many items on my running wish list is a way to ignore warnings in RAM.  Both Concrete and Steel have warnings that are more minor (i.e. composite % especially for short members) than capacity or serviceability issues, and the ability to ignore/pause them would be great so that larger issues can be addressed first.

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RE: STAAD: Bending moment in plate elements

March 14, 2014, 5:33 am
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HI Sirojt did you find what is My (line) ? if so please help me to learn about it..... (this is regarding the plate results along a line)

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