Checking max deflection?

0753.Canopy.std

Hello. I've been following this deflection post and have tried to write in commands to print max displacements in the output. I am having difficulties executing the displacements properly; may be writing it wrongly in the code.

See below the STAAD output - the displacement given does not correspond with the max displ. shown in the outputs. Why are they not the same? And why is the L/DISP value so large? I have also attached the STAAD input. Please help.

By the way, I've also tried to follow this but have found it confusing. Not sure how to properly use the SECTION command and PRINT SECTION.

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/9129/print-section-forces-in-staad-output

Thanks.

The displacements that you see at the 13 section locations for each member and for each load case, are the global displacements of these points from their initial positions to their finally displaced positions. So if you consider the member 212 and load case# 12, the 7th section which is at the middle of the member, has displaced in global Y by -0.2007 inches. The Max LOCAL DISP on the other hand, reports the maximum displacement of the section measured relative to a straight line joining the displaced positions of the start and the end nodes. That is 0.02482 inches in this case and a quick and approximate way to verify this is by subtracting the global displacement of that point from the average global displacement of the start/end nodes ( or the first and the 13th sections). So in this case the Max local displacement =  0.2007 - (0.1755 + 0.1764)/2 = 0.0248 in. The L/DISP is the ratio of the total member length and the max local displacement. So L/DISP = 100/0.02482 = 4029

The displacements that you see at the 13 section locations for each member and for each load case, are the global displacements of these points from their initial positions to their finally displaced positions. So if you consider the member 212 and load case# 12, the 7th section which is at the middle of the member, has displaced in global Y by -0.2007 inches. The Max LOCAL DISP on the other hand, reports the maximum displacement of the section measured relative to a straight line joining the displaced positions of the start and the end nodes. That is 0.02482 inches in this case and a quick and approximate way to verify this is by subtracting the global displacement of that point from the average global displacement of the start/end nodes ( or the first and the 13th sections). So in this case the Max local displacement =  0.2007 - (0.1755 + 0.1764)/2 = 0.0248 in. The L/DISP is the ratio of the total member length and the max local displacement. So L/DISP = 100/0.02482 = 4029

Thank you. Is there a way that the output can just tell me what is the global displacement and at what length along the member, with it's L/DISP? I don't want the LOCAL DISP. I know the GUI can tell me the deflection, as you showed earlier, but I'd be interested to see if the deflection can be stated in the output too.

The section displacements that you see in the analysis output file are all global displacements and these are reported at 13 equally spaced sections along the length of a member (including the start and end). The L/DISP however is reported for the max local displacement only and not for the global section displacements because when checking the deflection in a member and satisfying the allowable deflection requirements ( L/240, L/360 etc) you should use the local displacements.

Thank you Sye. This explanation helps.

Sye, as you said the displacements are at 13 section locations. Therefore, the max displacement should be 0.2007 - (0.1755+0.1834)/2 = 0.02125 per your approximation and it is not equal to the printed MAX LOCAL DISPLACEMENT. 

Also, I did a run on a structure and I compared the Report Table "Section Displacements" Max Disp, it does not match the printed (when using PRINT SECTION MAX DISPLACEMENT) Max Displacement which in turn does not match the approximation you mentioned above. 

Which one is correct - the result table or the print? The table is helpful in getting the critical beam. Otherwise, one has to go through the result output one by one to get the maximum. The graphical deflection does seem to follow your approximation of the local deflection above. However, it would be difficult to know which member is critical. Using the Global Deflection to filter out maximums may not be that accurate. 

To keep things simple here is a sample file ( EXAMP01_rev.STD) with respect to which I have explained the calculations. I have highlighted the max local displacement for member 6 for load case 1 obtained in different ways. If you look at the member section displacements output from the PRINT SECTION DISPL and PRINT SECTION MAX DISP output, both show the max local disp as 0.13606 inches as highlighted below

A rough calculation based on the section displacements reported in the output above also matches the value as shown next

Local Displacement at mid section = 1.0087 – (0.9048 + 0.8405)/2 = 0.13605 inches

The Maximum Relative Displacement table reports the same value as well as marked below