lateral deflection - RAM | STAAD Forum - RAM | STAAD - Bentley Communities

### lateral deflection

how can i check whether my column or beam has passed deflection check. for instance if span/180 is check for beam deflection how can i enter? should i use DFF command?

### 14 Replies (Most Recent Reply)

• DFF is only to check vertical deflection for beams. For columns, you need to check the node displacements. Lateral deflection of beam needs to be checked manually if there are no intermediate nodes or provide node at middle and check node displacement.

Actually, deflection checking is based on the resultant Beam Relative Displacement values, so the use of DFF will correctly evaluate lateral deflections in addition to vertical deflections.  I am jumping into a class right now, but I will post an example after I get out.

Cheers,

Chris

Here is a simple model to demonstrate that deflection checking is basing its result on the resultant deflection.

START JOB INFORMATION
ENGINEER DATE 22-Dec-09
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 30 0 0;
MEMBER INCIDENCES
1 1 2;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 TABLE ST W21X48
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
1 FIXED
2 FIXED BUT FX
1 CON Z 6 15
UNIT INCHES KIP
1 CON Y -10 180
UNIT FEET KIP
PERFORM ANALYSIS
UNIT INCHES KIP
PARAMETER 1
CODE AISC UNIFIED
FYLD 50 ALL
DFF 400 ALL
CHECK CODE ALL
FINISH

The output file shows a dff value of 276.  The ratio of DFF/dff = 400/276 = 1.449 (the controlling ratio).

The span of the beam is 360 inches.  360/276 = 1.304 inches.

Check the Beam Relative Displacement Detail table, and note that the Resultant displacement at midspan is 1.302 inches.

So to summarize, thedff value is being calculated based on the Resultant displacement, and the deflection check will work for lateral deflections as well as for vertical deflections.

Cheers,

Chris

what is the dff value? and which factors will it govern.
• In reply to Rohail Iyaz:

The value of dff (lowercase letters) is a deflection result that is calculated by "Deflection Length"/Max Deflection.

• "Deflection Length" defaults to member length, so in the case of members that consist of multiple analytical members connected by intermediate nodes, it becomes necessary to define a "Deflection Length" by defining a DJ1 and DJ2 parameter to identify the two ends of the "Deflection Length".  In my example in the previous post, "Deflection Length" = member length = 360 inches.
• Max Deflection is the maximum resultant deflection found to occur anywhere along the length of the member.  Whether studying a single member or a multi-segment member (using DJ1 and DJ2), the Max Deflection value will be normalized to remove the component of deflection that is attributable to the displacement of the end nodes of the member.  In my example in the previous post, the maximum resultant deflection = 1.304 inches.
• In my example in the previous post, dff = 276 = 360 inches / 1.304 inches.
• DFF is the user specifed limiting value of dff.  In my example in the previous post, DFF= 400.
• For the purpose of a unity check, the ratio is calculated as DFF/dff.   In my example in the previous post, DFF/dff = 400/276 = 1.449

When a code check is performed, the ratio of DFF/dff is calculated just as many other ratios are calculated for evaluating all of the applicable code clauses.  If the value of DFF/dff is found to be the controlling ratio, then it will be indicated as the controlling condition for that member.  If the value of DFF/dff is NOT found to be the controlling ratio, then STAAD.Pro will indicate something OTHER than deflection as being the controlling condition.  (Note that just because a member doesn't indicate deflection as its controlling condition, it does not necessarily mean that the member passes the deflection check.  It just means that something else had a higher ratio than deflection.)

I hope this helps.

Cheers,

Chris

Thank you so much for your response, it really cleared up the air. one thing i wanted to ask is usually we simple compare the deflection with span/(some constant value), cant we just do this. for example i have 20 meter span. i put DFF = 180 so the value becomes 20/180 = 0.111m or 111 mm. if deflection exceeds this value the rafter or column fails?
• In reply to Rohail Iyaz:

It's always our pleasure to help.

Yes, the value of DFF is your (some constant value).  So in that regard, STAAD.Pro works exactly the way you are asking.

On the other hand, if you asking for the ability to perform a deflection check without concurrently performing a stress/strength check, that is challenging.  Before I start down that path, I'll let you clarify your intent.

Cheers,

Chris

• In reply to Rohail Iyaz:

Hi Rohail,

Could you please try again to attach your model?  The way it is coming through, it is very difficult to interpret in a meaningful way.

In addition, here are a few things you can check:

1. Verify that the parameters are consistent with the units being used.

2. Create separate DJ1 and DJ2 parameters for each unique girder to be evaluated.

3. Note whether the ratio of 9 is being controlled by deflection or stress.

4. Verify that bracing parameters are defined and assigned correctly (K, L, UNT, UNB).

Cheers,

Chris

I am not able to attach file because an error message keeps popping up that file cannot be attached. secondly can you guide how to paste the staad input here because i just copied the the data and pasted here but it entered all jumbled up unlike the way you entered. 1. the parameters are consistent with units. 2. I also created DJ1 and DJ2 separately for each girder but same result. 3. the ratio 9 is controlled by deflection. 4. All bracing parameters are assigned correctly
• In reply to Rohail Iyaz:

What version of STAAD.Pro are you currently running?

The procedure for copying and pasting the content of the STAAD.Pro input file is as follows:

1. With the model open in STAAD.Pro, click Edit > Edit Input Command File in the Menu Bar.

2. Select the entire file.  (Either click at the top and drag to the bottom, or click anywhere within the file and then press CTRL + A.)

4. Move to the open Forum post form, right click, and choose Paste from the pop-up menu.

Regards,

Chris

• In reply to Rohail Iyaz:

I was able to manually parse your input file and have spent a good deal of time observing the ratios that are indicated as a result of numerous different model configurations.  Here is a list of my observations:

• Your model contains loading in all three global axis directions, therefore the most appropriate structure type would be SPACE as opposed to PLANE.
• In seismic definitions, the magnitudes of the specified weights are summed algebraically to arrive at the building weight.  Therefore, the seismic weights should always be specified as positive values.
• If your intent is only to check the roof members for deflection, then DFF 180 should not be assigned to columns, brackets, etc.
• The loading that is being applied to this frame is causing excessive deformations.
• The deflection check is not appropriate for this application because of the double-pitch of the roof member. Deflection check works for members that are collinear.
• When performing a deflection check, STAAD.Pro evaluates members at sections along the length of the member.  At each section, it considers deflection in all three global axis directions, and normalizes those deflections to account for the displacement at the DJ1 and DJ2 nodes.
• When the maximum resultant normalized displacement is identified between DJ1 and DJ2, then dff is calculated as "deflection length"/maximum resultant normalized displacement.
• Finally, the unity ratio is calculated as DFF/dff.

At this point, I would recommend completing the remainder of the model (or providing some out-of-plane bracing to resist the extreme displacements in the global Z direction), and then evaluate your deflections for this gable roof manually by observing nodal displacements along the ridge.  Note, however, that the maximum deflection may actually turn out to be at some distance from the ridge.  This is because the two sloping rafters tend to stiffen the roof at the ridge where they intersect.  Close study of the Beam Relative Displacement Detail table will help you locate the points of maximum displacement and decide if they are significant, or if it is close enough to just read the nodal displacements at the ridge.

I hope this is of some help to you.  Please don't hesitate to let us know if we can help further as your model develops.

Cheers,

Chris

Thank you again for your correspondence. It was quite helpful. I am using 2007.05.15 version and i followed the same steps in pasting the input file. I checked the deflection for each case and found that the frame was failing in z-axis as there was no bracing in that direction. So i modeled a 2 D frame and i got the right result.
• In reply to Rohail Iyaz:

Glad to hear that things worked out for you. A new release of STAAD.Pro is now available, if your SELECT agreement is current. Happy New Year! Chris