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.
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
1 0 0 0; 2 30 0 0;
1 1 2;
DEFINE MATERIAL START
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 TABLE ST W21X48
MATERIAL STEEL ALL
2 FIXED BUT FX
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
1 CON Z 6 15
UNIT INCHES KIP
1 CON Y -10 180
UNIT FEET KIP
UNIT INCHES KIP
CODE AISC UNIFIED
FYLD 50 ALL
DFF 400 ALL
CHECK CODE ALL
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.
The value of dff (lowercase letters) is a deflection result that is calculated by "Deflection Length"/Max Deflection.
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.
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.
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).
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.)
3. Click Edit > Copy in the STAAD Editor Menu Bar.
4. Move to the open Forum post form, right click, and choose Paste from the pop-up menu.
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:
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.