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.
STAAD SPACE 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 LOAD 1 LOADTYPE None TITLE LOAD CASE 1 MEMBER LOAD 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.
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.