# Rigid Diaphragm Consequences
An unintended consequence of using the RigidDiaphragm constraint (or other constraint) on a beam column element is a result of the fact that it enforces a condition of zero axial strain on the element. For sections where the neutral axis does not shift as a consequence of bending in the beam this constraint has little effect on the axial load in the beam, e.g. elastic or steel sections. For those beams where the neutral axis does shift, i.e. Reinforced Concrete beams modeled using fiber sections under moment, the moments in an unconstrained beam will cause the beam to undergo axial deformation. To constrain this axial deformation requires axial forces, the magnitude of which can be significant.
Here is a stupid made up example that illustrates my point:
```tcl foreach rigidConstraint {no yes} {
puts "RIGID CONSTRAINT: $rigidConstraint"
foreach matType {steel concrete} { foreach eleType {forceBeamColumn dispBeamColumn} {
wipe
model basic -ndm 2 -ndf 3
set width 360 set height 144
node 1 0.0 0.0 node 2 $width 0.0 node 3 0.0 $height node 4 $width $height
fix 1 1 1 1 fix 2 1 1 1
if {$rigidConstraint == "yes"} { equalDOF 3 4 1 }
if {$matType == "concrete"} {
uniaxialMaterial Concrete01 1 -6.0 -0.004 -5.0 -0.014
uniaxialMaterial Concrete01 2 -5.0 -0.002 0.0 -0.006
set fy 60.0; # Yield stress set E 30000.0; # Young's modulus
uniaxialMaterial Steel01 3 $fy $E 0.01
} else {
set fy 60.0; # Yield stress set E 30000.0; # Young's modulus
uniaxialMaterial Steel01 1 $fy $E 0.01 uniaxialMaterial Steel01 2 $fy $E 0.01 uniaxialMaterial Steel01 3 $fy $E 0.01 }
set colWidth 15 set colDepth 24
set cover 1.5 set As 0.60; # area of no. 7 bars
set y1 [expr $colDepth/2.0] set z1 [expr $colWidth/2.0]
section Fiber 1 {
patch rect 1 10 1 [expr $cover-$y1] [expr $cover-$z1] [expr $y1-$cover] [expr $z1-$cover]
patch rect 2 10 1 [expr -$y1] [expr $z1-$cover] $y1 $z1 patch rect 2 10 1 [expr -$y1] [expr -$z1] $y1 [expr $cover-$z1] patch rect 2 2 1 [expr -$y1] [expr $cover-$z1] [expr $cover-$y1] [expr $z1-$cover] patch rect 2 2 1 [expr $y1-$cover] [expr $cover-$z1] $y1 [expr $z1-$cover]
layer straight 3 3 $As [expr $y1-$cover] [expr $z1-$cover] [expr $y1-$cover] [expr $cover-$z1] layer straight 3 2 $As 0.0 [expr $z1-$cover] 0.0 [expr $cover-$z1] layer straight 3 3 $As [expr $cover-$y1] [expr $z1-$cover] [expr $cover-$y1] [expr $cover-$z1] }
geomTransf Linear 1 set np 5
element $eleType 1 1 3 $np 1 1 element $eleType 2 2 4 $np 1 1 element $eleType 3 3 4 $np 1 1
set P 180; # 10% of axial capacity of columns
pattern Plain 1 "Linear" {
load 3 0.0 [expr -$P] 0.0 load 4 0.0 [expr -$P] 0.0 }
system BandGeneral constraints Transformation numberer RCM test NormDispIncr 1.0e-12 10 3 algorithm Newton integrator LoadControl 0.1 analysis Static
analyze 10
loadConst -time 0.0
pattern Plain 2 "Linear" { load 3 1.0 0.0 0.0 load 4 1.0 0.0 0.0 }
integrator DisplacementControl 3 1 0.1 analyze 10 set strains [eleResponse 3 basicDeformation] set forces [eleResponse 3 forces] puts "eleType: $eleType matType $matType axialForce [lindex $forces 0] axialDeformation: [lindex $strains 0]" } } } ```
and the results when this script is run are:
```tcl RIGID CONSTRAINT: no eleType: forceBeamColumn matType steel axialForce -0.08875612010504939364 axialDeformation: 0.00000291960921383616 eleType: dispBeamColumn matType steel axialForce -0.00000000000010219973 axialDeformation: 0.00000000000000000000 eleType: forceBeamColumn matType concrete axialForce 1.25405861627505310629 axialDeformation: 0.12918648354852413362 eleType: dispBeamColumn matType concrete axialForce 3.61866611884458500015 axialDeformation: 0.17247077365271457072 RIGID CONSTRAINT: yes eleType: forceBeamColumn matType steel axialForce -0.00000000000010287569 axialDeformation: 0.00000000000000000000 eleType: dispBeamColumn matType steel axialForce -0.00000000000013310834 axialDeformation: 0.00000000000000000000 eleType: forceBeamColumn matType concrete axialForce 186.25340185913745472135 axialDeformation: 0.00000000000000000000 eleType: dispBeamColumn matType concrete axialForce 163.64106186688189836786 axialDeformation: 0.00000000000000000000
```