Node restraint data

The node restraint data is shown in the node properties data panel.

Node restraints are used to model the structure’s supports. They are sometimes referred to as boundary conditions.

Unrestrained nodes are generally free to move along or about any axis direction, however practical structures must be restrained to a footing in some way, otherwise instabilities would occur.

Nodes can be restrained about one or all of their six degrees of freedom and such a restraint may take the form of a fixed restraint or a flexible restraint. If a degree of freedom is given a flexible restraint then a spring stiffness must also be input. Fixing a degree of freedom has the effect of immobilizing that node movement, while specifying a flexible restraint causes the node movement to be a function of the spring stiffness.

Node restraints are specified by a six character code corresponding to restraints along X, Y and Z and about X, Y and Z respectively. "F" represents fixed, "R" represents released and "S" represents spring (or flexible). "D" restraints are no longer supported and "F" should be used instead.

For example, a pin-based support that prevents all translations but allows the node to rotate about X, Y or Z would have a restraint code of FFFRRR. Alternatively, a roller support that allows the node to move in the X direction only and rotate about X, Y or Z would have a restraint code of RFFRRR. A fully built-in (encastre) support would have a restraint code of FFFFFF. A restraint that prevents movement in the Z direction while allowing all other movements and rotations would have a restraint code of RRFRRR.

! IMPORTANT NOTE !

Member end fixities should not be confused with node restraints. Member end fixities specify how members are connected to their end nodes, while node restraints specify how nodes are connected to the footings or other supports. Note that completely rigid frame members should have member end fixities of "FFFFFF" regardless of whether the frame has pin based supports or not.

General restraint

The general restraint facility allows you to apply a restraint to all otherwise unrestrained nodes.

For example, if you have a frame with two pin based supports and you want to prevent all translations in the Z direction for all of its other nodes, you could apply restraints of FFFRRR to the two support nodes and specify a general restraint of RRFRRR.

In order to input a general restraint, you simply apply the desired restraint to any unrestrained node and then tick the "General" box (or select "Yes" in the General Restraint column if you are using a datasheet).

Using a general restraint saves data entry time and reduces the quantity of printed output. Note that output reports only show the general restraint code on one node, even though the analysis has assumed that it applies to all unrestrained nodes.

! IMPORTANT NOTE !

The general restraint facility should be used with great care and only if you are absolutely sure of the effect it has on your model! If you apply a general restraint early in the development of your model and then forget that it exists at some later stage when it is no longer appropriate, you could be over-restraining your model. This could happen if nodes are added that shouldn’t get the general restraint. It could also happen if you initially use a general restraint to prevent all out-of-plane movements in a 2D frame for example and then extend the frame to 3D and forget to remove the general restraint.

X, Y and Z axial stiffnesses

Axial spring stiffness for degrees of freedom restrained with "S". Axial spring stiffnesses must always be greater than zero.

When modelling the elastic properties of soil as a spring support, the spring stiffness is based on the modulus of subgrade reaction of the soil. This is a notoriously difficult parameter to get an accurate figure for. The following typical values of the modulus of subgrade reaction (to be used as a guide) are extracted from J. E. Bowles, "Foundation analysis and design", McGraw Hill 4th Edition, 1988.

Soil Type	Modulus of Subgrade Reaction
Loose sand:	4800 - 16000 kN/m3
Medium dense sand:	9600 – 80000 kN/m3
Dense sand:	64000 – 128000 kN/m3
Clayey medium dense sand:	32000 – 80000 kN/m3
Silty medium dense sand:	24000 – 48000 kN/m3
Clayey soil with qu < 200 kPa:	12000 – 24000 kN/m3
Clayey soil with qu in range 200 to 400 kPa:	24000 – 48000 kN/m3
Clayey soil with qu > 800 kPa:	> 48000 kN/m3

The spring stiffness to be input into SPACE GASS is simply equal to the modulus of subgrade reaction multiplied by the area of the footing that the spring is modelling. For example, if you have a 600mm wide strip footing supported on soil with a modulus of subgrade reaction of 80000 kN/m3 and the soil is modelled as springs spaced 500mm apart, the axial stiffness of each spring would be 80000 x 0.600 x 0.500 = 24000 kN/m. Units for the spring stiffness are shown in the headings of the node restraints datasheet.

X, Y and Z rotational stiffnesses

Rotational spring stiffness spring stiffnesses for degrees of freedom restrained with "S". Rotational spring stiffnesses must always be greater than zero.

Important note about restraining 2D frames

It is common practice amongst some engineers to restrain all out-of-plane movements in 2D frames. While this is generally appropriate for static analyses (provided there are no out-of-plane loads), it may not be appropriate for buckling and dynamic frequency analyses. This is because the frame may buckle or vibrate in an out-of-plane direction even though there are no loads in that direction. Of course, nodes that are braced in the out-of-plane direction should be restrained in that direction, however nodes that can move out-of-plane in the real structure should not be restrained in that direction in the model. Failure to do this could affect the buckling load factors, effective lengths and dynamic natural frequencies and mode shapes, and could result in unsafe designs.

For example, if a 2D frame rafter is sub-divided, the intermediate nodes should not be restrained in the out-of-plane direction unless they are braced in that direction in the real structure. Restraining them would prevent any out-of-plane buckling or vibration modes that could occur if the rafter member hadn’t been sub-divided.

Another example is a pin support for a 2D XY-plane frame column base which could be modelled with the standard 2D pin base restraint code of FFFFFR, however this would prevent rotations about the global X-axis. In reality, a column pin support would probably allow rotations about both horizontal axes and hence a restraint code of FFFRFR would be more appropriate. Restraining the rotation about the X-axis would affect the out-of-plane buckling and vibration modes of the column and could result in incorrect results.

The general rule to follow is that if a node is free to move or rotate in the real structure then it should not be restrained in that direction in the model. Be careful with the general restraint, as it is applied to all nodes that don’t have their own restraint, and for some nodes this may not be appropriate.

$image\ebx_-1773499217.gif$ If you have applied a general restraint and require some nodes to not have a restraint at all, you can prevent them from getting the general restraint by restraining them with a code of RRRRRR.