ETABS Users Manual - Volume Volum e 1
Unstable Unstabl e End Rel Rel eases eases Any combination of end releases may be specified for a frame element provided that the element remains stable. This assures that all load applied to the element is transferred to the rest of the structure. The following sets of releases are unstable, either alone or in combination, and are not permitted. ETABS checks for these conditions when you click the OK button in the Assign Frame Releases dialog box and if unstable releases are specified provides a message telling you this.
14
•
Releasing U1 (axial) at both ends.
•
Releasing U2 (shear force 2, major) at both ends.
•
Releasing U3 (shear force 3, minor) at both ends.
•
Releasing R1 (torsion) at both ends.
•
Releasing R2 (moment 22, minor) at both ends and U3 (shear force 3, minor) at either end.
•
Releasing R3 (moment 33, major) at both ends and U2 (shear force 3, major) at either end.
Frame Frame Rigid Rigid Offset Assignments Assign ments to Line Li ne Objects Objects Note:
Do not confuse rame member end joint offsets with end offsets along the length of the member. They are two entirely separate things.
14 - 24
In ETABS frame section properties are assigned to line objects. However, actual structural members have finite cross sectional dimensions. When two members, such as a beam and column, are connected at a point there is some overlap of the cross sections. In many structures the dimensions of the members are large and the length of the overlap can be a significant fraction of the total length of the frame element. ETABS provides the capa bility of defining rigid end offsets along the length of frame members to account for these finite dimensions of structural elements. See the subsection below titled "Rigid End Offsets Along the Length of Frame Elements" for more information.
Assi Assign gnme ment ntss to to Lin Linee Obje bjects cts
Chapter 14 - The ETABS Assign Menu
Note:
Rigid end off sets along the length of a rame element account for the inite size of beam and column intersections.
When a line object is used to model a frame section the line ob ject is assumed to be located at the centroid of the frame section. Thus when line objects (frame sections) intersect in a model it means that the centroids of the associated frame objects intersect. In a real structure this is not always the case. For example, it is not unusual for one or more floor beams in a building to frame eccentrically into a column. ETABS provides the capability of defining rigid frame end joint offsets to account for these eccentric connections. See the subsection below titled "Rigid Frame End Joint Offsets" for more information. Use the Assign menu > Frame/Line > Frame Rigid Offsets command to open the Assign Frame End Offsets dialog box where you can define both rigid end offsets along the length of frame elements and rigid frame end joint offsets. Be careful that you do not get these two types of rigid offsets confused. Any end offset assigned to a line object is ignored unless the line object also has a frame section assigned to it.
Rigid End Offsets Along the Length of Frame Elements Note:
The rigid zone actor for end offsets along the length of a rame element only applies to bending and shear deformations. It does not apply to axial and tor sional deformations.
Rigid end offsets along the length of frame members are defined in the End Offset Along Length area of the Assign Frame End Offsets dialog box. Use the Assign menu > Frame/Line > Frame Rigid Offsets command to open this dialog box. In the End Offset Along Length area you have the choice of having ETABS determine the end offset lengths automatically or specifying them yourself. You also can specify the rigid-zone factor. These items are described below.
Automatically Calculated End Offset Lengths ETABS automatically calculates offset lengths for beam and column-type frame elements. It assumes the offset length for all brace-type frame elements to be zero. (You can define your own non-zero offset lengths for brace elements if necessary.) Also, the dimensions of brace elements that frame into the ends of column and beam elements are not considered when calculating the end offset dimension for a column or a beam.
Assi gnments to Li ne Objects
14 - 25
14
ETABS Users Manual - Volume 1 When ETABS automatically calculates the end offsets along the length of a beam it bases the end offset length at an end of the beam on the maximum section dimensions of all columns that connect to that end of the beam. Similarly, when ETABS automatically calculates the end offsets along the length of a column it bases the end offset length at an end of the column on the maximum section dimensions of all beams that connect to that end of the column. Note:
14
ETABS outputs orces at the inside face of end offsets along the length of the member.
Note the following about ETABS automatically calculated end offsets along the length of frame members: •
When more than one beam frames into a column ETABS bases the end offset in the column on the deeper beam.
•
End offsets in beams are controlled by the size of the column below. The column above is not considered.
End Offset Properties and the Rigid-Zone Factor The rigid-zone factor specifies the fraction of each end offset assumed to be rigid for bending and shear deformations. When a fraction of the end offset is specified rigid the outside portion of the end offset is assumed rigid, that is, the portion at the end of the frame member. By default ETABS assumes the rigid end factor to be zero, that is, the end offsets are fully flexible and they have the same frame section properties as is assigned to the rest of the member. The rigid zones of the end offsets never affect axial and torsional deformations. The full element length is always assumed to be flexible for these deformations. Output forces for the end of a frame member are provided at the inside face of the end offset along the length of the member. No output forces are produced within the end offset.
14 - 26
Assignments to Line Objects
Chapter 14 - The ETABS Assign Menu
F i gur e 14-4:
xample rigid end oint offsets
Column Y X
14 Edge of slab
Spandrel beam
Plan Rigid Frame Joint Offsets Note:
When you specify member end joint offsets the local axes of the member are always based on the inal position of the member after the end oint offsets are applied.
Rigid frame joint offsets are defined in the Frame Joint Offset area of the Assign Frame End Offsets dialog box. Use the Assign menu > Frame/Line > Frame Rigid Offsets command to open this dialog box. In the Frame Joint Offset area you specify the global X, Y and Z joint offsets at each end point of the frame element. This feature is useful for modeling beams and columns when the beams do not frame into the center of the column. Frame mem ber joint offsets are always fully rigid. The floor plan shown in Figure 14-4 illustrates a concrete beam and slab system with such a condition. Note that all of the spandrel beams frame into the edge of the column, not the column center line. This circumstance can be modeled in ETABS by providing a joint offset to the top (j-end) and bottom (i-end) of each column in either the global X direction, global Y direction, or both directions depending on how the column is oriented.
Assi gnments to Li ne Objects
14 - 27
ETABS Users Manual - Volume 1
j
When you specify member joint offsets the loI mportant Note:
i j a) Beam with Joint Offset
cal axes of the member are always based on the final position of the member after the joint offsets are applied. Similarly, the location of loads assigned to the line object are based on the final length and location of the member after the joint offsets are ap plied.
i
j 1
Consider the example sketch shown to the left. Sketch a shows a 3 plan view of a beam that has the j-end joint offset. The end joint b) Original Position of Beam is offset such that the beam extends from i to j' rather than from i to j. j
i
14
1
3 c) Final Position of Beam
Sketch b shows the local axes for the beam when it is in its original position without the joint offset. Sketch c shows the local axes for the beam when it is in its final position with the joint offset. In both sketches b and c the local 2-axis points upward and thus does not show in the plan view sketches. ETABS bases the local axes of the beam on those shown in sketch c.
Frame Output Station Assignments to Line Objects Note:
When frame output stations are assigned to a line object a text value is displayed on the line object. If the text value is reported in arenthesis then it is the minimum number of output stations. If it is not reported in arenthesis then it is the maximum spacing between output stations.
14 - 28
Frame output stations are designated locations along a frame element. They are used as locations to report output forces, perform design and plotting points used for graphic display of force diagrams. When force diagrams are plotted, exact forces are plotted at each output station and then these points are connected by straight lines. Output stations occur at user-specified locations I mportant note: and at point load locations along a beam. Use the Assign menu > Frame/Line > Frame Output Stations command to designate the output stations for a frame element. Two options are available for defining output stations for a beam: •
Specify the minimum number of output stations along the beam: In this case ETABS will first equally space the specified number of stations within the clear length of the beam. Then a station is added for each point load that does not fall at one of the previously defined output station locations.
Assignments to Line Objects
