EXAMPLE 4

Correcting mistakes in SAP 2000

Solve the problem described in the figure below for the joint action of Εy, Ex

Input data

- Column EZ (both in 1st and 2nd floor) has dimensions 50/30 (X/Y) whereas the

remaining columns have dimensions 20/40 (X/Y)

- Beams have dimensions 20/60 (always the large dimensions refer to beam

height)

- The material will be concrete

5.0 m

x

y

z

A

B

Ex=220KN

4.0 m

3.0

m4.0

m

C

D

Ey=150KN

Z

ET

H

EXAMPLE 4: Correcting mistakes in SAP 2000 54

1. Geometry mistakes

1.1) Unit mistakes

If wrong units have been chosen during data input (e.g. kip-in instead of KN-m)

then correcting is very difficult and the modelling should start from the beginning.

1.2) Wrong element dimensions

The model is based on the Template models of SAP library according to the

following figure:

Figure 4.1. Determination of model geometry

The resulting structure is depicted in Figure 4.2.

Figure 4.2. Structural configuration

EXAMPLE 4: Correcting mistakes in SAP 2000 55

1.2.1) Errors

The resulting structure already has 2 important errors. The first concerns the

floor number which is 3 instead of 2 floors that were required from the problem

statement. The second is the length of the bay along Χ being 4m instead of 5m.

1.2.2) Correction of errors

a) We can always start again the model using the command FileNew model

from Template

β) We can delete the extra floor by deleting the corresponding elements.

Moreover, we can correct the length error if we select all implicated joints having

wrong coordinates (it doesn’t matter if we also select the related Frame elements)

and use the Move command to relocate them to the correct coordinates. In this

specific structure, we can select all joints of the frame located at X=2 and request

Move by 1m in the X direction.

In order to make the frame selection easier we can work in the right window in

2D yz plane at coordinate X=2.

Figure 4.3. Selection of frame at X=2 (yz view on the right)

After the selection of the proper frame it is quite easy to select Move by 1m

along the X axis.

The previous correction resulted in moving the structure along with its center of

gravity, which is not coinciding any more with the coordinate axes. In order to have

EXAMPLE 4: Correcting mistakes in SAP 2000 56

the centre of gravity at the X=0 and Y=0 coordinates we should select the whole

structure and Move along Χ=-0.5m.

Finally, we should also change the grid lines since the 2D views can only follow

the existing grid lines which are not at the same location with the new configuration

of our model. Grid lines of the X direction should be moved to +2.5 (from +2) and

to -2.5 (from -2).

It is obvious that correcting the model geometry is not always very easy,

requiring numerous commands in order to achieve the correct configurations. Since

we are still at the first stages of data input procedure, it might have been more

convenient to start from the beginning at this point. Nevertheless, there are cases

where the aforementioned modifications can not be avoided.

2. Errors in Frame section configuration

2.1) Errors in frame section dimensions

The correct way to determine the frame section dimensions is to use the local

axes configuration described in local axes appendix. If however something is wrong

we can also use some other options to reveal any important errors. Let’s suppose

that frame sections are initially given according to Figure 4.4.

Figure 4.4. Initial configuration of frame sections

EXAMPLE 4: Correcting mistakes in SAP 2000 57

Then each frame element is assigned with the corresponding frame section. In

order to perform frame section assignment in a more efficient way we can work as

follows:

- Using the tool the whole structure is selected and assigned to COLUMN

frame section

- Then activating the right window in XZ plane (the one that includes EZ

column), we select column ΕΖ (on both floors) and assign the COLEZ section

- Finally activating the right window in XY plane we select all beams in both

floors and assign them the BEAM section

In the 3D model depiction (left window) we can use Set elements and Show

Extrusions to obtain a nice visualization of the structure.

Figure 4.5. 3D structural view along with frame section dimensions

EXAMPLE 4: Correcting mistakes in SAP 2000 58

It is obvious that the beam section has been defined erroneously. Moreover, all

columns (except EZ) have wrong orientation since their large dimension should have

been along the Y axis.

2.2) Correcting the frame section dimensions

From DefineFrame Sections the correct dimensions are provided using the

Modify/Show option.

In the local axes notes the correct fields for each one of the related dimensions

are explicitly provided.

Figure 4.6. View of the structure after the corrections

3. Mistakes in load assignments

First 2 load cases ΕΧ and ΕΥ are created.

Also, a load combination is required to obtain the results for the action of both

loads on the structure.

EXAMPLE 4: Correcting mistakes in SAP 2000 59

A new node should be created at the center of the 1st floor using the right

window in XY plane view. The easiest way is to draw a new joint in a random

location using the tool . Then after right–clicking the joint, the proper

coordinates (here X=0 and Y=0) can be provided.

Figure 4.7. Relocation of the joint towards the center of the floor

This joint is also created at the 2nd floor using the Copy και Paste (Z=3)

command.

Afterwards the joint of the first floor is selected and a load in the ΕΥ load case is

assigned (Υ=220) as in Figure 4.8. Obviously, this is a mistake that should be

corrected.

EXAMPLE 4: Correcting mistakes in SAP 2000 60

Figure 4.8. Initial assignment of the load at the first floor

The same joint is selected again and assigned a load in the ΕΧ loading case

(Χ=220) using the option replace existing loads. After right-clicking on the joint, it is

revealed that load exists for both loading cases ΕΧ (load parallel to Χ=220 being in

the first field of Forces) and ΕΥ (load parallel to Υ=220 being the second field of

Forces) Figure 4.9.

Figure 4.9. Joint loads after the first correction effort

Therefore it is clear that the option “replace loads” does not interfere with the

other loading case (it only changes the loading properties of the specific loading

case that it is used).

EXAMPLE 4: Correcting mistakes in SAP 2000 61

In order to finally erase the ΕΥ load of the first floor joint we should again assign

loading in the ΕΥ load case, using zero (0) values for all fields and then use the

option “replace existing loads”. After that the only load of the joint is going to be the

one in the EX load case.

4. Mistakes in diaphragm assignment

The horizontal loads are assigned at the additional joint at the center of each

floor. In order to transfer this load to the rest of the structure it is very important to

include this specific joint when determining the floor diaphragm.

Note: If accidentally 2 diaphragms are given in a single joint then SAP 2000 will

not be able to run the analysis and will come up with an error message (ANALYSIS

INCOMPLETE). In this case, the diaphragm should be deleted and determined again

in the correct way.

EXAMPLE 4: Correcting mistakes in SAP 2000 62

Quick model checking

Materials

Materials take place in the analysis when they are assigned in a specific frame

section in DefineFrame sections where the correct material should be given. The

specific properties of each material can be viewed or changed in DefineMaterials.

Geometry

The dimensions of linear elements can be reviewed after right-clicking on each

specific element. The length of the element can change using the Move command to

relocate the Start or the End joint (or both).

The joint location can be also reviewed or changed after right-clicking on each

joint. Another way to relocate a joint is to use the Move command.

Dimensions of the cross-sections can only change using the command

DefineFrame sections. A quick visual inspection can be performed using the Set

ElementsShow Extrusions ( ) where some errors are revealed if the dimensions

of the cross-section differ substantially in the two directions (it is easy to distinguish

if a 20/80 cross-section is oriented erroneously – yet it is not that easy in the case

of a 35/40 cross-section). In all cases the best way is to always check the Local

axes when determining the cross-section properties.

Degrees of freedom – fixities (restraints) and constraints (diaphragms)

From AnalyzeSet options we should check if all required degrees of freedom

are available. (e.g. for a 3D structure all degrees of freedom should be available).

Joint Restraints are revealed after right-clicking a joint. The number and location

of the restrained field determines the type of fixity for each joint.

The joint constraints (e.g. diaphragms) can be reviewed after right-clicking the

joint.

In the same way, we can see any mass properties (mass degrees of freedom)

provided to a joint and the corresponding direction of movement.

EXAMPLE 4: Correcting mistakes in SAP 2000 63

Loading

Joint loads are easy to review after right-clicking on the joint. All loads are

presented in the corresponding loading case. It is also possible to review the loads

from DisplayShow LoadsJoint.

Loading of linear elements (e.g. beams) can be made visible after selecting each

load case from the command DisplayShow LoadsFrame.

Finally, all combinations should be checked to make sure they are correctly

defined.

Reviewing the results

Deformed shape-Joint displacements

DisplayShow deformed shape (or ) and select the load case (or

combination) for which the deformed shape is required.

Reinstate the initial (undeformed) shape using the command DisplayShow

Undeformed shape (or ).

Moment and shear force diagrams

DisplayShow element forces/stressesFrames (or ) and select the load case

(or combination) for which the diagrams are required.

The choice Fill diaphragm provides a colored diagram whereas the choice Show

Values provides values on the diagram.

In 3d problems it is better to review any diagrams in the 3d window.

Support (fixities) reactions

DisplayShow element forces/stressesJointsReactions (or ) and select the

load case (or combination) for which the support reactions are required.

EXAMPLE 4: Correcting mistakes in SAP 2000 64