How To Raise Or Drop A Beam In Elevation

By default, the top of the beam is always assumed to be atop of the storey in which  the beam is modelled.  To change the elevation of the beam, i.e. to lower or raise it from default, there are 2 methods :

1. Use e-z input in the Beam Properties > General tab  
   
2. Use delZ input in the Beam Properties > 3D tab

Method 1 "e-z" is recommended as it is easy and straightforward to cater for minor change in elevation, e.g. toilet drops.

"e-z" changes the elevation graphically (visual purposes) without affecting the elevation of the analytical wire of the member (analysis purposes). 

Method 2 "delZ" can be used if the change in level is significant to warrant a corresponding change in the analytical model.

"delZ" changes both the elevation graphically and elevation of the analytical wire of the member.

Method 1 :  Use e-z input in the Beam Properties

In the example model below, all the 3 beams in the right hand side are lowered (dropped) by 300mm.  

This is done by accessing the Beam Properties > General tab :

• e-z = -300mm
  
• h = beam depth = 500mm
  

e-z  value must be less than h, depth of the beam. Hence if the beam depth is 500mm, then the maximum absolute value of e-z can only be 499mm.  

If the drop or raise level is equal or more than the depth of the beam, then delZ method must be used (discussed later). 

The slab is lowered by the same amount by entering  Rel. Level = -300mm in the Slab Properties.

1. The analytical wire frame elevation of the lowered beams remains at the same elevation (no change).
   
2. The slab rigid diaphragm (grey lines) is created in the same floor plane and will constrain all the beams including the “lowered” beams as expected.
   
3. This means that all beams cannot shorten, elongate or bend in the plane of the slab and hence there will be no axial force, minor bending or minor shear in all the beams for all load cases.

Method 1 Conclusion

This is the recommended method as it is the easy and straightforward when the change in beam elevation is minor. For this reason, e-z value is restricted to be less than the depth of the beam.  

Method 2 :  Use delZ input in the Beam Properties

Using the same model, the beams can be lowered using the alternative delZ method.

This is done by accessing the Beam Properties > 3D tab :

• delZ -I = -300  (means lower the left end elevation of the beam by 300mm)
  
• delZ -J = -300 (means lower the right end elevation of the beam by 300mm)
  

The slab is similarly lowered by the same amount by entering Rel. Level = -300mm in the Slab Properties. Also, option "Apply Z to Analytical Model" in ticked to ensure the slab diaphragm is dropped analytically. 

Run building analysis and access the Analytical Model to examine the analytical wire-frame and results (as shown below) : 

1. The analytical wire frame elevation of the lowered beam is considered (lowered by 300mm).
   
2. Rigid link (dark blue) auto-created at the drop location, resulting in increase of stiffness at this beam column joint. 
3. Separate slab rigid diaphragms (grey lines) are created because option "Apply Z to Analytical Model" in ticked in slab properties. 
   
4. The slab diaphragm will constrain the beams in the same plane to prevent any axial forces, minor shear or minor bending forces from developing. 

Method 2  Conclusion

At first glance, this method seems to be more accurate. However, a more complex analytical model with rigid links is created, resulting in different structural stiffness. Further, if there are other members joining the lowered beams (e.g. secondary beam), care must be taken to apply the same del z, else the beams will not connect analytically.

It is recommended this method be used if the change in elevation is significant, example equal or more than the depth of the beam (in which case, the above e-z method will not work anyway).