Sloping hollow core or one way slabs in ETABS

[ranawaseem]

how can we model one way slab which is sloping too....We model slabs as SHELL when in slope but how can we make it one way too?

 

[johnbridge231]

I think (not absolutely sure) that you cannot do that. One way feature can only be added to membranes only. On the other hand you can try setting small modifiers to the bending properties of the area element m11, m22 and m12, choose then "One way load distribution" and see whether it works..

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[stressed]

ranawaseem, when you define your slab, checkbox the "special one-way load distribution" option. Alternatively, you can select the sloped slabs and assign an automesh or use Edit>Mesh areas and mesh something like 12 X 1 to create long strips in the direction of your one-way load. As johnbridge points out, you can specify modifiers to the slab property to reduce or eliminate bending (or axial) stiffness.

 

[MrHershey]

For slopes I usually model a flat slab and then select the corner points that need to be dropped and either align (edit>align points/lines/edges) the points to the desired elevation or move them (edit>move points/lines/areas).

Or, alternatively, if you already have corner points at the right location you can draw your shell object in 3d mode. ETABS will treat the shell as a ramp instead of a floor though.

A couple points:
- I found in ETABS you could not flip or rotate the local axes for ramps. You'll need to either pay attention to where your local axes are for each elements when applying loads (if you don't just specify gravity loads) or wind pressure coefficients.
- You cannot apply diaphragms to ramps in ETABS. This means that if you have a sloped roof, then your auto-wind loads will not account for the wind area at the roof level. If you have a one-story building, you will end up with no wind load at all if your roof is sloped.

 

[stressed]

Mark is correct if you are using diaphragm auto-wind loads on sloped roofs and he's also right that ETABS for some reason doesn't let you flip the local area axis for ramps. However, wind load generation from extents of diaphragm he describes is not the only way, or best way in many cases to handle wind load. ETABS allows you to assign wind load coefficients Cp to area elements (either finite elements or "null" none property areas) including sloped roofs using whatever coefficient is required. The user will have to determine the wind pressure coefficient since ETABS will not automatically check roof angles. Under wind load case lateral load definition, activate the "exposure from area objects" option.