Tilt Up Wall Design - Simultaneous in plane and out of plane loads on walls

[Bodgy Engineer]

Hi All,

Designing a Small, single story Tilt up building of mostly solid panels. The walls will be subject to both in plane and out of plane loads.

All design examples i could find seem to ignore this combined load case, they mostly design panels on out of plane loads and axial loads (ACI 14.8). Then just say that in plane loads are small (which is typically true)and design check for in plane shear only and panel overturning.

This is not going to be a problem for my building but it got me thinking. How should the load combination be analysed?

Should the typical wall stress (Pum/ag < 0.06*Fc), also include the in plane bending stress (assuming linear elastic as a rough indicator) and ensure the maximum stress < 0.06*FC. (lowish load cases only)

Computer software also treats the load cases in isolation (Bentley tilt up module software).


The local code (AS 3600) says for braced walls that are subject to simultaneous in and out of plane loads the slab shall be designed as either:

a slab , when in plane bending and axial force is < 0.03*Fc. I believe this assumes the loads are small and just design each case separately.

a column, do not want to do this for a low load.

 

[Aesur]

I assume your in plane loading is lateral loads, ie wind and seismic. If this is the case the controlling in plane loading occurs at a different time than the out of plane loading in the wall. Additionally, consider that out of plane wind loading is components and cladding and not main wind force which is used for lateral design. If you are talking about eccentric loading from the gravity framing, then I would consider combining the lateral design with those dead and live loads.

I see you are in Australia, I am not familiar with the codes and loading used in your local codes, the above information is based on US codes.

 

[EZBuilding]

ASCE 7-16, Figure 27.3.8 Case 3 and 4 are wind cases which would provide both in-plane and out of plane wind loads for a MWFRS configuration - when looking at a building in which the exterior walls are the lateral system. I have had this condition drive reinforcement design for some rectangular load bearing masonry buildings. Bi-axial moment design of your tilt-up panel for these cases are appropriate - even if possibly often not performed uniformly in industry.

 

[Aesur]

Sure, there are load effects where there is loading both in and out of plane at the same time, however in most cases I suspect you would find that if you design out of plane for C&C _+ gravity you won't have issues handling the MWFRS out of plane loading that applies at the same time as the in plane loading for shear walls. If you have time, you could run many load combinations to design for every possible case, however typically budgets nor deadlines allow for designing for every possible case without specialty software similar to that used by PEMB engineers so you have to pick the worst of each case. If you are taking utilization to 100% then maybe you should be checking the combined case.

Figure 27.3-8 reduces the MWFRS forces further for orthogonal effects, I believe 75% or less of the wind load. I suspect if you are designing using C&C for out of plane loading your wall can probably handle the reduced MWFRS loading. Maybe if your building had nothing but openings and a few slivers of concrete left for shear on each panel this could start being an issue.

 

[Aesur]

Please note that this is up to the engineer to make an engineering judgement for their specific project and I am stating why I believe many ignore the combination of in plane and out of plane loading.

 

[JoshPlumSE]

You could certainly do a "first principles" analysis and construct a bi-axial moment + axial force interaction diagram. That would be the most technically correct way to do it and would account for interaction between the two methods of bending.

However, I have a strong suspicion that this would be a good amount of work that would NOT affect the final design much if at all.

Obviously, this is subject to some engineering judgment. Where you will look at the forces that get into each portion of a wall for the difference load cases and compare them to the capacities. If you're really pushing the limit on some of them, it's not a bad idea to do a simple (Strong Direction Demand Capacity Ratio) + (Weak Direction Demand Capacity Ratio) < 1.0.

That's almost always conservative and it is much easier than doing the 3D interaction diagrams or such.

 

[Enhineyero]

Even if you combine the in-plane stress it will not be significant in the load combo, so why worry yourself. I'm assuming this a 1-2 storey industrial/office type of facility with perimeter walls, which is the typical application.