Lateral Load Considerations for Shoring/Formwork Supporting a Sloped Concrete Slab

[drakefinx2604]

What lateral loads must be considered when designing lateral bracing for shoring under a sloped concrete slab? For a flat slab, the lateral forces are generally taken as the greater of 100 lb/ft or 2% of the total vertical dead load on the form. What additional lateral forces – if any – should be considered when the slab has a large slope - 60% for example? Any references/guidelines would be greatly appreciated.

 

[jjl317]

Just to confirm, I am taking your question to mean about lateral loads developed during to the concrete placement process.

Vertical concrete formwork pressures are generally developed based on statics with the concrete considered being a liquid (at least initially). For sloped concrete, depending on the angle, and how the formwork is designed, following these same assumptions could be appropriate. However, for conditions with relatively small slopes, and/or with concrete that is relatively stiff, there may be little to no lateral pressure applied to the shoring system.

I worked in the concrete formwork and shoring business early in my career, and I was surprised that this topic didn't seem to be very well defined/developed from the design side. The only reference I am aware (in the US) is ACI SP-4 Formwork For Concrete. I have an older version (6th), which has only a pretty generic sketch showing an inclined shoring condition, with a note to add diagonal bracing to resist the lateral component of the pressure.

Looks like there is a newer version of this reference out there - perhaps they have expanded this topic.

I will be interested to see what other information may turn up on this thread.

 

[SlideRuleEra]

What lateral loads must be considered when designing lateral bracing for shoring under a sloped concrete slab?

If the slab is supported completely by shoring, it's a two-step solution:

Assume the formwork, concrete, and construction surcharge are gravity loads and act through their combined resultant.
This resultant load is supported by a single prop at the loading resultant location.

Step 1) Calculate normal and sliding components of the load.

Step 2) By definition (see above assumption), shoring vertical load magnitude is equal to loading resultant.
To obtain the lateral load coming from the slope, calculate the horizontal component of the (Step 1) sliding load.

This is easier to "do" (trigonometry) than to describe. Here is a worked hypothetical example for 60% slope. I get total lateral load is 66.2 Kips.

The above math works (as it must) from shored horizontal slabs (0 deg. slope) to "shored vertical" slabs (90 deg. slope)... and all points in between.