Where do the default values for cracked and uncracked properties of concrete come from?

Question

Where do the default values for cracked and uncracked properties of concrete come from?

Answer

1. Modification Factors

Analyse ribbon > Options > Modification Factors > Concrete allows you to define the factors for use in the different analysis types performed.

 

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These factors are applied to the various element types, based on whether the section is specified as cracked or not in their member properties. The factors reduce the full short-term stiffnesses of the members to their long-term values, which allows for long-term effects, such as creep, cracking, and shrinkage, and will generally have the effect of amplifying the deflections.

 

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1.1 Slabs

There are various general guidance available from design guides and documents as to what these values should be set to, but these generally provide a range of values - typically a range of 0.16-0.25. We have selected a value of 0.2 (for cracked sections) as the default, as this sits within these ranges and is generally considered to be quite reasonable. The 0.25 value is considered by some to be slightly unconservative for the office situation, so 0.2 seems more reasonable here, with 0.16 typically used for storage. However, if you want to set it to something else, then that is your engineering decision.

As mentioned above, there are various guidance documents available, but below are some more commonly available ones:

• “How to design reinforced concrete flat slabs using finite element analysis” by O Brooker  The Concrete Centre
• Technical Report (TR) 58
• CIRIA Report 110 (2nd edition)

1.1.1 Slabs on Beams

The modification factors for Slab on beams are set to 0.05 by default. This means the stiffness of these slabs is reduced to just 5% of the full short-term stiffness, in comparison to the Cracked Beam Modification Factors being set to 0.2 making the beam more stiff and so it deflects less under the same loading.

The reason the Slab on Beams Modification Factors are smaller than all other concrete elements comes down to how the beams should be designed. Traditionally, engineers would probably expect the slabs to decompose their loads into the beams first, and the beams would be designed effectively assuming they’re taking all the load from the slabs. When slabs are meshed in the 3D Analysis, the slabs and their stiffnesses are considered in the analysis. If the slabs and beams effectively have the same stiffness, then the slabs could bypass the beams and take some of their load directly to the columns and walls, so the beams would be designed for less load. By making the Modification Factors for the Slab On Beams lower, it makes them less stiff, forcing them to decompose their loads into the beams first, rather than going straight to the columns and walls.

1.2 Walls

Both the Eurocode and British Standard codes lack guidance in this area, however, the American codes provide more information and recommend doubling the factor for uncracked sections.

In the lack of any further guidance we have followed suit, but obviously, you can change the value if you wish.

 

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2. Engineering Decision

Ultimately the values should only be considered sensible initial defaults which we advise the engineer to always review and either accept or edit following their judgment.

2.1 Why can I only change the E, G, and t factors for Meshed Walls and Slab items?
 

Only 1D elements do we have an explicit inertia value because they follow beam theory.
2D elements on the other hand (meshed walls and slabs) do not have defined section properties and follow plate theory and its governing principles for a 2D elastic continuum.
 
Therefore, there are no section properties so the only way to alter their stiffness is via the material properties that govern this - E and G