Modification factors - origin of the default values for cracked and uncracked concrete properties

Environment

British Standard

Eurocode

Question

Where do the default values for Modification factors for cracked and uncracked properties of concrete originate?

Answer

1. Modification Factors

Analyze ribbon > Options > Modification Factors > Concrete allows you to define the factors for stiffness adjustments in the different analysis types performed.

These factors are applied to the various element types following the "Assume cracked" setting of the entity properties. The factors reduce the full (short-term) stiffnesses of members to reflect their likely long-term stiffness, making an allowance for long-term effects such as creep, cracking, and shrinkage. The general effect is to amplify the deflections.

1.1 Slabs & Beams

Various general guidance exists in design guides and documents for what the stiffness reduction values may be, generally giving a range of values - typically a range of 0.16-0.25. A value of 0.2 (for cracked sections) has been selected as the default for beams and flat slabs, as this sits within these ranges and is generally considered to be 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, the values are deliberately editable so you may set them to whatever values you consider appropriate following your engineering judgement.

Below are some of the more commonly available publications with guidance in this area:

“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 i.e. they are reduced to just 5% of the full short-term stiffness.

The reason for this high reduction relates to the idealized behaviour of slab on beam systems. Traditionally, engineers expect the slabs to decompose their loads directly to beams, with the beams being designed following this assumption. When slabs are meshed in the load decomposition analysis, their (out of plane) stiffness is necessarily included. If the slabs and beams had similar stiffnesses, some of the slab load could bypass the beams and span directly to supporting columns and walls. By making the Slab On Beams much less stiff than the beams, via the relatively much lower modification factor, the load decomposition tends towards the idealized behavior.

1.2 Walls & Columns

Both the Eurocode and British Standard codes lack guidance in this area. However, the American ACI code provides more guidance and recommends doubling the factor for uncracked sections. The default values follow this logic for both walls and columns.

In the absence of definitive guidance we have followed suit for the default values for all region codes, all of which can be changed as you require.

2. Engineering Decision

Ultimately the values should only be considered sensible initial defaults which we advise the engineer 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 for entities which are modeled with 1D elements are there explicit area and inertia values, since 1D elements follow beam theory.

For entities modeled with 2D elements on the other hand (meshed walls and slabs) there are no explicit section properties, since 2D elements follow plate theory and its governing principles for a 2D elastic continuum. Hence the only way to alter the stiffness of these entities is via the material properties E and G and/ or the thickness t that together govern their intrinsic stiffness.