Design checks

Design checks

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As a default, PREFAB runs checks for plastic limits, or design checks for shell elements of concrete and steel trusses. The design checks are run on import and vary depending on the chosen calculation method. Here's an example of the utilization check on import:

Here's a quick overview of the design checks:

Walls:

• Compression – minimum of Eurocode (concrete) or plastic limit on edge
• Tension – plastic limit on edge
• Shear – minimum of Eurocode (concrete) or plastic limit on edge
• Sliding – combined shear and compression check

Trusses, Based on FEM-Design:

• Tension of steel – minimum of Eurocode or plastic limit
• Compression of steel – minimum of Eurocode or plastic limit

Trusses, New optimized plastic redistribution:

• Tension and shear of steel – combined check with minimum of Eurocode or plastic limit

Whether the design checks are included in the PDF print can be specified in the settings.

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Wall design checks

Plastic limits

As a rule, PREFAB will always compare the connection forces to the plastic limits defined in the edge-connection properties. If a stiffness value is set to 0, PREFAB sees it as a plastic limit of 0.

For the edge-connection shown below, PREFAB will check the shear $V_{PL}$ (Kx', both directions) against $500\,\mathrm{kN/m}$, the compression $C_{PL}$ (Ky' - Compression) against $1600\,\mathrm{kN/m}$ and the tension limit $T_{PL}$ (Ky' - Tension) against $0\,\mathrm{kN/m}$, since the stiffness is set to 0.

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This isn't connected to the material used, and it only checks this for Kx' and Ky'.

Concrete wall specific checks

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PREFAB supports the National Annex for Norway, Denmark and Sweden. The Annex chosen in FEM-Design is the one used by PREFAB.

Tension

If the Ky' tension stiffness is 0, no tension is allowed, and PREFAB shows a $1000\,\mathrm{\%}$ utilization.

If the Ky' tension stiffness is greater than 0, tension is allowed and there's no concrete check. If a plastic limit is applied, the utilization is:

$$u = \frac{T_{Ed}}{T_{PL}}$$

Compression

The compression capacity is calculated as:

$$C_{Rd} = \frac{t \cdot \alpha_{cc} \cdot f_{ck}}{\gamma_c}$$

where $t$ is the thickness set in FEM-Design, $f_{ck}$ is from the material set in FEM-Design, $\gamma_c$ is from the material's U | Ua, Us setting in FEM-Design and $\alpha_{cc}$ is from the material setting in FEM-Design.

The utilization is calculated as:

$$u = \frac{C_{Ed}}{\min(C_{Rd}, C_{PL})}$$

Shear of concrete

The shear capacity of concrete is calculated as:

$$V_{Rd} = t \cdot 0.5 \cdot \nu \cdot \frac{f_{ck}}{\gamma_c}$$

where $t$ is the thickness set in FEM-Design, $f_{ck}$ is from the material set in FEM-Design, $\gamma_c$ is from the material's U | Ua, Us setting in FEM-Design and $\nu$ is an effectiveness factor that depends on the National Annex.

For DK:NA:

$$\nu = \max\left[\left(0.7 - \frac{f_{ck}}{200}\right), 0.45\right]$$

For all other supported Annexes:

$$\nu = 0.6 \cdot \left(1 - \frac{f_{ck}}{250}\right)$$

The utilization is calculated as:

$$u = \frac{V_{Ed}}{\min(V_{Rd}, V_{PL})}$$

Sliding

When Detach is activated on an edge, a combined shear and compression check is activated to check for sliding. The compression and shear of concrete are each checked individually as explained above.

The utilization is calculated as:

$$u = \frac{V_{Ed}}{\mu \cdot C_{Ed}}$$

where $\mu$ is the friction factor from the edge set in FEM-Design.

In case $u > 100\,\mathrm{\%}$, PREFAB will tell you the necessary mechanical anchorage needed to keep it from sliding.

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Truss design checks

Plastic limits

As a rule, PREFAB will always compare the normal forces to the plastic limits defined in the truss properties. For the truss shown below, PREFAB reads the plastic limits as a compression $C_{PL}$ of $5\,\mathrm{kN}$ and a tension $T_{PL}$ of $100\,\mathrm{kN}$.

Steel truss specific design checks

General information

When PREFAB gets the yield capacity of the truss element, it takes the smallest value defined for the material in FEM-Design. You can create a new material and specify the desired yield capacity in all the relevant fields.

For the design value, PREFAB reads the maximum $\gamma_s$ of the connected shell elements.

Based on FEM-Design: Tension/Compression of steel

The tension capacity is calculated as:

$$T_{Rd} = \frac{f_{yk}}{\gamma_s} \cdot A_s$$

and the utilization as:

$$u = \frac{T_{Ed}}{\min(T_{Rd}, T_{PL})}$$

The compression capacity is calculated as:

$$C_{Rd} = \frac{0.8 \cdot f_{yk}}{\gamma_s} \cdot A_s$$

and the utilization as:

$$u = \frac{C_{Ed}}{\min(C_{Rd}, C_{PL})}$$

New optimized redistribution: Tension and shear of steel

For the optimized redistribution, shear transfer is used if the element can't utilize the friction from compression between the elements alone. In this case, a combined tension and shear check is performed.

The utilization is calculated as:

$$u = \frac{T_{Ed}}{\min(T_{Rd}, T_{PL})} + \frac{V_{Ed}}{\mu \cdot \min(T_{Rd}, T_{PL})}$$