An elastic analysis will calculate the internal forces, stresses, deformations an reactions according to the elastic theory (= only the elastic material features (E and ν) will be used).
To start the elastic analysis:
- Click on
or go to the menu ‘Analyse – Elastic analysis’ or press the F9-key.
The following dialogue appears:
Tabpage “Structural”
Analysis
- 1st order analysis or 2nd order analysis
You have to determine if Diamonds should run a 1st or 2nd order calculation.
The difference between the two is explained here.
This article explains which analysis you should use for steel design. And this article explains it for reinforced concrete structures.In case of a 2nd order analysis, you have to enter the Precision for normal forces in bars and the Max. number of second order iterations.
Both determine when the 2nd order analysis finds convergence: the calculation stops when the additional normal force ΔN to the total normal force N reaches the desired accuracy (default value: ΔN/N < 1% for all bars) or when the maximum number of iterations has been reached. If after the maximum number of iterations the desired accuracy is not achieved, Diamonds indicates for which combinations the equilibrium could not be found.
Note: N is generally small in elements primarily subjected to flexion. Any additional normal force due to the second (how small it can be), generates a very big ΔN/N. To avoid this type of anomalies, the criterion described above (ΔN/N < 1%) will not be applied when Δσ=ΔN/A < 1 N/mm². - Max number of iterations for nonlinearities
The word “nonlinearities” refers to non-linear-behaviour of boundary conditions: supports that cannot bear compression/tension, tie rods,…
The “max number of iterations for nonlinearities” refers to the amount of attempts Diamonds has to find a solution so that all the loads are transferred to the foundation, taking all the imposed boundary conditions into account. - Take shear deformation into account
Global imperfections
- Size
Impose the size of the global imperfections. Diamonds uses a default value of 1/200, but you can adjust it depending on what the standard requires. - Direction
You can choose between:- Automatic (default): Diamonds tests 16 directions and keeps the most adverse direction (not the enveloppe of all directions). This option is the most complete calculation, but will also require the most calculating time.
- Av. horizontal: Diamonds applies the global imperfection in the horizontal direction in which the structure bends most.
If the structure does not undergo any horizontal displacement, Diamonds will fall back on the automatic determination of the direction. - Global X and Z: Diamonds applies the global imperfection in the direction of the global X-axis and Z-axis. Diamonds keeps the most adverse direction (not the enveloppe of all directions)
- Global X: Diamonds applies the global imperfection in the direction of the global X-axis.
- Global Z: Diamonds applies the global imperfection in the direction of the global Z-axis.
- Imposed + a rotation angle: you impose the direction in which the global imperfections must be applied. The rotation angle is the angle between the global X and Z axes.
Diamonds applies global imperfections using equivalent horizontal forces (EN 1993-1-1 §5.3.2). A validation example can be found here.
Joints
Connections can be send to PowerConnect to have their resistance and stiffness checked. Once you send a connection from PowerConnect back to Diamonds, the stiffness diagram of the connection will be pasted into Diamonds. The stiffness diagram can be viewed with the button 
. Since the stiffness diagram will be different from your initial assumption regarding the stiffness of the connection, all results will be erased. You’ll have to redo the elastic analyse. Well, using the parameters below, you can indicate how that rigidity diagram should behave.
Connections are classified into 3 categories (rigid, semi-rigid or flexible) depending on the slope of the stiffness diagram (= rico = the value of Sj,ini).
| Non-braced structures | Braced structures |
|---|---|
 |
 |
| Zone 1: rigid when Sj,ini ≥ 25 E Ib / Lb Zone 2: semi-rigid Zone 3: flexible when Sj,ini ≤ 0.5 E Ib / Lb |
Zone 1: rigid when Sj,ini ≥ 8 E Ib / Lb Zone 2: semi-rigid Zone 3: flexible when Sj,ini ≤ 0.5 E Ib / Lb |
If a connection is classified as either rigid or flexibel, you may:
- Either redo the elastic analysis based on perfectly fixed (or flexibel) connection. You may neglect the stiffness diagram completly. This results in a simpler model that calculates quickly.
- Either redo the elastic analysis based on the full stiffness diagram. This results in a more complex calculation and is even somewhat excessive, as Eurocode states that you may assume perfect behaviour in this case.
If a connection is classified as semi-regid, you have to redo the elastic analysis taking the full stiffness diagram into account. Due to the complexity of stiffness diagrams, this often results in iteration problems. To address this, Diamonds includes a number of options for simplifying the stiffness diagram. This example illustrates the different options.
We calculated all the slopes for each part of the diagram, based on the table values on the left.
When you choose one of following options, Diamonds will take the orange dashed stiffness diagram into account, instead of the original red one.
| Full stiffness diagram | |
|---|---|
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|
| Avoid plasticity – limit 30 | Avoid plasticity – limit 350 |
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| Stiffness Sj | Initial stiffness Sj,ini |
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Once Diamonds has complete the elastic analysis, you must verify if you were allowed to calculate with the simpliefied stiffness diagram (in case you did). This article explains it a little more.
Concrete cracking
- The deformations in a concrete section are non-lineair. This article explains how to calculate the cracked deformation in concrete and explains the purpose of these parameters.
Stainless steel
- The deformations in a stainless steel section are non-lineair. This article explains how to calculate the deformation in stainless steel and explains the purpose of these parameters.
Timber
- Take creep into account
If you check this option, the Young’s Modulus of timber E will be reduced by the factor kdef according to EN 1995-1-1 §2.3.2.2. (1). This will lead to larger deformations. More info.
Tabpage “Soil”
This tabpage will only be active when your model contains soil layers.
- Highest foundation level
This is the highest global Y-coordinate to which a soil layer profile has been assigned. This Y-coordinate is automatically determined by Diamonds. - Initial ground level
This is a global Y-coordinate indicating were the ground level is before you start excavating.
This way, Diamonds knows:- which soil layers are situated under the various foundations
- the excavation depth for each foundation, so that its resulting prestress can be taken into account.
The initial ground level must be equal or heigher than the highestest foundation level. Otherwise Diamonds displays a warning and the calculation will not start.
If you had to define the ground level with the button
for exemple for wind loads, then Diamonds will set the Initial ground level = ground level. The input field for the initial ground level will be disabled. - Reference load combination
A table with soil layers is not suitable for FEM-calculations. So the soil layers are translated into functions. Each mesh node in the foundation will have it’s own function, depending on the loads present in that mesh node… That’s why you have to choose a reference combination. The loads in that reference combination will determine the behaviour of the soil function in that mesh node.
Soil functions in Diamonds, can only be trained to do one thing: either understand how compressed soil works, or either understand how soil under tension works.By consequence, if you need the settlement in combinations that contains mainely vertical downward loads, you should take a reference combination that contains mainely vertical loads.
If your model also contains combinations that mainely consist of vertical upward loads (water pressue) then you should redo the calculation a second time, using a reference combination that also contains mainely vertical upward loads.We generally opt for a quasi-permanent load combination SLS QP1 to start with and see how far it gets us.
- Vertical resolution
There is little point in evaluating stress increase in a thin soil layer if the mesh in the foundation slab itself is quite coarse. Therefor the resolution equals the maximum mesh size by default. This way the soil layers will be combined into groups with thickness equal to the maximum mesh size.
We don’t recommend changing this value. - Maximum number of iterations
This is the number of attempts Diamonds has to translate the soil layer profile into a soil function.
In general, 2 to 3 iterations are sufficiant. - Stress tolerance
This option allows Diamonds to stop the soil calculations at a certain depth z for which the specified accuracy (in percent) for ∆p/p is reached:- ∆p is the effective vertical stress increase at depth z as a result of the surface load
- p is the original effective vertical stress at depth z
According to EN 1997-1-1 §6.6.2. (5) and (6) you may use a stress tolerance up to 20%. Soil investigation firms usually use a stress tolerance of 10%.
However this option often causes iteration problems. Eurocode (EN 1997-1-1:2005 §6.6.2) gives two conditions on which the soil calculations may be stopped:
- (6) This depth may normally be taken as the depth at which the effective vertical stress due to the foundation load is 20 % of the effective overburden stress.
- (7) For many cases this depth may also be roughly estimated as 1 to 2 times the foundation width, but may be reduced for lightly-loaded, wider foundation rafts.
So we recommend using EN 1997-1-1:2005 §6.6.2 (7): thus manually increasing the thickness of the bottom soil layer until the thickness of all the soil layers combined equal 1 to 2 times the foundation width. Uncheck the option “the bottom layer extends to inifity” when definign the soil layers. Leave the stress tolerance to 0%.
- Apply removed ground at interface
This option handles how the excavation should be taken into account.
The option is ticked off, then Diamonds assumes that the ground will be removed in different steps, leading to a reduction of stress, under as well as next to the excavation.
The option is ticked on, then Diamonds assumes that all removed ground is taken away at once, leading to a larger prestress at the excavation level.In different steps
All at once
Tabpage “Dynamic”
This tabpage will only be active when the model contains a dynamic or seismic load. It contains the same parameters as the dialogue for modal analysis.
