Environmental Occasional Loads

Occasional loads represent short-duration, high-intensity dynamic events—most commonly Wind and Seismic activity.

Rather than requiring the engineer to calculate and manually apply wind pressures to every individual pipe, the engine automatically generates 3D spatial load profiles based on the selected structural design codes.

Dynamic Restraint Behavior

During Occasional load cases, the engine first converges the full non-linear Operating (OPE) state, then performs a "Locked Linear" overlay for the environmental increment: the operating contact state (engaged one-way supports and closed gaps) is frozen, all dynamic snubbers instantly lock and act as rigid supports, and friction is omitted from the overlay — matching the standard commercial treatment of occasional and dynamic load cases. See Boundary Conditions §2.4.

1. Wind Loading

Wind forces are modeled as uniform distributed loads (UDLs) projected onto the exposed area of the pipe and its insulation. The velocity pressure increases non-linearly with the elevation ($z$) of the element.

The engine calculates the wind magnitude and then decomposes the resulting UDL into global $X$, $Y$, and $Z$ vectors based on the user-defined wind attack angle.

ASCE 7 (United States)

For ASCE 7, the engine calculates the velocity pressure ($q_z$) in $N/m^2$ using the standard formulation:

$q_z = 0.613 K_z K_{zt} K_d V^2$

(The topographic factor $K_{zt}$ is taken as $1.0$ and the directionality factor $K_d$ as $0.85$, the ASCE 7 default for round structural shapes.)

The velocity pressure exposure coefficient ($K_z$) is derived dynamically based on the element's centroid elevation ($z$) and the user-selected Exposure Category (B, C, or D):

$K_z = 2.01 \left( \frac{z}{z_g} \right)^{2/\alpha}$

The final wind UDL ($w_{wind}$) applied to the pipe is a function of the pressure, the coated outer diameter ($D_o$), and the cylindrical force coefficient ($C_f = 0.8$):

$w_{wind} = q_z D_o C_f$

AS/NZS 1170.2 (Australia / New Zealand)

For AS 1170.2, the design wind speed ($V_{des}$) incorporates regional multipliers:

$V_{des} = V_R M_d M_{z,cat} M_s M_t$

(The shielding multiplier $M_s$ is taken as $1.0$.)

The terrain/height multiplier ($M_{z,cat}$) is dynamically interpolated from Table 4.1 based on the element's elevation and the selected Terrain Category (1, 2, 3, or 4). The resulting pressure and UDL are calculated as:

$p = 0.5 \rho_{air} V_{des}^2 C_{fig}$

$w_{wind} = p D_o$

(Note: $\rho_{air}$ is taken as $1.2 \text{ kg/m}^3$ and the aerodynamic shape factor $C_{fig}$ for cylinders is taken as $1.2$).

2. Seismic Loading

Seismic events are modeled using the Equivalent Static Force method. The engine extracts the total operating mass ($m_{ope}$) of each element—including steel, coating, fluid, and inline components—and subjects it to a uniform lateral acceleration.

AS 1170.4 Calculation

If the AS 1170.4 code is selected, the engine calculates the seismic coefficient ($C_d$) based on the site hazard factor ($Z$), probability factor ($k_p$), and the site sub-soil class multiplier ($C_h(0)$):

$C_d = k_p Z C_h(0) \left( \frac{a_c}{R_c} \right)$

The resulting seismic UDL ($w_{seis}$) applied along the user-defined earthquake axis is:

$w_{seis} = m_{ope} C_d g$

(If the "Generic" seismic code is selected, the user provides a raw G-factor which is directly multiplied by the element mass and gravity).

3. Valve Actuator Torsion (Advanced)

A critical feature of the FEA engine is its automated handling of center-of-mass offsets during seismic events.

Heavy valve actuators are rarely perfectly centered on the pipe axis. When subjected to a lateral seismic acceleration ($F_{seis}$), this offset mass acts as a lever arm, inducing severe torsional twisting into the pipeline.

If a valve element carries an Actuator Mass ($m_{act}$) with vertical ($d_v$) or transverse ($d_t$) centre-of-gravity offsets, the engine resolves the actuator's seismic inertia force $F_{act} = m_{act}\, C_d\, g$ into the element's local axes and computes the induced torsional moment from the offset lever arms:

$M_{tor} = F_{act,\,vert}\; d_{v} \;-\; F_{act,\,trans}\; d_{t}$

where $F_{act,\,vert}$ and $F_{act,\,trans}$ are the local vertical and transverse components of the actuator inertia force. The moment is injected as a fixed-end torsional couple, split equally between the element's two end nodes. This guarantees that torsion at valve stations is captured without requiring the engineer to model rigid dummy-legs.