Load cases and structure loads (.ohllc)#

A load case combines a weather case, the wires' condition and load factors. The structure loads are the forces the wires and strings put on the structures at every point where strings hang, in each load case: the starting point of the structure checks.

Load case files (.ohllc)#

format = "ohl-load-cases"
version = 1
name = "Synthetic example load cases"

[[case]]
name = "wind"
description = "high wind, factored"
weather = "high wind"      # a weather case of the model, by name
condition = "initial"      # initial, creep, load or max_sag
vertical = 1.0             # factor on the vertical loads
transverse = 1.4           # factor on the transverse loads
longitudinal = 1.0         # factor on the longitudinal loads
strength = 1.0             # strength (resistance) factor, for the capacity checks
tags = ["ULS"]

Design codes (EN 50341 with a national annex, the Swiss LeV…) supply their combinations and partial factors as such files: a design-code plugin writes them from your parameter file (Design codes). OpenOHL ships no code tables: the factors come from the standard you hold. examples/synthetic-line/load-cases.ohllc only shows the format.

In a project, the load case files are kept in the project like the other libraries (Criteria › Load cases library edits them).

Structure loads#

Every section is solved in each load case with the finite elements (strings swing, each span its own tension), in the load case's condition and weather. At every point of the structure its strings hang on, the load is the force the section puts on the structure: the reaction there reversed (a V: one per point), or at the wire point of a wire fixed to the structure. Points used by several sections add up; sections sharing strings (a TAK) are solved together and their loads counted once. It is the whole bundle's load, and includes the strings' weight.

Each load is given:

• in the line's axes: x east, y north, z up, in N;
• in the structure's axes: transverse (along the structure's local x, to the right of the line looking ahead — orientation_deg), longitudinal (ahead), and vertical (down);
• unfactored, and factored by the load case's vertical, transverse and longitudinal factors.

A section that does not solve in a load case is listed, with why. Its loads are then missing, and every structure it ends at or hangs on (and, for a jumper held by strings, every structure holding it) is incomplete in that case: its loads are those computed — nothing is made up for the missing wire — but they lack that wire's forces. The incomplete structures are listed per case with the sections or jumpers they miss ("T3: incomplete — not checked: S3 not solved in ice"), and their rows on the Structure loads page are grey with the missing wires named.

In the application: Checks › Compute structure loads runs in the background (the model is not changed) and opens the Structure loads page. It shows one load case at a time (factored, or unfactored), a filter by structure, a structure's link to go to it, and Export CSV… for every case.

From the command line:

ohl line line.ohl.json --weather cases.ohlwc --load-cases loads.ohllc --loads-csv loads.csv

The CSV columns: case,structure,point,label,sets,x,y,z,transverse,longitudinal,vertical,factored_transverse,factored_longitudinal,factored_vertical (point: the point's label, empty for the wind on the structure; sets: the wire ends hanging there, separated by spaces).

Wind on the structures themselves: a structure type may give the area the wind blows on, by height band ([[wind.band]], see Structure libraries). In every load case with wind, each band adds a load across the line — wind pressure × drag coefficient × area, on the wind side of the weather case — at the band's mid-height on the structure's centre line, listed without a point ("wind on the structure"), factored by the transverse factor. The capacity checks' load trees are per point and leave these out, and so does the fe check: it takes the member wind on the profiles instead, when the members' sections come from the section library (see Member wind). Height extensions are not added to the bands yet.

Jumpers: the sections carry them at their clamps as Criteria › Calculation options says (an estimate by default — weight, ice and wind — or the jumper's own pull; line file solver.jumper_loads), and a jumper held by helping strings pulls on those strings, so on the structure holding them.

Structure usage (capacity checks)#

With the loads, OpenOHL checks how much of its capacity every structure uses, from its type's [capacity] (see Structure libraries). Two methods, each used when the type gives its data:

• Method 1, allowable spans. The structure's wind span (half the sum of its adjacent spans, the largest over its conductor positions) and weight span (the wires' vertical load at the points they hang on, without the strings, over their vertical load per metre in the load case — for a catenary, the distance between the low points of the adjacent spans, negative under uplift; the largest and the smallest over its conductor positions) against the type's allowable spans at the structure's line angle (the largest over the routes through it). The allowable spans are interpolated between the angles given; a structure turning more than the last angle fails ("not allowed"). Usage: the larger of wind span / allowable and weight span / allowable; a weight span below the minimum (uplift) is 100 % plus the shortfall over the allowable weight span. The strength factor does not apply: allowable spans are the type's design spans. A conductor position takes both sides of the structure whichever way they hang: one wire point carrying the back and ahead spans (a suspension point; a line end has only one side), or a dead-end's back and ahead strain strings — two wire points of the structure joined by a jumper whose two ends both end a section there, or hanging on the same strings (a TAK). A jumper from a branch's end to a point in another section's middle (a tee-off) joins nothing: the branch is a position of its own. Without the jumper, a dead-end's two strings are checked as two line ends.
• Method 2, allowable loads (load trees). At every point the type's load tree gives, the factored loads in the structure's axes against the allowable loads times the load case's strength factor: transverse and longitudinal either way, vertical downwards, uplift upwards (an upward load with no uplift given fails). Usage: the largest ratio of the point, or their sum with interaction = "linear"; the structure's is its worst point's.

Two more checks come from an analysis, when the type names a structure model or a pole (see Structure analysis):

• fe — the type's finite-element model (its members when it is a full design, or model = "….ohlfe") solved with the factored loads at its points (and the member wind on profiles from the section library); usage: the member using most of its capacity.
• pole — the type's pole (pole = "id"): bending moment along the pole from the factored loads and the wind on the pole, P-Delta amplified, against its moment capacity.

A structure's usage is the largest of its checks. A structure without a type, or whose type gives nothing to check against, is listed as not checked.

Incomplete structures. A structure whose loads in a load case lack a section or jumper that did not solve there is incomplete in that case: each of its checks (spans, loads, fe, pole) is computed from the loads there are and marked incomplete — the usage may be too low, so it is never shown as a pass, whatever its number. A structure every wire of which failed has no load at all and is listed all the same.

In the application: on the Structure loads page, Usage shows every structure worst first — for the governing load case of each (or one load case): usage (red over 100 %, orange from 90 %), each method's usage (spans, loads, fe, pole), what governs (e.g. "weight span 640 m of 600 m", "1:2 transverse 31.2 kN of 30.0 kN × 0.9"), the load case, line angle, wind and weight spans — and Export CSV…. An incomplete structure's row is grey and italic: its usage reads not checked (the usage of the loads there are on hover) and "governs" names what is missing, e.g. "not checked: S12 not solved in ice". In the governing view a structure incomplete in any load case is shown so, with those cases. The summary counts the structures not checked because of a wire not solved.

Colour structures by usage (a tick on the Usage view) colours the structures in the plan and 3D views by their usage in the load case chosen there (the governing one: their worst; incomplete in any case is incomplete): green below 90 %, orange from 90 % to 100 %, red over 100 %, a neutral grey-blue for not checked: a wire not solved, dark grey for no capacity to check. A legend sits at the bottom right of both views.

From the command line: the loads run prints the ten structures using most of their capacity (an incomplete one marked "(incomplete)") and lists the incomplete structures per case; --usage-csv usage.csv writes every load case and structure:

ohl line line.ohl.json --weather cases.ohlwc --load-cases loads.ohllc --usage-csv usage.csv

The CSV columns: case,structure,type,line_angle_deg,wind_span,weight_span_min,weight_span_max,spans_usage,loads_usage,fe_usage,pole_usage,usage,governs,notes,incomplete (usages in %). incomplete is empty when the structure's loads in that case are complete, else it names what is missing: incomplete: section S12 not solved (why); jumper J3 not solved (why) — the usages of that row are then those of the loads there are.