Structural Configuration of the Bridge Model Assessment

3.1 Model description

The bridge has a span of 100 m and a deck and width of 10 m. The main tie is UB 914 * 419 * 388 in twin arrangement along the span. The arch rib is CHS 610 * 20 with an arc length of about 107.5 m and an arch base inclination of about 75 degrees. The deck includes a cast in place concrete slab 0.5 m thick across the full width. There are 17 cables on each side of the arch, 34 in total. Cables are currently modelled as CHS 114.3 * 8 as a placeholder and will be replaced by cable elements with high strength wire specification in the next stage. Cross girders are UB 610 * 229 * 140 with transverse beams and X bracing. End anchorage is provided by tower structures that are 20 m high with 5 m embedded below ground to simulate foundation restraint.

To keep height clearance over traffic, the first cable from each end is about 1.5 m and the second cable is about 4.1 m. The transverse top strut that links the two arches starts from the third cable line so the clearance zone near the ends stays open. To stabilise the spacing of the two arches in this region, cross stabilising cables are arranged in an X layout from the top of each anchorage tower to the head of the second cable on the opposite arch.

 3.2 Materials and sections

Global steel is structural grade. Primary members are intended to match a grade equivalent to S355 for the arch rib and the main tie, and a grade equivalent to S350 for cross girders and bracing. The deck is normal weight concrete 0.5 m thick. Cables will be specified as high strength parallel wire or locked coil with tensile strength in the range 1860 to 1960 MPa. The current CHS placeholders for cables will be replaced with equivalent axial stiffness and with tension only behaviour.

3.3 Loading and analysis settings

Self-weight is applied to all members including the 0.5 m concrete deck. A representative live load on the deck will be added in the next set of runs, together with a one-sided live load to force an asymmetric response. Large deflection is used in the static analysis to capture second order effects. An eigenvalue buckling check is run to screen global stability. Cables are assigned initial pretension so that all main cables remain in tension for service combinations.

Brief Summary of Assessment Requirements

You are to set up and document a preliminary structural model of a 100 m-span tied-arch bridge with a 10 m-wide deck, covering three sections:

3.1 Model Description — structural configuration

• Twin UB 914×419×388 main ties along the span.

• CHS 610×20 arch rib (≈107.5 m arc length; ≈75° base inclination).

• Concrete deck slab 0.5 m thick (full width).

• 17 stay cables per side (34 total); currently CHS 114.3×8 placeholders, to be replaced with high-strength cable elements at next stage.

• UB 610×229×140 cross girders with transverse beams and X-bracing.

• End anchorage towers 20 m high (5 m embedded) to simulate foundation restraint.

• Clearance measures: first cable ≈1.5 m, second ≈4.1 m above traffic; transverse top strut begins at the third cable line; near-end arch spacing stabilised by cross stabilising cables (X-layout) between towers and opposite second-cable heads.

3.2 Materials and Sections — property specifications

• Structural steel overall; S355-equivalent for arch rib & main tie; S350-equivalent for cross girders & bracing.

• Deck: normal-weight concrete, 0.5 m.

• Cables: high-strength parallel wire/locked coil, 1860–1960 MPa; modelled with tension-only behaviour and axial stiffness equivalent to design.

3.3 Loading & Analysis Settings — analysis scope

• Apply self-weight to all members (incl. concrete deck).

• Add representative live load and one-sided live load (for asymmetric response) in next runs.

• Use large-deflection static analysis (second-order effects).

• Run eigenvalue buckling for global stability screening.

• Assign initial cable pretension so main cables remain in tension under service combos.

How the Academic Mentor Guided the Work (Step-by-Step)

Step 1 — Clarify scope & deliverables

• Reconfirmed the three required sections (3.1–3.3) and that this stage focuses on model setup and documentation, not detailed design.

• Agreed on naming conventions, units, and a simple folder structure for model files, figures, and notes.

Step 2 — Build the geometry (Section 3.1)

• Sketched the global layout (100 m span, 10 m deck width).

• Modelled twin main ties and the CHS arch with specified arc length and inclination.

• Placed 17 cable lines per side; positioned first and second cable elevations (≈1.5 m and ≈4.1 m) to respect traffic clearance.

• Introduced transverse top strut from the third cable line onward.

• Added X-layout cross-stabilising cables between towers and opposite second-cable heads.

• Created anchorage towers (20 m high, 5 m embedded) and defined boundary conditions to simulate foundation restraint.

Step 3 — Assign sections & materials (Section 3.2)

• Applied UB/CHS section sizes to ties, arch, cross girders, bracing, and placeholders for cables.

• Set material grades: S355-eq (arch/tie), S350-eq (girders/bracing), and concrete for the 0.5 m deck.

• For cables, documented the future replacement of placeholders with tension-only high-strength elements and matched EA (axial stiffness) to the intended wire/locked-coil specification.

Step 4 — Loads & analysis controls (Section 3.3)

• Activated self-weight for all parts, including the slab.

• Prepared load cases for uniform live load and one-sided live load (to be used in subsequent runs).

• Enabled large-deflection (geometric nonlinearity) to capture second-order effects.

• Set up eigenvalue buckling to screen global stability modes.

• Assigned initial pretension to cables and verified that service combinations keep cables in tension.

Step 5 — Sanity checks

• Verified member connectivity, cable directions, and support restraints.

• Checked basic reactions and deformed shapes under self-weight to ensure realistic behaviour (no compression in tension-only cables, no rigid-body motion).

• Logged assumptions and any items deferred to the “next stage” (e.g., live-load patterns, cable replacement).

Step 6 — Document the model

• Wrote concise text for 3.1, 3.2, 3.3 matching the brief; added figures (where available) and a short checklist of pending items for the next analysis run.

• Ensured language clearly states what is modelled now vs what will be updated next.

Outcome & Learning Objectives Achieved

Outcome (this stage):

• A clean, traceable preliminary model of the tied-arch bridge is set up with correct geometry, section properties, and materials.

• Analysis settings (large deflection, buckling screening, pretension) are in place; self-weight load case runs; live load cases are prepared for the next iteration.

• Clear documentation of assumptions, placeholders (cables), and future updates supports an efficient next stage.

Learning objectives covered:

• Structural modelling discipline: Translating a textual brief into a coherent finite-element/structural model with correct topology and connectivity.

• Property assignment & specification: Selecting appropriate section sizes and material grades; planning the transition from placeholder to tension-only cable elements with matching stiffness.

• Analysis strategy: Applying geometric nonlinearity, pretension, and eigenvalue buckling to capture realistic behaviour and stability.

• Load case planning: Setting up self-weight and preparing asymmetric live load to probe critical response.

• Verification mindset: Performing early sanity checks (supports, cable force states, deformed shapes) to catch modelling issues before detailed loading.

• Professional reporting: Producing concise, structured sections (3.1–3.3) that distinguish current state from next steps—mirroring industry documentation standards.

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