Construction, Structural
Structural Design of Steel Catwalks for MEP Maintenance Platforms
Nov
1. What is a structural catwalk?
A catwalk is a narrow elevated walkway used to reach mechanical and electrical equipment.
It usually sits above ground floor level, spanning between concrete walls or steel frames.
The surface is often open steel grating so dust and water can fall through.
In the reference design the catwalk:
- Runs in a long curved line at Level B1.
- Is suspended from, and supported on, a reinforced concrete slab.
- Uses steel grating as the walking surface.
- Includes handrails on exposed edges.
- Is braced in both directions to control sway.
The catwalk’s main purpose is safe maintenance access to MEP equipment at ground floor level.
2. Design criteria and standards
The engineer set clear design rules before any modelling work.
2.1 Main codes
- BS 5950-1:2000 – design of structural steel members and connections.
- UBC 97 – basis for the seismic loading.
- Manufacturers’ data – for steel grating load tables and for Hilti anchors.
These standards define load combinations, safety factors, member capacities and connection checks.
2.2 Design philosophy
The report states a simple but strong design philosophy:
- The catwalk is mainly for maintenance access.
- Light dead loads allow the use of small tube sections at grating level.
- Vertical loads travel through pipe columns into the concrete slab via base plates and anchors.
- Extra bracing bays appear about every 6 m along the length.
- In the short direction the catwalk behaves as a rigid frame.
- In the long direction it acts as simply supported beams between frames.
- Steel–concrete joints are modelled as pinned supports at base plates or slab fixings.
This approach balances safety, stiffness and ease of fabrication.
3. Materials and components
3.1 Structural materials
The calculation summary lists:
- Concrete with characteristic strength of 60 MPa.
- Steel grade S275 for beams, columns and braces.
- Bolts grade 4.6 plus Hilti proprietary anchors.
- Welding using electrode type E60.
These choices are typical for an indoor catwalk where corrosion is moderate but reliability is critical.
3.2 Main steel sections
From the design philosophy and drawings:
- Grating level tubes: 60 × 60 × 4.5 mm square hollow sections.
- Vertical posts: 42.4 × 4 mm circular hollow sections.
- Longitudinal beams: box or tube beams sized to carry grating loads.
- Braces: CHS elements in X-bracing form in both elevations.
Steel grating sits directly on the tube beams and is welded at regular spacings.
4. Design loads for the catwalk
The report lists several load cases.
4.1 Gravity loads
- Dead load on cross beams: 1 kN applied at mid-span.
- Live load on beams: 1 kN/m², converted to 1.5 kN/m over a 1.5 m tributary width.
- Self-weight: calculated automatically in ETABS using a factor equal to 1.0.
- Grating self-weight: added as 0.277 kN/m² and applied as 0.42 kN/m on beams.
These values match manufacturer data for typical industrial grating.
4.2 Handrail loads
A 0.5 kN horizontal load is applied at 1.0 m above floor level to represent people leaning on the handrail.
The handrail posts and their base plates must resist this load without excessive deflection.
4.3 Earthquake loads
The catwalk is within a seismically designed building.
Earthquake actions are generated in ETABS using UBC 97 parameters:
- Soil profile type SB.
- Seismic zone factor Z = 0.075.
- Importance factor I = 1.0.
- Ca and Cv coefficients 0.08 and 0.06.
The program calculates lateral forces and story shears for both X and Y directions.
These forces are then applied to the catwalk frame to check stability and brace forces.
5. ETABS computer model
The report includes many ETABS output pages and model views.
5.1 Geometry and framing
The 3D model shows:
- Longitudinal box beams following the curve of the catwalk.
- Regular transverse frames formed by vertical pipe posts and top beams.
- X-bracing between frames in both directions.
- Base restraints at concrete support points.
Each member is assigned a section property and material grade S275.
5.2 Load combinations
Typical load combinations include:
- Dead + live.
- Dead + live + wind.
- Dead + earthquake.
Factors follow BS 5950 recommendations for ULS and SLS checks.
5.3 Output checks
The model generates:
- Axial forces, shears and bending moments in beams, posts and braces.
- Nodal displacements and sway ratios.
- Story shears for seismic events.
Representative member design summaries are printed for the most critical tube and pipe sections.
6. Member design
6.1 Beams and grating supports
The longitudinal and transverse beams support the grating and live loads.
Design checks use BS 5950 formulas:
- Bending capacity Mpl compared with maximum moments from ETABS.
- Shear capacity Vpl checked at supports.
- Lateral torsional buckling length reduced by the grating, which braces the compression flange.
The report prints out capacities and utilisation ratios for the most heavily loaded beams.
Utilisations remain well below 1.0, giving good safety margins.
6.2 Columns and posts
Vertical CHS posts carry axial loads from the beams plus bending from handrail forces.
Checks include:
- Axial compression strength using buckling curves.
- Combined bending and axial compression interaction.
Because spans between frames are short, column forces are modest.
The catwalk therefore feels stiff and stable in service.
6.3 Bracing members
Braces resist sway and seismic actions.
Each brace is designed as a tension–compression member with slenderness checks.
The ETABS results show low lateral drifts, confirming that the chosen bracing arrangement is adequate.
7. Connection and anchor design
The most critical part of the system is the interface with the concrete structure.
7.1 Base plates and posts
Catwalk posts sit on steel base plates anchored to concrete.
The Hilti design sheets show:
- Plan geometry of plate and anchors.
- Resulting design loads from ETABS (shear, tension and moment).
- Design capacities for steel, concrete cone and pull-out.
Utilisation ratios for tension and shear are low, and each line is marked OK.
7.2 Anchor performance
The Hilti anchor report checks:
- Steel failure of the anchor bolt.
- Concrete cone failure under tension.
- Pry-out failure under shear.
- Combined tension and shear using interaction equations.
The final note states “Fastening meets the design criteria.”
This confirms safe transfer of loads from catwalk into the slab.
7.3 Welds and bolted connections
The document references welding electrode E60 and standard bolt grades.
Weld sizes are chosen so that throat shear stresses remain below allowable values under factored loads.
8. Serviceability: sway, vibration and comfort
The check list in the front of the report highlights the need to consider sway and installation requirements.
The ETABS model addresses this by:
- Including full frame and brace stiffness.
- Applying handrail and live loads at correct locations.
- Reviewing lateral displacements for each load case.
Because bay lengths are short and bracing is frequent, deflections are small.
This improves comfort for users and avoids a “bouncy” feeling in the walkway.
9. Construction and method statements
The reviewer’s comments emphasise that calculations alone are not enough.
Key points include:
- Provide full splice and field connection details.
- Consider erection sequence and temporary stability.
- Coordinate with MEP layouts to avoid clashes.
- Develop a method statement for installing the catwalk and fixing anchors into the slab.
Clear drawings at the end of the report show:
- Catwalk access plan and curved alignment.
- Structural elevation with X-braces.
- Detailed sections at connections and typical base plate layouts.
This package allows a fabrication shop and site team to build the catwalk exactly as designed.
10. Lessons for structural engineers
From this catwalk design we can draw several practical lessons:
- Start with a clear philosophy. Decide early how loads flow through beams, posts and braces.
- Use 3D modelling wisely. ETABS gives a realistic picture of stiffness and sway for irregular layouts.
- Check member and connection capacities under realistic load combinations, including seismic effects where needed.
- Treat anchors as critical elements. Use manufacturer design tools and ensure good edge distances and embedment.
- Think about constructability. Short standard bays, simple base plates and regular bracing make fabrication and erection easier.
A well-designed catwalk is not just a bridge between machines.
It is a carefully engineered structure that keeps maintenance staff safe for the life of the building.