Minimum design loads for steel structures in Dubai: dead, superimposed dead, live, construction and thermal

Where the loads come from

Clause F.7.1 defines loads by ASCE/SEI 7-16 together with the rest of section F.7, and F.7.2 factors and combines them using Chapter 2 of ASCE/SEI 7-16. Results of compatible linear analyses of individual load cases may be combined and summed algebraically.¹

Part F requires the Engineer to state the codes and standards used, by number, title and version, in calculations, drawings, specifications and basis-of-design reports, and where Part F and a referenced standard conflict, the more restrictive requirement governs.⁶

Dead loads and default densities

Dead loads are calculated from the densities and volumes of the materials in the construction. For the materials a steel frame usually carries, the Code’s default densities are 78 kN/m³ for steel, 25 kN/m³ for normal-weight and 18 kN/m³ for lightweight reinforced concrete, for example on composite decking, and 20 kN/m³ for screed. Table F.7 also gives defaults for blockwork and glass.²

More refined densities, and densities of other materials, can come from the referenced standards, material data sheets or testing, and the engineer is responsible for making the assumed densities match the specified materials. Where appropriate, dead loads also allow for the extra weight concentrated at structural connections.²

Superimposed dead loads: partitions, finishes, services and cladding

Superimposed dead loads cover components that are neither live nor dead load, including anticipated internal partitions, floor and ceiling finishes, facades and external cladding. The engineer determines them and applies them as point loads, uniform loads on plan or uniform loads on elevation, as appropriate.³

For the maximum load effect, the Code sets minimum allowances as average uniform loads on plan: 0.75 kN/m² for internal drywall partitions, 0.50 kN/m² for suspended services with ceiling finishes and 0.30 kN/m² for suspended services without ceiling finishes.³

Where the minimum load effect is critical, such as a buoyancy check, an appropriate minimum load is assumed instead, and features that are easy or likely to be removed should be taken as zero.³

The engineer provides calculations supporting the superimposed dead loads. If they are not provided, the vertical loads in Table F.8 are included for the partitions instead: they are given for block and drywall partitions, including finishes, and range from 4.0 kN/m² for drywall to 5.5 kN/m² for normal-weight block walls. Loading plan drawings showing the load provision should be submitted for approval, the assumptions are validated against the systems actually installed, and late changes are not made without new approval.³

Live loads

Minimum uniform live loads are not less than those in Chapter 4 of ASCE/SEI 7-16. On top of that, Part F requires concentrated live loads to be evaluated for objects creating significant point loads, including machinery, vehicles and storage racks.⁴

Car parks and garages take at least 3.5 kN/m², validated against the vehicles that will use them, and the engineer also accounts for emergency vehicles along their designated route. Drained floors around a swimming pool take 2 kN/m², while pool tanks and other areas that can flood are designed for at least the maximum retained head of water. The supporting structure is designed for mechanical, electrical and plumbing equipment in its designated areas and along the equipment access route.⁴

Helipads and heliports take the minimum live load allowance of Section 4.11 of ASCE/SEI 7-16. Structural elements resist the accidental impact loads of Section 4.6 of ASCE/SEI 7-16, and a secondary protection system should be provided to all primary structural members against accidental impact.⁷

Storage racks are outside the Code’s scope as structures,⁸ but their point loads on the floor that supports them still have to be evaluated.⁴

Soil and water on roofs, podiums and below ground

Soil and water loads are not taken below the minimum densities of Table F.9, for example 19 kN/m³ for saturated planter soil and 10 kN/m³ for water; the table also covers compacted trafficable soil and brackish water, and lower values need project-specific justification.⁹

Below ground, the design water table is established in the geotechnical interpretive report, considering its fluctuation over the design life, including dewatering in neighbouring areas, seasonal variation, future developments and climate change. As a minimum it is taken at ±0.00 m Dubai Municipality datum plus 1.0 m near bodies of water, and elsewhere at the actual water table defined in that report, with dewatering taken into account, plus 1.0 m for seasonal and tidal variation. For structures below the water table, hydrostatic uplift and lateral pressure are checked under Section 10 of BS EN 1997-1:2004 with its UK National Annex, and a dewatering system counts as temporary unless it can be shown reliable for the whole design life.⁹

Construction-stage loads

Minimum design loads during construction follow ASCE 37, which Part F lists among its essential references.⁷¹⁰ The engineer includes construction-stage loads, including stresses locked in by the construction sequence, states the construction load assumptions on the detailed design drawings, and verifies that the structure can carry them at every stage. Any departure by the contractor from those assumptions is assessed.⁷

Temperature and self-straining effects

Structures are designed for self-straining forces from contraction or expansion, which can come from shrinkage, creep or moisture change in the materials, including the soil, at any stage of the structure’s life. Details that relieve these forces may be used where practical and where they do not break an assumed load path; permanent ones are durable and maintainable.⁵

Thermally induced forces are derived from the structural restraints and the temperature changes across construction and operation, and across seasons and days. For steel members, and any member exposed to solar radiation, the range is taken as the maximum temperature swing; a default range of ±25 °C may be used for protected steel. For exposed steelwork the range can come from hourly maxima and minima, and the engineer assesses it case by case, considering the construction stages, the exposure and when the steel structure is locked into its final configuration.⁵

For covered concrete members the Code uses mean monthly temperatures, with default ranges of ±20 °C for exposed elements above ground and ±15 °C for elements that are not exposed, such as basements. It points to the National Centre for Meteorology and Seismology for site temperature and humidity data.⁵¹¹

Putting the loads together

Strength design uses the factored loads in the load combinations defined in Part F without exceeding the strength limit states of the material, and the basis of design includes, as a minimum, deflection, crack control in concrete, building movement due to wind and to earthquake, movement joints and separation, vibration, lateral acceleration, fatigue and extra requirements for transfer elements.¹²

Scope and limits