Serviceability of steel structures in Dubai: deflection, wind drift, vibration, movement joints and fatigue

What serviceability covers

Part F’s basis of design includes, as a minimum, deflection control in steel structures, building movement and motion due to wind, movement due to earthquake, movement joints and building separation, vibration, lateral acceleration, fatigue and additional requirements for transfer elements, alongside strength.⁷

AISC 360 treats serviceability as preserving a building’s function, appearance, maintainability, durability and occupant comfort in normal use. Its limiting values, such as maximum deflections and accelerations, are to be chosen for the intended function of the structure and checked with the applicable load combinations; a user note points to Appendix C of ASCE/SEI 7 for serviceability loads and combinations, and says the reduced stiffnesses of the direct analysis method are not meant for these checks.²

Deflection of steel members

Deflection limits are adopted so that the building keeps working as intended, and may be governed by furniture, finishes, fixtures and fittings. Vertical and horizontal deflections stay within the limits of the referenced design standards and any project-specific requirements, for example from cladding, crane girders or members supporting sensitive machinery. The most adverse realistic combination and arrangement of serviceability loads is used, and the Engineer verifies that the adopted limits keep the building functional.⁸

For steel, the limits follow AISC 360 and AISC Design Guide 3. A cantilever may be treated as a span of twice its length, and the limits do not take account of precamber, which may be used to reduce the effect of total deflection.¹ AISC 360 requires deflections of members and systems, and drift, to be limited so they do not impair serviceability, without giving numbers.²

For comparison, the Code does put a number on concrete: the incremental deflection of concrete slabs and beams after partitions and finishes are installed is limited to 20 mm.⁹ It states no equivalent figure for steelwork.

Wind drift and building motion

Overall building displacement is calculated with 10-year return wind pressures and the total building height H, and inter-storey movement with 10-year pressures and the floor-to-floor height Hs; the clause, as printed, calls the second an inter-storey shear calculation. Table F.11 limits overall displacement to H/500 and inter-storey drift to between Hs/400 and Hs/600. The overall limit is there to protect cladding, partitions and other non-structural components from overstress, fatigue damage or loss of effectiveness, and inter-storey displacement may be estimated from inter-storey drift or calculated to include other deformations.³ Buildings taller than 120 m or more than five times as tall as they are wide are wind tunnel tested; see the wind guide.³

Accelerations of the uppermost occupied floor are assessed for 1-year and 10-year return periods and compared with recognised criteria, such as ISO 10137 or the ASCE guide on wind-induced motion of tall buildings; the Code shows the ISO 10137 horizontal-acceleration curves for offices and residences as Figure F.17, which is not reproduced here.¹⁰ Unless the design adds damping, the damping estimates of Table F.12 for normal buildings are used for both serviceability and strength: for steel structures, 0.75% to 1% of critical for serviceability and 1% to 1.5% for strength design, with intermediate values for composite steel and concrete.⁴

Floor vibration

The natural frequency of steel floor systems for normal occupancy is evaluated to AISC Design Guide 11 or SCI P354.⁵ AISC 360 requires the effect of vibration on occupant comfort and on the function of the structure to be considered, including occupant loading and vibrating machinery, and likewise the effect of wind-induced motion on occupants.¹¹

Movement joints, thermal movement and connection slip

Movement joints are provided where necessary for expansion or deflection of above-ground structures, and the separation from any surrounding obstruction is at least the total maximum displacement defined in Section 12.12 of ASCE/SEI 7-16. Because movement joints are a common source of water ingress, structures below ground should be designed without them.¹² The separation is found from the Code’s Eq. F.7 and F.8: the total is the square root of the sum of the squares of the two adjacent structures’ maximum inelastic displacements at their facing edges, and each inelastic displacement is the elastic analysis displacement multiplied by the deflection amplification factor Cd and divided by the importance factor Ie, both from ASCE/SEI 7-16.¹² Using the elastic analysis output directly understates the separation.

Drift and deformation are assessed over the whole life of the structure, including construction stages. Out-of-plumb under permanent gravity loads is verified by the Engineer and the Contractor against Section 7 of ACI 117-10, with any setting-out correction specified by the Engineer. For multi-storey buildings with different stresses in cores and perimeter columns, asymmetric plans or massing, stepped setbacks or outriggers, the Code requires, as a minimum, a non-linear construction-sequence analysis of differential vertical shortening, with the method agreed with the Authority and site corrections submitted for review and approval. Seismic drift and deformation follow Section 12.12 of ASCE/SEI 7-16, and wind drift follows F.7.12.4.¹³

AISC 360 requires the effects of thermal expansion and contraction of a building to be considered, and the effects of slip in bolted connections to be included where slip could impair serviceability, with slip-critical design where appropriate.¹¹ In Dubai, F.7.11 allows a default range of ±25 °C only for protected steel; exposed steel is assessed case by case.¹⁴

Fatigue

Members supporting vibrating machinery, vehicles or plant should be checked for fatigue. Stress changes from normal wind fluctuations need not be included, but wind-induced oscillation is allowed for where aerodynamic instability can occur. Where fatigue is critical, steel is checked to Appendix 3 of AISC 360, and all details are fully defined, including workmanship and quality assurance tests.⁶

AISC 360 requires fatigue to be considered for members and connections under repeated loading, but not for seismic effects or for wind on typical building lateral systems and enclosures; it also requires components to tolerate corrosion, or be protected, where corrosion could impair strength or serviceability.¹⁵ Under Appendix 3, the fatigue resistance of shapes and plate is determined when live load is applied more than 20,000 times; hollow sections in building-type structures under code wind loads need no fatigue evaluation; the peak stress from cyclic loads is limited to 0.66Fy; and the method applies only to steel with suitable corrosion protection, or in mildly corrosive conditions, at temperatures not above 150 °C. No further evaluation is needed where the applied stress range is below the threshold stress range of Table A-3.1; under stress reversal the range is the numerical sum of the maximum repeated tensile and compressive stresses, or of the maximum shearing stresses of opposite direction; and the engineer of record either gives complete details, including weld sizes, or specifies the planned cycle life and the maximum ranges of moment, shear and reaction for the connections.¹⁶

Transfer beams and trusses

Any beam, slab or truss that redirects the gravity or lateral load path of upper storeys onto different vertical structure below is a transfer element, and the Code says transfer structures should be avoided where possible. Where they are used: transfer beams have at least two direct supports; the column axis may not be eccentric to the beam axis; the load taken is not less than a tributary-area hand calculation; elements supporting planted columns whose failure could cause progressive collapse are treated as key elements; the planted structure has alternative load paths; the transfer structure resists the maximum reactions attached components can reasonably transmit; members not otherwise covered are analysed for the possible critical combinations; and members supported by the transfer element meet the F.8.3 deflection limits. The Code also asks for the long-term deflection of transfer members and their supports to be considered.¹⁷

Ponding on roofs

A roof system must be analysed for strength and stability under ponding unless the roof surface is configured to prevent water accumulating, with methods in Appendix 2 of AISC 360.¹⁵

Scope and limits