Steel portal-frame warehouses in Dubai: what governs the design

Is a portal-frame warehouse within the Dubai Building Code?

The Code applies to new buildings and to changes to existing buildings as set out in Table A.1, except where an alternative solution is permitted, and Table A.2 lists the occupancies it covers: storage use is described as warehouse, and industrial use as factory and workshop.¹ Part F applies to the design and construction of buildings and of structures or appurtenances connected to a building.¹⁴

The Code does not apply to structures such as tents, cranes, storage racks, scaffolding, formwork and falsework, nor to structures with specific loading conditions such as silos and chimneys.²

Separately, structural elements have to be designed for the accidental impact loading conditions specified in Section 4.6 of ASCE/SEI 7-16, and the Code recommends a secondary protection system for every primary structural member to guard it against accidental impact.¹⁶ The clause is not limited to any type of building or vehicle. ASCE/SEI 7-16 is not held by this library, so what its Section 4.6 covers is not stated here.

The design basis, and where the other guides take over

Structural steel is designed to AISC 360 and the codes and standards it references, with seismic design to AISC 341, materials to ASTM standards and welding to AWS standards.³ Loads are defined by ASCE/SEI 7-16 together with section F.7 of the Code, and factored and combined to Chapter 2 of ASCE/SEI 7-16.⁴

Wind and uplift on light roofs

The Engineer calculates wind effects taking into account strength, for life safety, and serviceability, where building movements affect cladding or occupant comfort, based on ASCE/SEI 7-16 with the supporting information and additional rules the Code gives for Dubai.⁵ The Code’s reference wind speeds are used instead of the ASCE/SEI 7-16 wind maps, and the same clause sets a minimum design wind pressure that applies in all cases.⁵ The speeds, the minimum pressure and how the risk category selects the row are in the wind guide and are not repeated here.

SCI P399, a UK guide to portal frames designed to Eurocode 3, describes the behaviour. Under uplift the column moment reverses and the side rails then restrain the column’s compression flange,¹⁷ while in the rafter most of the bottom flange goes into compression, the purlins do not restrain it, and the rafter has to be verified between torsional restraints.¹⁸ This is UK Eurocode practice, cited for the behaviour only; it is not a Dubai design basis.

Frame stability in the plane of the frames

AISC 360 requires stability for the structure as a whole and for each element, with second-order effects, imperfections and stiffness reductions accounted for, and permits the direct analysis method for all structures,⁶ with the effective length and first-order methods of Appendix 7 as alternatives within that appendix’s limits.¹⁹ The stability guide, Load paths, stability and bracing of steel structures in Dubai, sets out those requirements, notional loads and stiffness reductions in full; this section keeps to what is particular to portal frames.

One of the conditions under which AISC 360 lets P-δ be left out of the structure’s overall response is that no more than one-third of the total gravity load is carried by columns that are part of moment-resisting frames in the direction being considered.¹⁹

SCI P399 explains why the two directions are treated differently. A portal frame building is a series of unbraced transverse frames, braced longitudinally; in the plane of each frame the rigid eaves and apex joints resist the loads, so in-plane stability is considered for the frame as a whole.²⁰²¹ Out of plane, the longitudinal bracing and secondary steelwork provide points of lateral restraint that set the lengths over which the columns and rafters can buckle, so their out-of-plane stability can be considered member by member.²¹ This is UK Eurocode practice, cited for behaviour, not as a design basis.

Under the direct analysis method, members are then checked to Chapters D to K with no further check of overall stability, taking the unbraced length as the effective length for flexural buckling unless rational analysis justifies less, and any bracing that defines those lengths needs enough stiffness and strength to control movement at the braced points.⁷ The effective length method works differently: the effective length is KL, and in moment frames, where the columns’ bending stiffness provides lateral stability as a portal’s columns do in the plane of the frame, K or the elastic buckling stress of those columns comes from a sidesway buckling analysis of the structure. K = 1.0 may be used for all columns only where the ratio of maximum second-order to maximum first-order drift, both for LRFD or 1.6 × ASD combinations, is 1.1 or less in every storey.²² The method itself is limited to structures carrying gravity mainly through nominally vertical elements and with that drift ratio, on the same basis and with unreduced stiffness, of 1.5 or less in every storey.²³

Restraint of rafters and columns out of plane

The Code requires every feature with a critical influence on overall stability to be identified and included in the design, including all members that restrain critical members in compression, and requires continuous load paths from where each action is applied to the ground.⁸

Appendix 6 of AISC 360 sets the minimum strength and stiffness of the braces that define unbraced lengths, and a bracing system that braces several members is designed for the sum of their required strengths, with the stiffness of its connections and anchorage included.²⁴ The stability guide sets out its equations; three of its points bear directly on portal frames.

First, a user note on column bracing says lateral bracing is assumed to act at the shear centre: bracing that does not prevent twist leaves the column open to torsional buckling, and bracing offset from the shear centre to constrained-axis torsional buckling.²⁵ Side rails fixed to a column’s outer flange are offset bracing of that kind (our reading).

Second, a braced point between supports has to prevent twist, and in double-curvature bending the inflection point is not a braced point unless bracing is provided there.²⁶ Lateral bracing is attached at or near the compression flange, with two exceptions: at the free end of a cantilever it goes at or near the top, tension, flange, and at the brace nearest the inflection point of a beam in double curvature it goes at or near both flanges.²⁶ Torsional bracing may be attached anywhere on the section.²⁷

Third, Section 6.3 is written for I-shaped members with zero net axial force; for a beam-column such as a rafter, Section 6.4 adds the axial and flexural requirements together, and where axial force and bending put both flanges in compression, both flanges have to be restrained.²⁶²⁸²⁹

SCI P399 gives the portal-frame picture in UK practice. Under gravity load the rafter hogs near the eaves, with its bottom flange in compression, and sags near the apex.¹⁷ The side rails are fixed to the outer flange of the column, so the inner compression flange is unrestrained unless restraints are added, and a side rail interrupted by industrial doors cannot be relied on for torsional restraint.³⁰ The guide recommends a torsional restraint at the underside of the haunch and warns that a separate member restraining the inner flanges must be tied to the outer flange or to the foundation, because the aim is to stop the section twisting, not merely to connect the inner flanges together.³¹ Restraint to inner flanges is commonly made with diagonal stays from purlins or sheeting rails; where it can come from one side only, the stay has to be able to carry compression; and its effectiveness depends on the stiffness of the system, especially of the purlins.³² This is UK Eurocode practice, cited for principle only.

Built-up and tapered members

Steel plate for built-up sections has to be at least 6 mm thick.⁹

Eaves, apex and splice connections

Bolted connections are designed and constructed to the AISC Steel Construction Manual, including the specification for structural joints using ASTM A325 or A490 bolts.³⁶ Welding follows AWS D1.1 or BS EN ISO 15614; welding should not be done on site, and where it cannot be avoided the method of work, certificates and tests are submitted for approval.³⁶ No connection may be detailed to rely on welds and bolts together.³⁶

P399 notes, as UK practice, that deep portal members with relatively thin webs usually need a compression stiffener in the column at the eaves, that the column web panel and the column flange in the tension zone may need strengthening, and that a significant reversed moment under uplift may need a stiffener at the top of the column in line with the rafter’s top flange.³⁷ Cited for principle only.

Serviceability, cladding and doors

Deflection limits have to meet the referenced design standards and the project-specific requirements set by features such as the cladding, crane girders and members supporting sensitive machinery.¹⁰ For steelwork the Code says the limits conform to AISC 360 and AISC Design Guide 3, which this library does not hold; a cantilever may be taken as a span of twice its length, and the limits do not account for precamber.¹¹ Overall wind displacement and inter-storey drift, calculated with 10-year return wind pressures, are checked against Table F.11; the Code says the overall displacement limit protects cladding, partitions and other non-structural components.³⁸ The serviceability guide covers these clauses and how the drift limits read for a single-storey shed.

SCI P399 notes, as UK practice, that the acceptable deflections of a portal frame depend on its appearance, use and cladding type and should be agreed with the client; that differential deflection between adjacent frames has to be limited so the sheeting fixings are not overstrained, tearing the sheeting and causing leaks; that steel sheeting stiffens the frame as a stressed skin, so the bare-frame deflection overstates the real one; and that a sheeted or braced gable is very stiff in its own plane, so the calculated differential deflection between the gable and the first internal frame can be large.³⁹

Thermal movement of long sheds

For steel members and members exposed to the sun, the Code takes the thermal range as the maximum temperature swing, allows a default of ±25 °C for protected steel, and requires the Engineer to assess the thermal range and the thermally induced forces case by case, considering the construction stages, exposure and when the steel structure is locked into its final configuration.¹² AISC 360 also requires the effects of thermal expansion and contraction of a building to be considered.⁴¹ The loads guide covers these clauses and the Code’s self-straining rules.

P399 gives expansion-joint spacings for steel-framed industrial buildings on the stated basis that temperature movements in the UK are generally small.⁴² The Dubai Code instead sets a default range of ±25 °C for protected steel and case-by-case ranges for exposed steelwork,¹² so those spacings are not transferred here. The same guide observes that satisfactory expansion joints are neither easy nor cheap, that it is usually better to detail the structure so joints are avoided, for example by resisting expansion with braced bays, and that care should be taken to detail any joints provided so that differential movement cannot cause cladding leaks.⁴³ It lists the positions wall bracing may take: at one or both ends of the building, within its length, or in each portion between expansion joints.⁴⁴

Column bases, anchor rods and post-installed anchors

Table F.5 lists anchor rods to ASTM F1554 at 380 N/mm², with a second figure of 517 printed as the yield strength where a supplementary weldability requirement applies;⁹ the reading the Authority applies to those figures has not been confirmed, so confirm the anchor-rod grade and yield value for the project. AISC 360 approves several anchor-rod specifications and notes that ASTM F1554 is the preferred one.⁴⁵

AISC 360 requires anchor rods to provide the required resistance at column bases, including the net tension from any bending moment, designed as threaded parts, with the transfer of force into the concrete to ACI 318 or ACI 349.⁴⁶ Where anchor rods resist horizontal force, the hole size, the anchor-rod setting tolerance and the horizontal movement of the column have to be considered, and larger oversized or slotted base-plate holes are permitted with ASTM F844 washers or plate washers bridging the hole.⁴⁶ A user note points to AISC Design Guide 1 for column base design,⁴⁶ which this library does not hold.

Post-installed anchors into reinforced concrete may support steelwork where cast-in anchors are impractical.³⁶ They are designed to ACI 318-19 for every conceivable failure mechanism, which Figure F.14, adapted from ACI 318-19, illustrates as steel, pullout, concrete and bond failures under tension and steel, pryout and concrete failures under shear.³⁶

P399 records UK practice for bases: most portal bases are nominally pinned, because a rigid base makes the foundation resist moment and costs significantly more,³⁷ but they should be designed for the small moment from wind on a free-standing column during erection so that temporary bracing is not needed, and where crane girders are carried by the column, moment-resisting bases may be needed to limit deflections.⁴⁷ Cited for principle only.

Building permit and inspections with Dubai Municipality

Dubai Municipality’s procedure for industrial buildings has three online steps through the Dubai BPS system, each shown as the responsibility of the consultant and the contractor.¹³ First, the building permit: the applicant attaches all required drawings and documents, fills in the building card information and pays the permit fees; the government authorities taking part review the application through the unified window and upload their NOCs, and the Municipality issues the permit and stamps the drawings, which can then be printed online.¹³ For this step the sheet cites Circular (224) of 2018.¹³

Second, structural inspection, requested for shoring works and foundations and for cast concrete slabs or steel structures; a note says inspection of steel structures is done for heights larger than G+1.¹³ The Building Control Department inspects, and foundations are inspected by a unified committee of the Building Control and Survey Departments.¹³ Third, the final inspection for completion and services connection: the consultant and contractor notify the Municipality that the work is finished, a joint team of the authorities concerned (Building Control, Survey, Dubai Civil Defence, RTA, etisalat and du) inspects, after which the completion certificate is issued and the connections are made.¹³

Scope and limits