Seismic design concepts and standard details for drive-in pallet rack structures

Technical Abstract

E1: The design of Drive-In Pallet Rack Structures for seismic loads requires consideration of several secondary factors which affect both the response of the structure and sizing of members.

E2: Tensioned wire bracing systems can be an effective means of providing lateral support to the overall rack structure;
(i) Along side walls in the longitudinal direction;

(ii) Acting in conjunction with upright frames, in the transverse direction;

(iii) As a roof level diaphragm, taking longitudinal loads from near the central aisle back to the rear wall bracing.

A flexible rear plane or side wall bracing system is recommended, as allowance for increased structural deflection reduces the overall structural response of the rack to seismic loading.

E3: The design of upright frame (columns) to face loading is strongly influenced by;
(i) The range and variability of column top deflections at various positions in the rack i.e. dynamic amplifications factor;

(ii) Column top and “sag” deflection under dace load i.e. P-delta effects;

(iii) Amount of “fixity” achievable at column top support and base plate level;

(iv) Whether localized “hydraulic” loading from product spillage is seen as a design condition.

E4: Factors affecting bracing design and performance:-
(i) The response of column elements to seismic loading is strongly dependant on the range and variability of the column top deflections, which comprise a roof level diaphragm and a rear plane bracing deflection component. Increased deviations of the deflected shape from the average deflection correspond to increased dynamic amplification of loadings in elements at those locations.

(ii) Rear plane or side-wall bracing provides support to the structure under longitudinal earthquake loading. A flexible rear plane or sidewall bracing system is recommended, as allowing for increased structure deflection reduces the overall response of the rack structure to seismic loading.

(iii) Tensioned wire bracing is an effective rear plane bracing system. Bracing stiffness increases with wire pretension, so the level of wire pretension in the rear plane bracing should normally be minimal. Note, however, that wires should be evenly tensioned to ensure even distribution of loading. A minimum factor of safety of 2 against wire breakage should apply.

(iv) In situations where the roof level diaphragm is supported in the longitudinal direction along one edge only, the upright frames acting in conjunction with supplementary bracing can be detailed to provide the required torsional resistance.

(v) It is recommended that, where possible, the roof level diaphragm be continuous across the centre aisle and span between rear plane or side-wall bracing lines, to minimize eccentric seismic loading effects.

E5: Factors affecting upright frame design and performance:-
(i) Design of upright frame column members in strong axis bending under longitudinal seismic face loading must take account of P-delta and dynamic amplification effects, as well as any earthquake induced axial loads. Load carrying capacity is increased by detailing for end restraints to provide at least partial end fixity.

(ii) P-delta actions have a significant effect in the design of drive-in rack structures and are not adequately provided for by reliance on the provisions of code interaction design formulae.

(iii) Columns may need to be checked for possible “hydraulic” loading, which may occur due to product spillage under earthquake loadings. Depending on the nature of product and method of storage, this effect can be severe.

E6: the magnitude of secondary effects have been inferred from trial analyses as:-
(i) P-delta actions may typically amplify column moments by about 50 to 100%, and column top reactions by about 30 to 60%. The higher amplifications would normally apply to columns with high earthquake induced axial loadings adding to gravity loads.

(ii) Typically, dynamic amplifications effects only need to be considered in design of columns and those members directly supporting columns with greater than the average deflection. Loadings should be determined from an appropriate dynamic analysis or,  alternatively, a dynamic amplification factor of 1.5 should be applied to the results of an equivalent static analysis.

(iii) For a typical drive-in rack structure column design moments as found from an equivalent static linear analysis may be amplified by a factor of 1.5 for dynamic effects and 1.5 – 2.0 for non-linear effects, giving a total amplification of say 2.0 to 3.0.

These indicate that a significant increase is required in the sizes of members typically used by the industry in upright frames, to comply with current seismic codes e.g. NZS4203:1984.

E7: In the design of rack systems, adequate separation should be maintained between that rack structure, and the fabric of the enclosing building, to permit predicted earthquake deformations to occur.

E8: Experimental work, and field experience has shown that:-
(i) To ensure satisfactory frame performance under lateral seismic loading, careful attention should be given to design and detailing. Experimental verification of the design is required, unless the design and detailing complies with the requirements of an approved “means of compliance” document. Tests on “standard” frames not specifically detailed for seismic loading have indicated relatively poor seismic performance.

(ii) Bracing details must be adequate to ensure proper bracing of column members in flexural and flexural-torsional buckling considerations. Bolted connections must be fully tightened for end restraints and bracing connections to be fully effective.

(iii) Column base anchorage should be designed to yield, and overstrength provided against brittle failure mechanisms. In some cases the base anchorage may be detailed to allow frames to rock under seismic loading.

E9: Notwithstanding the mandatory code provisions applying to pallet rack structures (including the effects derived from E5), the insurance needs to justify economically the necessity for increased standards of seismic resistance above those presently adopted by the industry. Relevant factors here include:-

(i) Risk to life (as opposed to property);
(ii) Relative cost of palletized goods, and storage rack;
(iii) Difficulty in preventing “spillage” of palletized goods in earthquakes, regardless of seismic design standards;
(iv) Temporary nature of the rack structure (as a component in a materials handling system);
(v) Damage from other sources e.g. forklifts;
(vi) Economics.