pattern loading

[Skicat]

Can someone direct me to where in what code I can find the guidelines for pattern loading? Specifically, I am trying to find out what the dead load to live load ratio is wherein pattern loading must be utilized. Off the top of my head I'm thinking that if the LL is >= DL, pattern loading is not required but I wouldn't bet much on that. Thanks for your help.

 

[frv]

Skicat-

Quite the opposite. If your LL is > DL, then the effects of LL would be more significant and pattern loading becomes more important.

Assuming you want an answer more detailed than "load alternate bays, then every other pair of contiguous bays and cover all possibilities," I direct you to ACI 8.9.2 and ASCE 7-05 C4.6 (p.270).

 

[Lion06]

You could draw a qualitative influence line (muller breslau, I believe) for moment and shear and apply the LL as shown by the influence line

 

[asixth]

Depends on what code you are referring to, I don't have ACI318 on me right now, but I can talk about the theory of patterned loading and AS3600.

AS3600 (Australia Code) calls it up in Section 7 "Methods of Structural Analysis". Section 7.2 "Idealised Frame", I believe this is what Americans call equivalent frame, requires 0.75 of the design live load to be applied on alternate spans, whereas Section 7.6 "Linear Static Analysis", calls up full design live loads on alternate spans. Basically this is an inconsistancy by the code writers, and I understand the revision of AS3600 will require full design live loading to be applied to alternate spans, for both ultimate and service limit states.

Don't quote me on this but I believe ACI318 requires 0.75 design live load on alternate spans, but ACI318 also uses larger load combination factors than we do here in Australia, we use 1.2 Dead + 1.5 Live.

The theory behind patterned live loads is that you are generating a moment envelope that theoretically cannot be exceeding for the given design loading. 75% of the design live load is used to account for the fact that you will effectively have a maximum positive moment and maximum negative moment that occur for seperate load cases and therefore, can never co-exist.

It is also good practice to consider patterend live loads when you are calculting service deflections.

 

[tngolfer]

ACI 318-05

General 8.9.2
8.9.2 It shall be permitted to assume that the arrangement of live load is limited to combinations of:
(a)Factored dead load on all spans with full factored live load on two adjacent spans; and
(b)Factored dead load on all spans with full factored live load on alternate spans.

Two-way slabs 13.7.6:
13.7.6.1 When the loading pattern is known, the equivalent frame shall be analyzed for that load.

13.7.6.2 When live load is variable but does not exceed 3/4 of the dead load, or the nature of live load is such that all panels will be loaded simultaneously, it shall be permitted to assume that maximum factored moments occur at all sections with full factored live load on entire slab system.

13.7.6.3 For loading conditions other than those defined in 13.7.6.2, it shall be permitted to assume that maximum positive factored moment near midspan of a panel occurs with 3/4 of the full factored live load on teh panel and on alternate panels; and it shal be permitted to assume that maximum negative factored moment in the slab at a support occurs with 3/4 of the full live load on adjacent panels only.

13.7.6.4 Factored moments shall be taken not less than those occurring with full factored live load on all panels.