Design loads for decorative ornamental Railings

[amain002]

Hello everyone. I am seeking for some advice and help to design a decorative ornamental railings. While reviewing the load requirement in the IBC and ASCE, I found these to the basic loading criteria:
i) A uniform load of 50 PLF
ii) A concentrated load of 200 lbs placed at the top of the handrail or guard
iii) Infill to be designed for 50 lbs on an area of 1 square foot.

The railings are basically designed as a custom shape with no end or intermediate posts in between. (I have attached a picture I found in google which appears to describe a similar scenario). In the past, when I designed railing, I applied load at the railings post location and designed post as a cantilevered member.

In this case I am not sure how to distribute the load and design the members since I don't have any posts and my member sizes are typically 1/2" square bars. Does anyone have any experience in designing such railings system. Also, this is for a commercial building and not residential house if it does make a difference.

Thank you all for the help.

 

[jayrod12]

It may be best to model it outright to confirm any calculation assumptions made, however I personally feel that top rail is going to share the 200lb load to at least 2 of the groupings, since a single vertical is shared by two groupings, I feel it is reasonable to assume that 5 of the pickets contribute to the overturning resistance. Those should be designed as a cantilevered member.

 

[Ron247]

My first question is the small posts you show. Are they fixed-based or pinned? If pinned, it looks like the main structural component for lateral loads is the handrail itself spanning a long distance between 2 points. At 50 plf, that is a large load. I think some manufacturers test their rails rather than calculate the loads. Design, build, test. If the system can sustain the design load without total failure is their proof. I do not think they worry too much about how much it deflects.

In any case, your design loads need to be near the top of railing system, since your are modeling one or more people leaning on it, or one person climbing over it.

 

[CANPRO]

I would design each vertical (more specifically, each vertical that is connected at the base) for its tributary share of the 50 lbs/ft on the top rail.

I assume your rail is similar in the sense that all of the verticals are anchored at the base?

I think some manufacturers test their rails rather than calculate the loads. Design, build, test. If the system can sustain the design load without total failure is their proof. I do not think they worry too much about how much it deflects.

I agree, and I'd like to also add that they don't really care how its anchored at the base either - which is where all of the fun is. I'm sure in their test it is anchored in the most favorable way possible...but I can guarantee that somewhere in the brochure or technical specs for every pre-manufactured rail available, there is a note stating the actual connection is by others.

 

[amain002]

Thank you all for your input.

@jayrod12: I was thinking the same, may be distribute the load to at least 2 to 3 members.

@Ron247: The members are welded to a embed plate mounted on the side of the stairs. So it will be a fixed base.

@CANPRO: I agree with you on the 50 plf load on top of the rails but how will I justify the code official for the 200 lbs concentrated load. This is where i am struggling.

 

[dhengr]

Amain002:
The 200lbs. concentrated at any point on the handrail, the 50lbs. per ft. on the handrail, and these can act both vert. or horiz., and the 50lbs. on one sq.ft. on the balusters are the important IBC loadings, along with the 4” ‘ball passing through’ spacing. The 200 and the 50lbs/ft. loadings are why you so often see main post spacings at about 4’, where the infill balusters, lower horiz. member and the handrail are designed to span 4’ btwn. the posts. Significant deflections are allowed, but not well defined at these loads, but the system should not fail (should prevent people from falling off the deck or stairway. There are those who will argue that 200lbs. is not particularly conservative when you consider a few drunken football linemen, at a frat part, trying to impress the girls with their ‘bull rushing’ and strength abilities. Given your arrangement, you have to make some engineering judgement about the strength and stiffness of your handrail (guard rail) as this relates to distributing the 200lb. load to sever of the individual sub-assemblies of balusters and the way they are made to act as an individual structural canti. systems or elements. Then, you say, that 2, 3 or 4 balusters are connected at their base by some sort of a connector pl., and made to act as a unit. This unit and its balusters must be able to withstand the 200lb. canti. load at 36 to 42”, or some portion thereof, and transmit it into some real structural system which can take that loading. You can not (should not, but many do) just screw these into a skirt board or a rim joist, because that loading will likely just peel the rim joist off the main structure. These canti. support elements must be tied back into sufficient real structure.

 

[CANPRO]

The way the concentrated load gets distributed to the vertical members is a function of the bending stiffness of the top rail (bending horizontally) and the bending stiffness of the verticals (cantilevering from the base). Think of two extreme cases:

1) The top rail is extremely stiff compared to the verticals. In this case the top rail will deflect but because it is so stiff will remain a straight line, which means all the verticals must deflect equally, which means they all share the load equally.

2) The verticals are extremely stiff compared to the top rail. In this case, the load is first resisted by the 1 or 2 verticals nearest the concentrated load and will deflect very little - the top rail is too flexible to redistribute loads, and those 1 or 2 verticals take all of the concentrated load.

Long story short, your top rail is essentially a beam supported on an elastic foundation, where the elastic foundation is the vertical members. Stiff beam/soft supports = lots of load distribution, soft beam/stiff supports = very little load distribution.

 

[amain002]

Thank you dhengr and CANPRO for the explanation. I like the concept of the stiff beam and a soft support. I have to make sure that the top rails are designed adequately so that it can distribute the forces to multiple vertical members. Thank you all. I think this will get me started.