Steel Base Plate Design 1

[KarlT]

I have a situation where there is a 2 inch +/- diameter HSS tube welded to a 5"x5" base plate & cast into concrete. The tube is a vertical guardrail post, which translates a significant moment to the base and a shear force as well.

I wanted to put a single nelson stud anchor centered on the plate below the HSS tube to resist the large uplift tension from the moment couple and the small amount of shear.

Now, I am having trouble with making valid assumtions on the base plate design as listed below:

1. To establish the tension in the stud, I need to work out the location of the compression resultant from the centerline of the stud. How does everyone else establish this?

2. On way of doing this is to locate the compressive force resultant a full bearing plate thickness beyond the compression face of the attached element (per PCA anchor design method). However, with only a 2" diameter tube I'd get one heck of a large tensile force. If I design the base plate so that it won't yield under the full moment from the post can I move the compressive load outwards towards the edge of the plate?

3. How would the plate actually fail? Would it yield along the compression edge of the HSS or would it yield along the stud location? Can i assume the full 5" of plate width in calculating the moment resistance?

Karl T

 

[eureka]

A very good source for base plate design is:
Design in Structural Steel, by John E. Lothers
Chapter 6

However, in NO case would I depend on a SINGLE BOLT for a base plate connection.

 

[CTSeng]

I'd have to agree with eureka I would not rely on a single headed stud. You might consider boring into the concrete and casting the rail posts 4" min in an epoxy cement, however water can damage the posts if not well protected.

I have designed some of the 2-bolt railing base plates using the moment base plate method from 'Design of Welded Structures, Blodgett' or similarly 'AISC Design Guide 1, Column Base Plates'. The tension in the anchors does get large quickly (3/8 or 1/2 wedge anchors) as well as a thick plate (3/8" or more).

Four anchors does seem like overkill but is probably the lost lasting luxury option although I'd still specify the cast in place as the cheapest.

 

[KarlT]

Ya, I know one stud is a little risky, especially if the stud is not welded on properly. (I actually don't like it, but our senior engineer claims he has got it to work so I thought I'd check the numbers for fun!) If I went with two studs then there is the danger that the contractor will place the plate in the wrong orientation and then one stud ends up with almost no cover and would blow out the concrete if it went into tension.

In order to meet the minimum requirements for side cover on the studs I would end up having to use a fairly large plate (likely a 6"x6" plate or more) with four studs, which starts to look stupid with only a 2 inch post sitting on it. Not only that but with 4 anchors there are often conflicts with the wall or slab reinforcing steel within the first couple of inches of the edge of concrete.

 

[ishvaaag]

You may design it as if of a reinforced concrete rectangular section was the case, with the full size of your plate, provided the plate is strong enough.

Hence, assume a rectangular stress block in the bearing plate in equilibrium with the moment for a T-C pair of forces. Then use the compressive stress to check the plate at the worst yield line in bending, more or less for a pipe a bit inside the pipe face (think to remember made equivalent as if square 0.85 of diameter, hence the position). For strength yielding check at proper safety factor is enough, even if it would be better a triangular stress block (elastic assumption etc) for a less stressed plate.

Respect the failure, will depend upon the design of the plate, weld and stud. The stud must be properly designed for T tension, and also the weld, as any insert in concrete would be. Some even would recommend to let unbonded some length of the stud just under the plate to ensure that instead of a severe stress concentration there the variation of stresses along the life of the connection get distributed along the unbonded length. This may be or not good depending upon the actual construction fitting and soundness of the anchorage beyond.