span of beams supported by columns of section 9.17 of OBC?

[sunyaer]

This is from Ontario building code Division B part 9:
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Section 9.17. Columns
9.17.1. Scope
9.17.1.1. Application
(1) This Section applies to columns used to support,
(a) beams carrying loads from not more than 2 wood frame floors where,
(i) the supported length of joists bearing on such beams does not exceed 5 m, and
(ii) the live load on any floor does not exceed 2.4 kPa,
(b) beams or header joists carrying loads from not more than 2 levels of wood frame balconies, decks or other
accessible exterior platforms, or 1 level and the roof, where,
(i) the supported length of joists bearing on such beams or joists does not exceed 5 m,
(ii) the sum of the specified snow load and the load due to use and occupancy does not exceed 4.8 kPa, and
(iii) the platform serves only a single suite of residential occupancy, or
(c) carport roofs.
(2) Columns for applications other than as described in Sentence (1) shall be designed in accordance with Part 4.
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Question:
What is the span of beams supported by columns in this section? (the span of beams would affect the loads on supporting columns)

 

[bill1952]

What is the span of beams supported by columns in this section? (the span of beams would affect the loads on supporting columns

The span is whatever the plans or building measures. Measure from bearing to bearing - perhaps column to column or sill plate to column - or whatever. I would hope if there are plans it is dimensioned. And then of course figure out what loads are tributary to that beam and how much of that is on the column. It's a drawing exercise.

 

[sunyaer]

The span is whatever the plans or building measures. Measure from bearing to bearing - perhaps column to column or sill plate to column - or whatever. I would hope if there are plans it is dimensioned. And then of course figure out what loads are tributary to that beam and how much of that is on the column. It's a drawing exercise.

What I was trying to say is that design of all columns, at least steel columns, have to be in compliance with part 4, since there is no prescriptive design in part 9 regarding columns, correct?

 

[tmurray]

See 9.17 for column design, including steel. Provided the steel beam is selected using Part 9, the columns can remain in Part 9 as well.

If the beam is designed using Part 4, the columns and column pad must be designed using Part 4 as well.

 

[sunyaer]

See 9.17 for column design, including steel. Provided the steel beam is selected using Part 9, the columns can remain in Part 9 as well.

If the beam is designed using Part 4, the columns and column pad must be designed using Part 4 as well.

Agreed.

But It looks like I can't find a table in part 9 for designing a steel column based on its loads or beam(s) span it supports? Am I missing something?

 

[tmurray]

Part 9 doesn't appear to take loading into account for any column design. This is interesting as all other associated structural members (beams, joists, and foundations) do.

 

[sunyaer]

Part 9 doesn't appear to take loading into account for any column design. This is interesting as all other associated structural members (beams, joists, and foundations) do.

So basically a steel column (in part 9) design has to be using part 4, correct?

 

[tmurray]

So basically a steel column (in part 9) design has to be using part 4, correct?

No, if everything the column is supporting is coming from Part 9, the column just needs to meet Part 9.

 

[sunyaer]

No, if everything the column is supporting is coming from Part 9, the column just needs to meet Part 9.

The only sentences for steel column design in OBC is as follows:
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9.17.3. Steel Columns
9.17.3.1. Size and Thickness
(1) Except as permitted by Sentence (2), steel pipe columns shall have an outside diameter of not less than 73 mm and a wall thickness of not less than 4.76 mm.

(2) columns of sizes other than as specified in Sentence (1) are permitted to be used where the loadbearing capacities are shown to be adequate.
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So if steel pipe column is selected for situations of column design in part 9, a pipe column of 73 mm in diameter with 4.76mm wall thickness would cover all scenarios, assuming I do not use other sizes as permitted in (2), is this the way to design a pipe column in part 9?

 

[tmurray]

The only sentences for steel column design in OBC is as follows:
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9.17.3. Steel Columns
9.17.3.1. Size and Thickness
(1) Except as permitted by Sentence (2), steel pipe columns shall have an outside diameter of not less than 73 mm and a wall thickness of not less than 4.76 mm.

(2) columns of sizes other than as specified in Sentence (1) are permitted to be used where the loadbearing capacities are shown to be adequate.
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So if steel pipe column is selected for situations of column design in part 9, a pipe column of 73 mm in diameter with 4.76mm wall thickness would cover all scenarios, assuming I do not use other sizes as permitted in (2), is this the way to design a pipe column in part 9?

Correct.

 

[Mark K]

The span of beams resting on a column is not useful in designing the column except in the simplest of situations. What is relevant is the loads transferred to the column.

 

[sunyaer]

The span of beams resting on a column is not useful in designing the column except in the simplest of situations. What is relevant is the loads transferred to the column.

A beam carries loads from joists, the longer the span of the beam, the greater load the beam will carries from the joists, then the greater load will go to the column.

 

[tmurray]

The span of beams resting on a column is not useful in designing the column except in the simplest of situations. What is relevant is the loads transferred to the column.

The part of the code this comes from contains prescriptive live and dead loads for structures, so one could easily develop an anticipated column load based on the tributary area.

Sunyaer's questions on this topic are not as outlandish as they would appear.

 

[sunyaer]

Correct.

If you look at round hollow column sections factored Axial compressive resistance, Cr (KN) in "Handbook of Steel Construction", HSS 73 X4.8 for 3.0 m unsupported length, Cr is about 100 KN, for 10 m span of beam and 5 m span of beam supported joist, this column is OK, but when the span of beam or / and joists increase, this column does not really work. I use a floor factored load of 4.0 kpa.

Do you see any issue with my calculation?

 

[tmurray]

If you look at round hollow column sections factored Axial compressive resistance, Cr (KN) in "Handbook of Steel Construction", HSS 73 X4.8 for 3.0 m unsupported length, Cr is about 100 KN, for 10 m span of beam and 5 m span of beam supported joist, this column is OK, but when the span of beam or / and joists increase, this column does not really work. I use a floor factored load of 4.0 kpa.

Do you see any issue with my calculation?

Nothing wrong with the math, just keep in mind that the beams and joists would need to come from Part 9. The largest beam span we can have is 5.99m of douglas fir-larch at 5 ply 2x12 Structural Select grade, but that only allows a supported joist length of 2.4m. As you increase the span of the floor joists, the maximum beam span is reduced, maintaining a similar tributary area.

Our maximum tributary area under Part 9 is somewhere around 14.5 square meters. Adding the 0.5 dead load and 1.9 live load, we get a maximum load of 38.4 kN.

The big thing restraining the anticipated load on these columns are the beams.

 

[sunyaer]

Nothing wrong with the math, just keep in mind that the beams and joists would need to come from Part 9. The largest beam span we can have is 5.99m of douglas fir-larch at 5 ply 2x12 Structural Select grade, but that only allows a supported joist length of 2.4m. As you increase the span of the floor joists, the maximum beam span is reduced, maintaining a similar tributary area.

Our maximum tributary area under Part 9 is somewhere around 14.5 square meters. Adding the 0.5 dead load and 1.9 live load, we get a maximum load of 38.4 kN.

The big thing restraining the anticipated load on these columns are the beams.

Very insight input, so, we got all the things solved in this tricky column design, right?

 

[Inspector Gadget]

Just popping in to plug the Canadian Wood Council's "The Span Book" which provides Code-accepted spans for all sorts of assemblies (inc. three-ply 2x6 not in the 2015 NBC tables.)
Also gives both metric and imperial tables. Stupendously useful.

 

[sunyaer]

Nothing wrong with the math, just keep in mind that the beams and joists would need to come from Part 9. The largest beam span we can have is 5.99m of douglas fir-larch at 5 ply 2x12 Structural Select grade, but that only allows a supported joist length of 2.4m. As you increase the span of the floor joists, the maximum beam span is reduced, maintaining a similar tributary area.

Our maximum tributary area under Part 9 is somewhere around 14.5 square meters. Adding the 0.5 dead load and 1.9 live load, we get a maximum load of 38.4 kN.

The big thing restraining the anticipated load on these columns are the beams.

A big piece of information may have been missed:

Table 9.23.4.3.
Maximum Spans for Steel Beams Supporting Floors in Dwelling Units(1)
Forming Part of Sentence 9.23.4.3.(1)

W310 × 39: Two Storeys Supported, span 6.7 m, supporting 6.0 m span joists, total tributary area is 40.2 square meters, total factored load: 40.2 * 3.25 =130.65 Kn, greater than Cr of HSS 73 X4.8 for 3.0 m unsupported length.

Everything is in and complies to part 9, but the desgin ends up with an unsafe column, where is wrong?

 

[tmurray]

A big piece of information may have been missed:

Table 9.23.4.3.
Maximum Spans for Steel Beams Supporting Floors in Dwelling Units(1)
Forming Part of Sentence 9.23.4.3.(1)

W310 × 39: Two Storeys Supported, span 6.7 m, supporting 6.0 m span joists, total tributary area is 40.2 square meters, total factored load: 40.2 * 3.25 =130.65 Kn, greater than Cr of HSS 73 X4.8 for 3.0 m unsupported length.

Everything is in and complies to part 9, but the desgin ends up with an unsafe column, where is wrong?

Good point. It may actually be worse than you are pointing out. Dead and live loads would be from both stories in this case and may include dead and live loads from the roof.

Ultimately it is in the code, so I as a building official must accept it. It does not mean the designer or contractor shouldn't exercise their own judgement.

 

[sunyaer]

...

... It does not mean the designer or contractor shouldn't exercise their own judgement.

The designer or contractor is not engineer, they should not be expected to have judgement of engineers' level, should they?

 

[tmurray]

The designer or contractor is not engineer, they should not be expected to have judgement of engineers' level, should they?

No, but many have practical experience that tells them what should work or not. A lot of things allowable by the code as a "minimum standard" are unthinkable to many designers and contractors.

The main issue is related to the expected level of performance. Despite something meeting the code, it may not meet the owner's expected level or performance.

 

[sunyaer]

No, but many have practical experience that tells them what should work or not. A lot of things allowable by the code as a "minimum standard" are unthinkable to many designers and contractors.

The main issue is related to the expected level of performance. Despite something meeting the code, it may not meet the owner's expected level or performance.

For this particular column design, it seems to me the code needs to be revised, doesn't it?