Masonry chimney
A rectangular masonry chimney above the roof, cantilevered from the roof line or braced at one level above it, under wind (ASCE 7-22 29.4) and its seismic force as a nonstructural component (ASCE 7-22 13.3). The section is checked at the roof line, at the brace and in the braced span by TMS 402-22, reinforced or unreinforced, strength or allowable stress design (the switch in the toolbar). The prescriptive limits of IBC 2021 2113 (the same as IRC R1003 for dwellings) are checked as reference. US and metric units.
Edition. IBC 2021 2113 and ASCE 7-22 29.4, Figure 29.4-1, 13.3.1 and Table 13.5-1 were read in the library copies. IRC 2018 R1003 is in the library only as an image scan and was not read: its rules are taken as the IBC's (to confirm against the code text). TMS 402-22 is used through the shared masonry library.
Inputs of the chimney #
- Chimney: outside plan width Bx and depth By; the wall thickness (at least 4 in. nominal, 2113.10); hollow units grouted solid or solid units (sets the modulus of rupture); the masonry unit weight and the weight of the liner and cap per foot; f′m; mortar; reinforced or not; the vertical bar, the number of bars (four at the corners, the rest along the faces Bx) and grade; the tie spacing.
- Above the roof: the height above the roof; the roof height at the chimney (it sets qz and z/h); the highest part of the building within 10 ft of the chimney, above the roof penetration (termination, 2113.9); bracing at one level and its height. A brace above the centre of mass makes the chimney "braced above its centre of mass" in Table 13.5-1 (CAR 1.0 instead of 2.2).
- Wind: a strength level pressure, or the basic wind speed, exposure, Kzt, Ke, Kd (0.90 for square chimneys) and Cf of Figure 29.4-1 for a square section with the wind normal to a face or along the diagonal (h/D with h the height above the roof and D the least plan dimension), or Cf entered. qz is taken at the centroid of the part above the roof.
- Seismic: the seismic design category (exempt in SDC A, and B with Ip = 1.0); SDS; Ip; Hf by Eq. 13.3-5 or 13.3-4 at z/h = 1 (attached at the roof); Rμ = 1.3 or from R, Ω0 and Ie.
Auto-design (editor toolbar) varies the number and size of the bars and the wall thickness. Objective: the least steel, then the thinnest walls. Plan size, height, brace, loads and f′m are kept.
Checks performed #
| Check | Basis |
|---|---|
| Wind | p = qz Kd G Cf (Eq. 29.4-1), G = 0.85, on the projected width, both axes |
| Seismic | Fp = 0.4 SDS Ip Wp (Hf/Rμ)(CAR/Rpo) within 0.3 and 1.6 SDS Ip Wp, uniform with the mass; vertical effect 0.2 SDS on the weight |
| Statics | Cantilever M = w H²/2; braced at a: propped cantilever with overhang c, M at the roof = w a²/8 − w c²/4, brace reaction (w H²/2 − M)/a, the largest moment between |
| Flexure, reinforced | Strength: strain compatibility of the hollow section at Pn = Pu/φ (0.9D or 1.2D with Ev), φ of Table 9.1.4, bars not tied; allowable stress: cracked elastic section with the weight, fb ≤ 0.45 f′m, fs ≤ Fs (8.3) |
| Flexure, unreinforced | M/S − P/A at most φ fr (Table 9.1.9.2) or the allowable flexural tension (Table 8.2.4.2) |
| Shear | On the two walls parallel to the load (9.3.3.1.2 or 8.3.5; 9.2.6 or 8.2.6 unreinforced) |
| IBC 2113 | Reinforcement in SDC C to F (2113.3); four No. 4 bars up to 40 in. wide plus two per further 40 in. (2113.3.1); ties at most 18 in. (2113.3.2); walls at least 4 in. nominal (2113.10); at least 3 ft above the roof and 2 ft above the building within 10 ft (2113.9); the roof-line straps of 2113.4 stated |
The drawings are the elevation above the roof (with the brace when there is one) and the plan section with the bars.
Limitations #
- Rectangular chimneys with one flue; Figure 29.4-1 is for square sections, so for a rectangle the least dimension is used for h/D (enter Cf for a different reading).
- The brace, its connections and the roof-line anchorage (2113.4 straps or a designed connection) are not designed; the brace reaction and the roof-line moment are reported.
- The chimney below the roof, its footing (2113.2), the liner, cap, corbels and offsets are not covered.
- One brace level; the brace is taken as rigid.
Verification examples #
Verification status: 9 values compared in 3 examples (0 published, 3 hand calculations from the cited equations): 9 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: not yet recorded for this module.
No published example is on hand: hand calculations of the wind pressure and moment, the weight and seismic force and the statics of the braced chimney. A published worked chimney design (TMS, BIA or CMHA) is listed as a document needed.