Civil Engineering/Outstanding Issue
QUESTION: Dear Sir,
I would like to get an advice from you regarding the following issues which is unclear to me. Appreciate your help in advance.
1.ASCE 7-10 Clause 126.96.36.199 Rigid Diaphragm assumption.
“Diaphragms of concrete slabs or concrete filled
metal deck with span-to-depth ratios of 3 or less in
structures that have no horizontal irregularities are
permitted to be idealized as rigid.”
My concern is the statement "span - depth ratios of 3 or less"
For example a slab with a span of 4m and thickness of slab is
150mm, taking the ratio 4/0.15 = 26.67 > 3 thus not a rigid diaphragm? Or this is only applies to Metal deck or the span to depth ratio the dimension will be taken from plan view that is 4/1 = 4 > 3 thus not a rigid diaphragm? Please discuss.
2. ASCE 7-10 Clause 12.13.4 Reduction of Foundation Overturning
Overturning effects at the soil–foundation interface are permitted to be reduced by 25 percent
for foundations of structures that satisfy both of the
a. The structure is designed in accordance with the
Equivalent Lateral Force Analysis as set forth in
b. The structure is not an inverted pendulum or
cantilevered column type structure.
Overturning effects at the soil–foundation
interface are permitted to be reduced by 10 percent
for foundations of structures designed in accordance
with the modal analysis requirements of Section 12.9.
Is this applies to Soil Bearing capacity?
3. Serviceability limit for seismic? In ASCE 7-10 Cl. 12.8.6- Storey Drift Determaination.
"The design story drift (Δ) shall be computed as
the difference of the deflections at the centers of mass
at the top and bottom of the story under consideration.
See Fig. 12.8-2. Where centers of mass do not align
vertically, it is permitted to compute the deflection at
the bottom of the story based on the vertical projection
of the center of mass at the top of the story.
Where allowable stress design is used, Δ shall be computed using the strength level seismic forces
specified in Section 12.8 without reduction for allowable stress design."
Referring to above clause, does it mean in checking for Seismic drift we will consider factored loadings? "Δ shall be computed using the strength level seismic forces"
But a bit confused with the condition "where allowable stress design is used" since I did not use allowable stress design method. Pls. clarify on this.
Thanks and regards,
ANSWER: Dear Ronald
1- The Span to depth ratio is considered on the plan.
2-The reduction in overturning is applied on soil stress and foundation design both.
3-When designing under seimic effect every force and displacement has to be considered to behave in extreme limit even if you use allowable stress methos as a design method. Then The displacements have to be computed using extreme forces. If you have reduced the seismic forces to apply the allowable stress you should modify the resulted displacement to draw out the limit state drift .
I hope it would be clear . Please dont hesitate to ask further or discuss the matters it would be unclear to you.
---------- FOLLOW-UP ----------
QUESTION: Dear Sir
Thank you for your reply.
Item 1 & 2 is clear to me. However, for item 3 because other believes that to get the seismic drift only SLS loadings need to be consider not ULS loadings. Do we have a reference other than above mention clause in ASCE, the debate is clause 12.8.6 is only applied when "Allowable stress design is used,otherwise the following clause is not applicable. Please clarify more on this.
Thanks & Regards,
It is a matter of USD and ASD. You know that seismic forces are reduced by R factor according to US codes. This reduction is a redoction of force and not a reduction of displacement. For calculation of ultimate displacement you need to modify the displacement from ABI/R by some factor say R, 0.4R or 0.7R. Take a look at UBC97 aand IBC to get familiar with the concept of seismic displacement.
Any doubt? Then ask